The gut is a profoundly important (yet often underappreciated) organ that needs to function well for athletes to optimize training, performance, and recovery.

Patrick Wilson, PhD, is one of the world’s foremost experts on the connections between exercise, nutrition, and the gut and is the author of the recently released book, : The Inside Science of Digestion, Nutrition, and Stomach Distress.

In this series of articles, Patrick will shed light on the inner workings of the gut, why certain athletes are prone to digestive issues, and how they can go about tackling these gut issues using nutrition, training and psychological strategies.

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When it comes to optimizing performance, recovery, and health, many of us are naturally attracted to quick remedies. The use of dietary supplements is one of the best examples of this quick fix culture. In the United States today, people spend more than on supplements that promise to do everything from extend longevity to get you shredded at the gym. It’s not clear-cut why we so often turn to pills when we wish to dull an ache, boost our mood, or get stronger in the gym, but to some extent this predilection to instant remedies seems to be a part of our DNA.

As someone who has written extensively on gut issues in athletes, I often get questions about which supplements hold promise when it comes to improving gut function and reducing the chances of gut distress during exercise. To the disappointment of many of these answer seekers, I usually point out that most supplements have little scientific weight behind them, and for the handful that do work, the actual benefits are often smaller than what people hope for.

Still, dietary supplements are just one of many tools that an athlete can pull out of the toolbox to attempt to manage gut issues, so long as they are used smartly and for the right reasons. In this article, we’ll take a closer look at the scientific oomph behind some of the supplements that are commonly used to influence gut function in the context of exercise.

Probiotics

The gut (particularly the colon) is home to trillions of bacteria that come from perhaps . In fact, the number of bacterial cells colonizing the human body slightly outnumber human cells. Over the past couple of decades, literally hundreds of studies have documented undeniable links between this collection of microbes—called the microbiome—and different aspects of human health and function. Recent research has even tied different gut microbiome profiles to levels of . It comes as no surprise, then, that probiotics—live microorganisms taken to manipulate the gut microbiome and health—are one of the trendiest supplements used by athletes.

Although it’s intuitive to think that probiotics can prevent or manage gut woes, the science on this topic is frustratingly inconsistent, at least when it comes to gut troubles in athletes. A few studies have found modest , but others have not, and one even found likely increases in the number and duration of GI symptom episodes with a . It’s also important to recognize that the magnitude of benefit observed in the positive studies has generally been pretty small. In other words, even if a probiotic does work, you shouldn’t expect that it will completely ameliorate all your gut problems, or even reduce them by half in most cases.

Even though probiotics likely have minor effects on the severity of gut symptoms in most athletes, there is more positive data when it comes to their impact on the respiratory system. Placebo-controlled experiments in both athletes and non-athletes show that , which, by reducing the odds of catching infections. (In case you’re wondering about the risk of SARS-CoV-2 infection and probiotics, we don’t currently know if they offer any protection.)

If you do decide to take a probiotic, be aware that varieties from the Lactobacillus and Bifidobacterium genera have the strongest records of benefit across a variety of applications. Fermented foods like yogurt, kefir, and kombucha are potential sources of probiotics, but supplements are probably a better route when it comes to targeting specific dosages and strains. There is no minimum dosage that applies to all probiotic supplements but using one that provides at least one billion colony-forming units is often recommended as a starting point.

Ginger

Ginger has been used as an antidote for a queasy stomach for hundreds, if not thousands, of years. Ancient Chinese sailors purportedly used ginger during voyages on the high seas to ward off seasickness. In more contemporary times, its anti-barf properties have been trialed for everything from . The mega-popular TV show MythBusters even put ginger to the test in one of its episodes. For host Adam Savage and sidekick Grant Imahara, ginger prevented motion sickness even though they each spent half an hour in a slowly rotating chair. And among a few experiments that have evaluated , the results have generally been favorable. Ginger is thought to work as an anti-nausea remedy by blocking the activation of receptors in the gut that bind with serotonin. This is the same pathway by which some prescription antinausea medications work (e.g., ondansetron, brand name Zofran).

Of all athletes, ultrarunners are especially well-acquainted with nausea. In a study of competitors from the Western States 100-mile Endurance Race (WSER), . The WSER is often held in sweltering conditions, which may contribute to the remarkably high rates of nausea there; regardless, practically every ultrarunner has dealt with nausea at some point while training or competing.

So, what do we know about ginger’s potential as a nausea remedy for ultrarunners and other athletes? Unfortunately, direct evidence that ginger prevents nausea during exercise is lacking, so we’re largely left in the dark on this issue. Ginger is generally recognized as safe by the Food and Drug Administration and has a pretty solid safety record, though it can trigger other gut symptoms like and affect blood clotting, meaning those who have clotting disorders or who take blood thinners should be extra-cautious. In terms of dosage, 1–2 grams an hour before an expected nausea-provoking event could be a starting point. For many runners, though, nausea doesn’t rear its ugly head until hours into competition, so ingesting some at the midway point of a race could be more effective. Many foods containing ginger (ales, snaps, etc.) probably don’t contain enough active compounds to quell nausea; if you’re looking to get a real physiological effect, it’s probably best to go with a supplement or to make your own beverage or food.

Glutamine

Your gut has a hearty appetite for the amino acid glutamine. After getting absorbed in the gut, most amino acids—the building blocks of protein—enter the blood, but , meaning it gets locked up by gut tissue. Much of this sequestered glutamine, it turns out, is . For this reason—as well as its effects on immune cell function—glutamine is often touted as a gut health promoter.

Exercise—especially when it’s intense or prolonged—poses a challenge to gut function. Blood, oxygen, and nutrient delivery to the gut can all become compromised as these precious resources are redirected to the skeletal muscle and skin (for cooling purposes). As a consequence, the normal gut barrier that prevents outside stuff from entering your body can become leaky and dysfunctional. This gut leakiness can lead to bacterial translocation inside the body. In turn, your body identifies these trespassers and releases inflammatory molecules to help keep the invasion at bay.

It’s been hypothesized that these changes in gut barrier integrity contribute to gut symptoms like nausea and perhaps even heat illnesses, though scientists still debate to what extent this theory is true. Regardless, glutamine supplementation has been tested in several studies to lessen gut leakiness during exercise (see , , and for examples). In one representative study, glutamine ingested two hours before a 60-minute run in the heat (86℉) likely attenuated the typical . While sports scientists like to geek out over changes in physiological biomarkers, most athletes are more interested in actual performance and perceptual improvements. Somewhat disappointingly, the studies cited above either did not measure gut symptoms or failed to find any clear improvements with glutamine when they did assess said perceptions. Perhaps more importantly, plain old carbohydrate is known to , which begs the question as to whether the extra expense of supplementing with glutamine is largely redundant. As an added benefit, carbohydrate ingestion can improve performance when exercise lasts longer than 60 to 90 minutes.

I’m hopeful that future research will help clarify what advantages, if any, glutamine offers over carbohydrate ingestion, but at this juncture, I tend to favor focusing on other strategies for dealing with gut issues instead of turning to glutamine supplements. If you decide you still want to give glutamine a try, bear in mind that doses used in research range substantially but often fall somewhere between 15 and 50 grams. To minimize the likelihood of any stomach upset, it’s usually taken at least one hour before starting exercise.

Summary

Although dietary supplements are inherently an attractive means of easing gut problems (what’s easier than taking a pill?), the honest truth is that they are unlikely to be a home run for most athletes. Most of the time, taking supplements to fix the gut is more akin to hitting a slap single in the ninth inning when your team is down by five runs. If you do decide to use any of the supplements discussed in this article (or other supplements), it’s most definitely worth your effort to do some research on the company selling the product to make sure they are serious about quality and safety (for some resources on this process, see and ). Oftentimes, however, people can achieve better outcomes with managing gut problems through changing their diet, getting sufficient sleep, and managing life stress and anxiety. Plus, improving these other aspects of your life are much more likely to extend to other aspects of your health and performance than taking a pill.

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Runners face numerous gut problems during training and competition, and there are undoubtedly numerous potential explanations for each. Indeed, there are so many possible causes that I wrote a on the topic. For those who want to cut to the chase, here are five of the leading ways runners trigger exercise-related gut woes.

1) They Gobble NSAIDs

It’s no surprise that lots of runners struggle with daily aches and overuse issues. In of 844 novice and intermediate-level runners participating in a 13-week training program, about three out of every 10 reported a running-related injury. A published a decade later found a similar injury incidence among a cohort of 200 recreational runners who were followed for 12 weeks. If you follow runners for even longer periods of time, say a couple of years, upwards of suffer from at least one overuse injury.

Surely these figures aren’t a revelation to any veteran runner or running coach, but they do help explain why the rate of anti-inflammatory medication use is astronomically high among this population. Indeed, a recent recruited through Parkrun UK (an organization that facilitates weekly 5-kilometer runs across the United Kingdom) found that about 9 out of every 10 runners used a non-steroidal anti-inflammatory drug (NSAID) in the past year, and 80 percent had taken NSAIDs in the previous four weeks.

While popping a few over-the-counter NSAID pills may seem relatively harmless, these drugs can impinge on many of the body’s organ systems, and the gut is no exception. In that study of Parkrun UK runners, roughly one-third of those who had used NSAIDs said they experienced at least one adverse drug reaction that impacted the gut (nausea, discomfort, heartburn, GI bleeding, etc.). Likewise, a found that runners who used pain relievers like NSAIDs pre-race had a tenfold higher rate of intestinal cramps than non-users and were about twice as likely to withdraw from the race because of gut cramps.

2) Their Hydration Is Imbalanced

If you are training or competing for an hour or less, drinking solely based on your perception of thirst is usually the simplest and most effective hydration strategy. Once exercise extends beyond a couple of hours, though, hydrating to optimize performance can become a real conundrum.

I say this because both overhydrating and underhydrating in these situations can cause major issues. On the underhydration side, large fluid deficits can compromise your blood volume and ultimately the delivery of oxygen and fuel to your muscles, as well as to your gut. Some research has even documented that exercising in a dehydrated state from the stomach and like nausea and stomach cramps. In other words, if you become dehydrated because of a failure to drink enough fluid before and during exercise, your gut’s function may become compromised. And that could make it even harder to correct the problem, as any fluid you ingest will sit like a brick in your stomach.

On the other end of the spectrum, downing bottle after bottle of fluid can also cause major gut problems. On the most basic level, drinking fluid at a high rate heightens perceptions of fullness, bloating, and nausea, which are generally not conducive to performing at your peak. In one , drinking fluid at an average rate of 1.4 liters per hour (which was done to minimize dehydration levels) did not lead to better half-marathon performance in comparison to simply drinking to thirst (~0.4 liters per hour), despite the fact that drinking more reduced body weight losses (1.3 percent vs. 3.1 percent) and lowered heart rates by 4 beats per minute. Importantly, drinking well above thirst doubled the severity of gut discomfort, which likely overshadowed any cardiovascular or thermoregulatory benefits from drinking the additional fluid.

So, if drinking to thirst worked well in the previous study, then why not just recommend it across the board for all runners? Personally, I’m hesitant to say that drinking to thirst works best in every situation because as exercise duration gets longer, small mismatches between sweat rates and drinking rates can add up to a hefty total. If you’re not sure what I mean, it can be helpful to consider an example. Say a 150-pound runner who drinks based on his thirst ingests 0.5 liter per hour and sweats 1 liter per hour during a 5-hour trail marathon. Ultimately, this mismatch in drinking and sweating may not add up to much 2 hours into the race (i.e., a couple of pounds), but by race’s end, his net loss would be about 5.5 pounds of fluid, which has the potential to impact his performance.

Bottom line, there is no one-size-fits-all hydration strategy that will work for every athlete in every situation. While drinking to thirst is a good option for many runners, especially during relatively brief runs, a regimented approach may offer some advantages for longer exercise bouts. On a final note, you should never drink above your sweat rate during exercise, as this can raise the risk of complications like hyponatremia. (Here’s an example of how to .)

3) They Ignore Pre-Race Nerves

Anecdotally, many athletes believe that excessive nerves are a source of their gut woes during competition. Although scientific studies confirming this are few and far between, one of London Paces Distance Club members revealed that 43 percent said they experienced “nervous diarrhea” before competition. It wasn’t until very recently that more systematic research attempted to quantify the relationships between psychological stress, anxiety, and gut complaints in athletes. It just so happens that I’ve published a few of the only existent studies on this topic.

The first one was published in the Journal of Sports Science in 2017. In short, I had 150 seasoned runners track their gut symptoms after every run for a month, and at month’s end, they completed questionnaires on life stress and anxiety. After crunching the numbers, I found that scores on the stress and anxiety questionnaires were positively correlated with the percentage of monthly runs that participants reported at least one substantial GI symptom. It’s important to note that the correlations in this study were not super strong, meaning that other factors were undoubtedly involved in the runners’ gut problems. Still, the correlations were as big as those from other studies looking at predictors of gut troubles in athletes. In reality, we should rarely expect any single factor to fully explain why a runner gets gut distress.

Based on these initial results, two of my colleagues and I carried out a follow-up study to see if this same phenomenon occurs during endurance races specifically. In the , we reported that having higher levels of anxiety increased the odds of suffering from several GI issues (nausea, reflux, cramping) during races. Moreover, being anxious on the morning of a race seemed to be somewhat more predictive of gut problems than just having general life anxiety.

Before we get carried away, it’s important to remember that these findings are correlational. In other words, we did not prove with 100 percent certainty that stress and anxiety are causing these gut symptoms. It’s certainly possible that athletes who are stressed or anxious are engaging in other behaviors that provoke their gut. Similarly, these athletes’ anxieties may be the result of — rather than a cause of — their gut woes.

We are currently planning follow-up studies to see whether easing anxiety leads to fewer gut complaints in runners and other athletes, so hopefully we’ll have more concrete evidence in coming years. Even so, there are few, if any, possible downsides to managing anxiety through various interventions like slow deep breathing and meditation.

4) They Don’t Prepare for Hot Conditions

Acclimatizing to the heat is crucial to performing well as the summer months roll around each year. The physiological adaptations your body makes to repeated training in hot conditions — larger blood volume, enhanced sweating, less cardiovascular strain, et cetera — not only help you perform well but may just also lower your chances of being plagued by gut disturbances during exercise.

Runners who don’t acclimatize to sweltering conditions can experience upper gut symptoms like nausea, bloating, and burping at when they exercise in hot conditions as compared to when they exercise in temperate environments. Although there are several explanations for this observation, a drop in gut blood flow is perhaps the most important one. A showed that exercising in super-hot conditions (110°F) diminished blood flow to the liver (which reflects gut blood flow generally) more so than exercising in moderate conditions (78°F). This is partly because a notable share of blood flow is diverted to the skin in hot conditions, leaving table scraps for the gut.

The details of any heat acclimatization plan will vary depending on the athlete’s experience, goals, and access to resources. For more context on how to implement heat acclimatization training, check out this other .

5) They Fast or Under-Eat

You don’t need a scientific study to tell you that overeating — especially within 30–60 minutes of starting intense exercise — is a recipe for digestive difficulties. (If for some reason you doubt this, check out from The Office.) What’s perhaps less intuitive is that undereating or outright fasting before exercise can also provoke certain gut symptoms, the most prominent of which is nausea.

Surprisingly, few studies have looked at how working out on an empty tank influences gut perceptions despite the growing popularity of fasted training. Even so, we do have a decent amount of circumstantial evidence from other arenas that can help inform us. Another situation where people often fast is before surgery, which is usually done to lower the risk of aspirating foodstuffs into the lungs under anesthesia. Traditionally, patients getting surgery are told to fast overnight, which typically equates to at least 8–12 hours sans eating. More recently, however, some of these guidelines have shifted, in part because that long fasts before and after surgery.

Obviously, these studies aren’t completely applicable to exercise, but they do bolster anecdotes of heightened nausea that some people report with fasted workouts. Plus, has shown that doing high-intensity cycling for 20 minutes provokes more nausea if a person fasts overnight in comparison to exercising 60 minutes after eating.

Exactly why fasting may inflame nausea is still being explored, but one of the likeliest explanations is that it boosts the secretion of stress hormones like adrenaline into your bloodstream. These hormones can act on receptors in the brain to induce nausea. Coincidentally, a rise in these hormones in the blood may help explain why super-stressful life events and high-dose caffeine ingestion are also capable of exacerbating nausea in certain people.

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Patrick Wilson, PhD, RD, is a registered dietitian, associate professor of exercise science, and author of . He has published over 50 peer-reviewed scientific articles, many of which are on the causes of gut issues in endurance athletes.

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Caffeine is perhaps the most widely used mind-altering substance in the world, though unlike some of humanity’s other favorite psychoactive chemicals, caffeine is neither illegal nor demonized. Indeed, most people have very positive views of caffeine-containing foods and beverages. (How many “I need my coffee” memes and GIFs are there? I’m too unmotivated to count, but I’ll venture to guess thousands.)

Like the general public, many athletes also love their coffee, energy drinks, and caffeine. that looked at caffeine in urine samples analyzed at the Spanish Anti-Doping Laboratory, roughly three-quarters of over 20,000 samples received over a 5-year period contained traces of caffeine. Interestingly, endurance athletes were found to have the highest levels, suggesting they’re more likely to ingest larger amounts of caffeine before or during competition than other types of athletes.

As with any nutritional strategy, though, caffeine use comes with some potential downsides. These drawbacks, to a large extent, depend on dosage and the situation, and they vary quite a bit between people. The body organs affected by caffeine are many in number, so we’re going to narrow our lens to focus on the gut, my area of expertise. Whether you’re a crazed coffee enthusiast or just the occasional caffeine consumer, you’ll hopefully find something useful to take away from this overview of how caffeine impacts digestive function and gut symptoms.

Caffeine’s Effects on the Gut

Anecdotally speaking, most coffee drinkers know that having a cup of joe in the morning can open the metaphorical freeway that is their bowels. Perhaps up to one-third of people believe that for them, and some experiments confirm that drinking coffee (i.e., colon and rectum), at least in a subset of people. Part of this effect is due to caffeine, though even decaffeinated java can spur motor activity in the colon, so coffee’s poo-inducing effects . Still, studies in humans and other animals () show that caffeine impacts large intestine activity and that larger doses produce more profound changes. Furthermore, diarrhea and loose stools are of ingesting toxic (extremely high) amounts of caffeine.

Caffeine can undoubtedly influence the occurrence of lower-gut symptoms, but it’s also capable of provoking problems that affect the upper half of your digestive tract. Of any upper-gut problems that an athlete can experience before or during competition, nausea is unquestionably one of the most troublesome. Nausea tends to be more prevalent with super-intense, relatively brief exercise bouts (think a 400-meter dash) or extremely prolonged exercise (ultramarathons).

The underlying origins of nausea differ depending on the situation, but high-dose caffeine ingestion is one possible culprit, particularly when it comes to nausea that arises with high-intensity anaerobic exercise. These brief bouts of super-intense activity cause loads of catecholamines (a.k.a. adrenaline and noradrenaline) to be released by the body, and these hormones can exacerbate nausea via their effects on the brain and nervous system. Caffeine, as it turns out, is one of the things that amplifies the body’s secretion of catecholamines (), which, in theory, could provoke greater feelings of nausea in some athletes.

It should be noted that most people don’t experience much, if any, nausea after ingesting moderate caffeine doses (e.g., 3-5 milligrams per kilogram of body mass, or about 2-4 cups’ worth of brewed coffee for the average person). In addition, many studies have shown caffeine to be performance-enhancing across a broad range of athletic endeavors, which further supports the notion that its gut-related side effects are a major problem for only a minority of athletes. That said, there are a few things athletes can do to minimize the likelihood that caffeine will cause them digestive troubles before and during competition.

• Avoid taking caffeine in high doses (>7 mg per kg of body mass) or with other stimulants. found that using a dose of 9 mg per kg of body mass led to gut problems in 31% of exercising subjects versus only 8% when lower doses were used. In addition, that mixing caffeine with other stimulants can elicit nausea during high-intensity exercise.
• If you’re typically a nervous Nellie before or during competition, consider skipping the caffeine or using just a little. It’s worth keeping in mind that nearly all the research supporting caffeine’s performance benefits has been done using simulated competition, and I highly doubt the testing done in these situations was as stress-provoking as real-life competition.
• Instead of ingesting your brew just before competition, down your coffee 60-90 minutes earlier. Coffee’s colon-stimulating properties probably peak over the initial 30 minutes after ingestion.
• Make sure you aren’t totally fasted when taking caffeine before competition. Your body responds to fasting by , which, as I’ve already mentioned, can worsen perceptions of nausea. In basic terms, taking loads of caffeine on an empty stomach before a grueling workout or competition can be a perfect recipe for being stricken with nausea.

Daily Caffeine Use and Tolerance

The above discussion of caffeine’s effects on the gut is largely based on studies that have acutely administered caffeine, oftentimes after making people avoid caffeine for a day or two. You might be wondering, then, whether we can extrapolate those findings to situations where people habitually consume caffeinated beverages and foods every day. Unfortunately, there aren’t many studies that have specifically looked at the gastrointestinal implications of chronic caffeine consumption.

The findings of one recent investigation, however, do seem to confirm that daily caffeine ingestion leads to more digestive problems in some people. For the study, , investigators had 11 participants consume caffeine (3 mg per kg of body mass or about 2 cups of coffee) or a placebo for 20 days. After their first treatment period, the participants were then reassigned to the opposite treatment for another 20-day period. The researchers were interested in seeing whether the participants would develop a tolerance to caffeine and whether the potential negative impacts of its consumption (blood pressure elevations, nervousness, insomnia, etc.) would wear off over time. Interestingly, caffeine’s blood pressure-raising effects seemed to diminish somewhat after about 10 days, while its propensity to cause nervousness did not seem to dissipate over the 20-day period.

The researchers also asked participants about their “gastrointestinal distress” levels, though they didn’t elaborate on what exactly that phrase meant. Gut distress was assessed on a 1-to-10-point scale every 2-3 days, and overall, average distress levels were likely higher in the caffeine condition than in the placebo condition at several points throughout the 20 days, including as far as 15 days into the supplementation period. Notably, average ratings of gut distress during the caffeine condition were typically between 2 and 3 (in comparison to 1-1.5 with the placebo), although there were clearly some subjects who reported ratings of greater than 5. This re-enforces the idea that although most people experience only mild GI disturbances from caffeine ingestion, a subset occasionally deals with moderate-to-severe gut troubles.

Final Thoughts

Decades of research verifies that caffeine can be used as a performance-booster, so long as one considers the factors that increase the likelihood of negative effects such as gut distress. At high doses, caffeine can trigger nausea, urges to go nĂşmero dos, and perhaps other symptoms like reflux and abdominal cramping.

As with many nutritional strategies, the responses to caffeine ingestion can vary tremendously between individuals, so trial and error is crucial for athletes who want to determine how to get the most out of their caffeine use. One important yet undiscussed factor that may influence your response to caffeine is your genetics, which is a topic that will be covered in a follow-up article. In the name of science, I decided to have my genes tested for important caffeine metabolism pathways. and an overview of a burgeoning research field on the genetics of caffeine responses.

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The sport of ultrarunning was essentially nonexistent 50 years ago. Even as the running boom took hold in the 1970s, races longer than the standard 26.2-mile marathon remained a rarity. The origins of perhaps the most famed ultrarace in the United States, the (WSER), date back to 1974 when Gordy Ansleigh ran on foot against horseback riders in the Western States Trail Ride, but the first official WSER wasn’t held until 1977.

The popularity of ultrarunning has grown dramatically in the intervening years, particularly in the past two decades. According to , there were over 2,400 ultrarunning races in North America in 2019, with 128,685 finishes. In contrast, there were just over 200 such races and 13,852 finishes in 2000. These are still relatively small counts compared to shorter running races, but there’s no reason to think that the number of people competing in ultras won’t continue to grow precipitously in the coming decades.

Because ultrarunning places such extreme energy and metabolic demands on the body, fueling is undoubtedly one of the most important elements to crossing the finish line of these races. Paradoxically, however, the extreme durations involved in these contests also make them a particularly tough nut for scientists to crack. And although this is changing ever so slowly, to a large degree we scientists are akin to a person wandering around in the dark looking for a light switch. And ultimately, this lack of data explains why personal anecdotes and trial and error play such a huge role in informing nutrition practices in ultrarunning communities.

Why the Lack of Data?

From the simplest of perspectives, studying nutrition in ultrarunning is so vexing because of the exercise durations involved. As a scientist, one of my major concerns when designing sports nutrition experiments is participant recruitment. If you can’t get enough people for a good-sized sample, there’s no point in even carrying out your experiment, as you won’t be able to say whether any changes you see are due to the intervention or simply from day-to-day variation in performance. The definition of an adequate sample varies depending on the scientific field and a study’s methodology, but any experiment with less than 10 people is usually highly questionable.

In most metropolitan areas, it’s not hard to find a few dozen committed ultrarunners, if not more. Convincing them to volunteer hours of their time, often for no money, while being poked and prodded like a lab rat, is another matter. The best studies also ask participants to standardize their diet and training, meaning that these human guinea pigs must give up control over these things for at least a few weeks. It’s safe to say most runners are not big fans of relinquishing control over these particulars.

To put these issues into more concrete terms, it can be helpful to consider a hypothetical example. Say I wanted to understand how two different in-race fueling strategies impact 50-mile ultra performance. Finishing times for 50-mile ultras vary a lot depending on the course, but assume I design a protocol on a treadmill that takes eight hours to finish on average. Most of these types of studies ask participants to complete all the study interventions over a period of weeks or months, which effectively means that each person serves as their own control. In addition, each person would ideally complete a baseline performance test prior to receiving any intervention. To recap, that means each volunteer is expected to complete three separate 50-mile runs on a treadmill. Not only that, but if you want to understand whether an intervention actually impacts performance, your volunteers need to give a good effort each time.

Even if a researcher somehow miraculously found enough volunteers to do this sort of study, there are more issues to consider. As any ultrarunner could tell you, replicating all the facets of a real-life race is next to impossible in a lab (changes in elevation, the isolation on certain parts of the course, the extremes in environmental temperature, etc.). And that begs the question as to how well any result from a lab study translates to the real world. A seemingly simple solution to this problem would be to do an experiment at an actual ultrarace, but perhaps unsurprisingly, runners often don’t want to risk volunteering for a study where they must stick to a rigid fueling plan they may be unaccustomed to. Furthermore, the optimal fueling plan for an elite runner who competes for a win is inherently different from someone running near the middle or back of the pack.

Long story short, there are few published experiments that have scrutinized the effects of various nutrition interventions on ultra-exercise performance. This was succinctly summarized by the authors of on single-stage ultramarathoning that was recently published by the International Society of Sports Nutrition.

“The data informing our recommendations are incomplete . . . for several reasons. Firstly, despite the growing popularity of ultra-marathon, participant numbers are still relatively low. Moreover, runners are often reluctant to compromise their race preparation and/or recovery to volunteer for data-collection . . . ”

What’s more, Dr. Martin Hoffman, who has served as the research director for the WSER and has led numerous ultrarunning research studies, recently combed the scientific literature and found that, from 1999 to 2019, there were only on the topic of ultrarunning.

So How Do Most Ultrarunners Fuel?

Despite the lack of experimental data on ultramarathon fueling, we do have a decent amount of research available on what these athletes choose to do of their own volition. One of the was an examination of what the Greek ultrarunner Yiannis Kouros ate during the 1985 Sydney–Melbourne Ultramarathon. Kouros is widely considered as one of the greatest ultrarunners of all time and has sometimes been called the “Running God” (he even starred as Pheidippides in the movie The Story of the Marathon: A Hero’s Journey). During his 5-day, 5-hour Sydney–Melbourne Ultramarathon effort, Kouros downed just over 55,000 kcal, or about 10,500 kcal per day. The vast majority of his energy intake (96 percent) came from carbohydrate (Greek sweets, dried fruit, fruit and sport drinks, etc.).

It’s certainly easier to maintain energy balance during multi-day ultramarathons like the one Kouros did in 1985, so we need to examine the fueling practices of single-stage races through a different lens. Several organizations such as the American College of Sports Medicine say that athletes participating in ultra events that last more than 2.5 hours can consume up to 90 grams of carbohydrate per hour, whereas intake rates of 30–60 grams per hour are more realistic for shorter events. The difference in recommendations is largely because it is easier to consume more food and carbohydrate during longer events (at least in theory), as the average exercise intensity is lower.

So, where do ultrarunners tend to fall when it comes to carbohydrate intakes? Well, most eat nowhere near 90 grams of carbohydrate per hour. Instead, many studies find average ingestion rates closer to 30 to 40 grams per hour (see and ). The reasons for these lowish intake rates are varied. Many of the runners studied were average to back-of-the-pack finishers, who likely needed to eat less carbohydrate on a per hour basis than highly competitive runners. In addition, gut distress (including nausea) is very common in ultraraces and can interfere with a runner’s ability to tolerate eating and drinking. In one study, for instance, intakes of energy and carbohydrate were negatively correlated with (vomiting, heartburn, diarrhea) during a 60-kilometer ultramarathon.

Although most investigations find that ultrarunners tend to eat 20 to 40 grams of carbohydrate per hour during competition, there is some interesting evidence that higher intake rates can be tolerated by certain athletes. One of the studies I referenced earlier included a runner who consumed 108 grams of carbohydrate per hour during a 120-kilometer race. Another found that a handful of them ate about 120 grams of carbohydrate per hour, which is equivalent to 5 to 6 sports gels every hour! A group of Spanish researchers recently took these field observations even further and found that relatively well during a mountain marathon so long as they trained their guts before competition.

As a caveat, it’s important to note that these studies can’t tell us much about carbohydrate dosage and performance. To put it another way, does downing 120 grams of carbohydrate per hour—if tolerated well by the gut—lead to better performance than a more modest intake (30–60 grams per hour)? Unfortunately, we do not yet have good experimental evidence to answer this question, at least when it comes to ultra racing. And honestly, the answer will likely depend on a number of factors, including the specific race, individual differences in metabolism, and whether the athlete has trained their gut to tolerate high ingestion rates.

Formulating Fueling Recommendations in the Face of Slim Evidence

Given that the cupboard of science is pretty bare when it comes to ultrarunning and fueling, recommendations from nutritionists and dietitians tend to focus on individualism, athlete experience, and trial and error. To illustrate this point further, consider that the recent position stand from the International Society of Sports Nutrition on included references to individualization and personal tolerance more than a dozen times. In other words, there are no quick and easy answers.

While we wait (hopefully not in vain) for more and better experimental science to emerge on how to fuel for an ultra, I would urge athletes to regularly document their feelings, perceptions, and challenges with fueling during training and racing. This doesn’t have to be a high-tech endeavor; paper and pen will suffice in most cases. The key is to keep some record of how your various nutrition plan adjustments and iterations affect how you feel, recover, and perform. Ultimately, this sort of regimented approach may help speed up the learning process that comes naturally with experience over time.

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Few topics in sports nutrition are in vogue today as the gut microbiota, with numerous companies now selling tests that come straight to your front door. Supposedly all you need to do is drop some of your poo in the mail, and presto, you’re on your way to your best performances ever. Indeed, some of these companies outright claim these tests can help improve performance, enhance sleep, hasten recovery, and prevent illnesses.

Supplement companies have also jumped on the microbiota bandwagon, creating a seemingly ever-expanding marketplace offering everything from probiotics to prebiotics to synbiotics. In fact, the global probiotics market was estimated to be . Much like with the poo-testing companies, many of these supplement producers make bold claims. After one minute of searching online, I found a company that claimed their probiotic could “support energy, stamina, and endurance at the microbial level,” whatever the heck that means.

To be sure, there is reason to feel optimistic that the gut microbiome may have the power to impact athlete performance and health. But, as with many novel and exciting topics in the field of sport science, the hype seems to be outpacing real scientific inquiry. I loathe to be a hater, but this is one topic that needs to be brought back down to earth.

Defining and Assessing the Gut Microbiota

Before we look at microbiota research that is relevant to athletes, we first need to define what we mean by the phrase gut microbiota. Like any of your body’s external surfaces, your gut is populated with numerous micro-organisms: bacteria, viruses, fungi, and the like. We’ve known the gut serves as a home to bacteria since at least the 1840s, when a surgeon named in the ejected vomit of a 19-year-old patient. These days it’s recognized that bacteria residing in the gut do more than just hang out. These bacteria—known collectively as the gut microbiota—play a role in . A slightly different phrase, the gut microbiome, describes not only the microbes themselves but also their genes and the entire habitat in which they reside.

Of any place in the gut, the colon is undoubtedly the most abundantly populated with bacteria. The exact count varies some from person to person, but the average man or woman might with over 95 percent in the colon. You may find it hard to believe, but you have more than one bacteria cell in and on your body for every one of your own cells.

When it comes to characterizing a person’s gut microbiota, the most common approach is taking a sample of caca. While this is relatively easy (and somewhat gross), fecal samples are just a . Indeed, microbes predominating in stool and those that tend to live in close proximity to the mucosa (the lining of the gut) . This issue—along with other methodological difficulties—has made it challenging to figure out which microbes populate the gut and how these populations change in response to various lifestyle and environmental factors.

Even so, there are some points of agreement among scientists in this field. First, the gut microbiota tend to develop in a somewhat predictable manner early in life. For example, in the first days after birth, an infant’s gut microbiota shifts to become richer in , and this persists until they transition to eating solid foods. Second, it also seems true that the gut throughout much of adulthood, though it does shift somewhat in response to diet changes, weight loss, and antibiotic use. Simply put, the gut microbiota seem to have both stability and responsiveness characteristics.

The Athlete’s Gut Microbiome Is Different. But Why?

One of the first studies to look at the microorganisms residing in . In comparison to non-athletes, the rugby players had more diverse microbiotas based on several measures. Notably, they had greater proportions of 48 bacteria phyla than non-athletes of similar body sizes, and just one phylum type, Bacteroidetes, was less plentiful in the athletes. Somewhat similar results were found in a later study of elite Polish marathoners, in that they had a . In contrast, runners had greater abundance of Prevotella than non-athletes. The findings of these studies also reflect the overall body of research, as a 2020 review from the International Society of Sports Nutrition concluded that, “active individual’s microbiota display a . . .and increased diversity.”

One issue that scientists are grappling with is exactly where this extra microbiota diversity comes from. Is it the athletes’ training, diet, or something else? As you can imagine, it is exceedingly difficult to tease these factors apart and make broad-sweeping conclusions. Still, some findings are fairly consistent between studies. The first is that higher intakes of fiber and other complex carbohydrates are associated with greater abundances of certain microorganisms, some of which are linked with good health. Further, experiments enhances the abundance of Bifidobacterium and Lactobacillus species. When people exercise a lot, they often up their carbohydrate consumption, which may partly explain the greater microbiota diversity observed in athletes.

Beyond fiber and complex carbohydrates, protein intake is often higher in athletes. Interestingly, that study of Irish rugby players found that protein consumption positively correlated with microbiota diversity. Amino acids (the products of protein digestion) can be broken down by certain bacteria in the colon if they are left unabsorbed (which is more likely with high protein intakes). Although this may seem like a positive, we need to interpret this finding with a good dose of caution. Some of the byproducts of this colonic amino acid metabolism may residing there.

Additionally, adding extra protein to the diet may directly impact the types of bacteria that thrive in the gut, sometimes not in a good way. One study of endurance athletes, for instance, found that supplementing with protein (a blend of whey isolate and beef hydrolysate) such as Roseburia and Bifidobacterium. Ultimately, the effects of protein on the microbiota will undoubtedly vary based on dietary source (plant-based, meat, fish, dairy), and we’re still trying to flesh out what each of these different sources does to the gut microbiome.

Does the Gut Microbiota Matter for Athletic Success?

It’s well and good to know that some of the microbes living in athletes’ guts differ from people who are sedentary, but does this matter on a practical level? There are various ways to answer this question, but one simple approach is to see if gut microbiota markers correlate with aspects of athletic prowess. A 2016 investigation did exactly that, finding that performance on a species richness among young adults. Other studies have also found that certain gut microbiota profiles correlate with aerobic fitness levels in healthy non-athletes (see and ).

Yet, these types of studies cannot prove the gut microbiota directly modifies aerobic fitness. It’s equally, if not more plausible, that the fitter people ended up with different gut microbiotas because of their diet and training. In other words, their gut microbiota could simply be a byproduct of their lifestyle. To really figure out whether the microbiota is vital to athletic success, the ideal approach would be to experimentally manipulate the microbes living in athletes’ guts.

Supplementing with probiotics is one such option, but disappointingly, experiments that have administered them to athletes and tested performance have, by and large, (though respiratory immune benefits are not uncommon). It’s conceivable that these studies didn’t use the right type or dose of probiotic, but it could also be because bacterial metabolism in your gut just simply isn’t powerful enough to affect your muscles or heart during vigorous exercise. Regardless, probiotics are pretty low on the list of evidence-based strategies an athlete should try to directly improve their performance.

A Performance Probiotic on the Horizon?

One criticism of the abovementioned research is that the probiotics weren’t chosen with the express purpose of improving performance. Instead, the specific species were typically selected because of their supposed health properties or simply because they were used in earlier investigations.

In 2019, a group of scientists published a paper in Nature Medicine that took a more methodical approach to identifying . In a first step, the researchers evaluated the microbiotas of 15 runners from the 2015 Boston Marathon, which involved collecting stool samples over several days preceding and following the race. Overall, the runners showed increases in Veillonella microbes after the marathon relative to pre-marathon. The scientists replicated these results in ultramarathoners and elite rowers, and they went on to speculate that Veillonella species could directly modify running performance because they are known to metabolize lactate into short-chain fatty acids. In theory, this could lower lactate levels in the body and provide another source of fuel in the form of short-chain fatty acids.

In a final part of their investigations, the scientists tried proving that Veillonella bacteria can directly enhance performance. They did this by inserting Veillonella atypica into the GI tracts of mice and making them run on a treadmill several hours later. The speed of the treadmill began at 5 meters per minute and was accelerated by 1 meter per minute every minute thereafter. In comparison to when they were treated with a control microbe (Lactobacillus bulgaricus), the mice ran 13 percent longer when they got the Veillonella treatment.

I was impressed with this research for several reasons. It combined observations from human athletes and showed that, in principle, resistance to fatigue can be improved through manipulating gut microbes. It’s worth mentioning, though, that some of the authors work for or hold equity in a company called , which is working to commercialize a product off of this research. There is nothing inherently wrong with that, but it does raise my level of skepticism a notch, and I’d like to see the results replicated by other groups.

More importantly, though, “performance improvements” in mice do not mean the same thing will happen in humans. As has been pointed out by others, . Also, given that endurance athletes already see substantial jumps in Veillonella with exercise, it’s debatable whether they would benefit from a supplement that attempts to add more Veillonella. In the next few years, I’m guessing we will have more concrete data on this potential performance-boosting micro-bug as more human experiments are carried out.

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When a runner complains of a troublesome gut, a well-meaning coach or dietitian will often quickly search for answers by looking at the standard perpetrators: , underhydration, ingestion of gut-provoking medications, inadequate training. In other cases, it turns out that a runner’s gut distress arises because of a more serious issue like Celiac disease or inflammatory bowel disease.

Oftentimes, though, the origins of gut woes can be maddeningly hard to pin down. In some respects, medical conditions like irritable bowel syndrome (IBS) and dyspepsia fall into this enigmatic diagnostic category, in that there is usually no obvious anatomical defect to the digestive tract. As such, these conditions are often labelled as functional gut disorders because it is the function, not the structure, of the gut that seems most affected.

Although much is still to be learned about IBS, dyspepsia, and other functional gut disorders, there is at least one important shared feature to the pathology of these diseases: visceral hypersensitivity. In basic terms, hypersensitivity means a person experiences discomfort/pain in response to a typically non-painful stimulus, or that they report heightened pain from something that causes most people mild discomfort. More specifically, visceral hypersensitivity refers to these sorts of pains/discomforts in the body’s abdominal region.

Importantly, a person can exacerbate or lessen visceral hypersensitivity based on the lifestyle choices they make. For runners, this has potentially important implications for preventing and managing some types of gut distress during training and competition. In this article, we’ll take a close look at some of these lifestyle choices and how they just might lesson a runner’s odds of being stricken by gut troubles.

Visceral Hypersensitivity and Functional Gut Disorders

If you’re not familiar with IBS, it’s a disorder that predominantly impacts that lower half of the digestive tract, with common symptoms being the backdoor trots, straining on the loo, abdominal cramping, and bloating. Many studies have found that people with IBS are often viscerally hypersensitive. To figure this out, researchers conduct fun-sounding procedures (insert sarcasm) like rectal barostat testing. Lucky volunteers start by undergoing a bowel cleansing protocol, which is followed by the insertion of a balloon into the rectum. Said balloon is then gradually inflated until the volunteer can’t take the pressure any longer. In one illustrative study, people with IBS handled only about . In another study, the severity of IBS symptoms was correlated with . All in all, these and other experiments strongly support the idea that visceral hypersensitivity is an important contributor to IBS.

Dyspepsia is an exceedingly common gut disorder, with estimates of about being afflicted. In Greek, the prefix dys- means bad and peptos translates loosely to digestion; hence, the phrase dyspepsia has historically been used to describe many a condition marked by indigestion or an upset stomach. In contrast to IBS, dyspepsia’s symptoms generally originate from the upper gut and include excessive or premature fullness, upper abdominal discomfort, and heartburn. Although dyspepsia can be due to readily identifiable causes like ulcers, in many instances there is no structural defect to the gut, and the term functional dyspepsia is used to describe these cases.

Like with IBS, visceral hypersensitivity has been documented in studies of dyspeptic patients. Take for example an experiment that administered as well as healthy folks. Capsaicin is a major constituent of chili peppers and gives them their spicy kick. The study volunteers ingested 0.5-gram capsules of capsaicin every 15 minutes until they reported moderate pain. In the end, two-thirds of the dyspepsia patients reported moderate pain after just one capsule, while only half of the healthy people did. Interestingly, about 10 percent of the healthy volunteers were able to handle four or more capsules while none of the people with dyspepsia could.

Because visceral hypersensitivity plays an important role in both IBS and functional dyspepsia, it is perhaps not overly surprising that many people with one of these disorders also has the other. In one analysis of outpatients from a gastroenterology clinic who had been diagnosed with at least one of these ailments, for having both disorders based on a standardized questionnaire.

Nervous System Dysregulation

Sensations that you perceive in your gut and visceral region of your body are controlled by your nervous system. Sensory nerves in and around your gut detect chemical and mechanical stimuli, and this information is ultimately relayed to your spinal cord and brain. This means that aberrations anywhere along this chain can make your gut hypersensitive, and that visceral hypersensitivity is due to a combination of the following: 1) sensitization of sensory nerve endings in the gut wall, 2) greater flow of sensory information through nerves of the spinal cord, and 3) amplification of sensory information in the brain. Regardless of what the true mechanisms are, we know a person’s visceral sensitivity level can, to a degree, change over time based on their environment and lifestyle choices.

One of the most consistent things that induces visceral hypersensitivity is acute stress. In patients with IBS, for instance, placing a hand and forearm into a tub of ice-cold to electrical current in the rectum. (These hypersensitivity experiments sound like a treat, huh?) Dipping your hand in bitterly cold water is a sort of mixed physical-psychological stressor, so perhaps you’re wondering if a similar response would be seen with a purer psychological stressor. It turns out that simply listening to one type of music (folk) in one ear while simultaneously listening to another type of music (rock) in the other ear can also .

Beyond acute physical and psychological stress, what else can aggravate visceral hypersensitivity in people with IBS, dyspepsia, or other gut disorders? There are many possibilities, but sleep loss and dietary factors are at the top of the list. Individuals with acid-reflux disease were found in one experiment to after one night of getting less than three hours of sleep, which jibes with other research showing that . On the dietary side of things, it’s well documented that among patients with IBS, lowering the intake of what are called FODMAPs (fermentable oligosaccharides, disaccharides, monosaccharides, and polyols) is often associated with symptom improvement. FODMAPs are short-chain carbohydrates that aren’t digested and absorbed particularly well in some folks. Recent experiments have even pointed to less visceral hypersensitivity as a possible explanation for symptom improvements with low-FODMAP diets (see and ).

Targeting the Nervous System and Mind to Reduce Visceral Hypersensitivity

Clearly, visceral hypersensitivity is more likely to be a problem for runners who suffer from IBS, dyspepsia, regular reflux, and the like. Still, there is reason to suspect that many people, even those who don’t have one of these illnesses, are susceptible to becoming a bit in certain situations.

Race morning is a prime example of when visceral hypersensitivity could rear its ugly head. This may be precisely why some runners are unable to consume their regularly sized pre-exercise meal; they are so stressed and nervous that their gut has become oversensitive to even the smallest food portions. Obviously, this could have implications for being able to carry out pre-race and in-race nutrition plans.

What, then, are some strategies runners can implement to decrease visceral hypersensitivity or the chances they will encounter this problem in the future? Obviously, avoiding undue psychological stress and sleep deprivation are important pieces of the puzzle, but in both life and sport, there are many situations that are unavoidably stressful. In these circumstances, one simple intervention that may diminish visceral hypersensitivity is (e.g., six breaths per minute), which may stem from its capacity to activate the parasympathetic (“rest and digest”) branch of your nervous system. To ensure that you are fully activating the parasympathetic branch of your nervous system, it’s important to exhale fully and slowly (i.e., over 4 to 5 seconds) with each breath cycle. Even a few minutes of slow deep breathing is enough to get the parasympathetic nervous system going, though doing it for longer may elicit greater effects.

A way to get potentially even more benefits out of slow deep breathing is to add in a mindfulness component. The practice of being mindful basically means trying to focus on the present moment instead of the past or future. Constantly ruminating on the past or excessively worrying about the future often leads to mental and physical problems, so there is a strong rationale underpinning the practice of mindfulness. Your focus of attention when doing mindfulness training can be on bodily sensations, thoughts, feelings, or things in your environment. A key aspect of most mindfulness techniques is to avoid judgment when your mind eventually wanders. One of the simplest ways to be more mindful is to , which can easily be paired with slow deep breathing. The evidence behind mindfulness and the research quality has not always been great, but the chance of any harm from trying it is low.

Diet and Medication Fixes

For people with IBS, following a low-FODMAP diet may bring some relief, though the long-term health effects of eating this way are largely unknown. An alternative to a long-term low-FODMAP diet is to just for the day or two before an important training session or race. From a practical point of view, low-FODMAP diets can be a bit complicated to follow, in large part because FODMAPs are found in most food groups and there is not a simple rule of thumb when it comes to the types of foods that should be avoided. (Information on what foods contain FODMAPs can be found .) In addition, people have different thresholds in terms of the amount of FODMAPs they can ingest before they start to experience gut problems. If you can afford it, consulting with an experienced dietitian or nutritionist would be ideal when implementing a FODMAP-restricted regimen.

Finally, athletes who have been diagnosed with IBS or functional dyspepsia could potentially benefit from , such as antidepressants, that have reduced visceral hypersensitivity in some studies, and interested individuals should talk to their doctor. If you haven’t been formally diagnosed with IBS but suspect you could be suffering from it, you could take the following .

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The first man-made sweetener was discovered—or more accurately, stumbled upon—over a century ago by a graduate student named Constantin Fahlberg at Johns Hopkins University. As the story goes, Fahlberg noticed a sweet taste on the bread he was eating at dinner one evening in 1879. Earlier that day, Falhberg had been investigating the chemistry of coal tar derivatives and spilled some on his hands. He subsequently figured out that saccharin was the chemical responsible for the delightfully sweet taste of his dinner rolls. Just think, saccharin may have never been happened upon if Fahlberg was of a more hygienic disposition.

Today, numerous artificial as well as natural low-calorie sweeteners are used by food producers throughout the world. According to one representative survey of Americans that was done from 2009 to 2012, s were consuming low-calorie sweeteners, and these intake rates represented a 200 percent surge in consumption for kids and a 54 percent increase among adults from just 10 years earlier. Although similar data in athletes is hard to come by, it’s almost a certitude that they are, on average, consuming more artificial and low-calorie sweeteners than in years past.

In large part, the surging use of these sugar replacers is a byproduct of the obesity epidemic. At the beginning of the 1960s, the prevalence of obesity among adults was 13 percent in the United States. Fast forward 50 years, and the prevalence had skyrocketed almost threefold, to Perhaps even more disturbing is that about and adolescents is now obese. While there are lots of complex theories about why this rise in overweightness has occurred in such a short period, on the most basic level it is due to an imbalance in energy intake and energy expenditure on a population-wide scale.

People have all sorts of questions and concerns about these supposedly guilt-free sweeteners. Alas, we don’t have time to debate all the pros and cons of said sweeteners. Instead, the aim of this article is to review how these sweeteners can potentially impact gut function and symptoms from an athlete’s perspective.

Sugar Substitutes Are Not One and the Same

It turns out there are many different energy-free and low-calorie sweeteners, so before we can talk about how they might affect your gut, it will be valuable to briefly review some of them individually.

Saccharin, the original artificial sweetener, is several hundred times sweeter than table sugar (sucrose), but it also has a somewhat unpleasant aftertaste. Sweet’N Low® is probably the most recognized brand of saccharin-based sweetening products. Aspartame is another popular synthetic sweetener that was unearthed in 1965. Interestingly, its discovery story is akin to saccharin’s; while working on anti-ulcer drug candidates and related chemicals, a chemist named Jim Schlatter accidently detected aspartame’s sweetness after licking his finger before picking up a piece of paper. Aspartame goes by the brand names Nutrasweet® and Equal® and is about 200 times sweeter than regular-old sugar.

Another artificial sweetener, sucralose (aka Splenda®), is even sweeter than saccharin and aspartame, coming in at 600 times the potency of sugar. Several other artificial sweeteners are approved by the Food and Drug Administration (e.g., acesulfame potassium, neotame, etc.), but we will focus on saccharin, aspartame, and sucralose because they are the most popular.

şÚÁĎłÔąĎÍř of these manmade sweeteners, natural low-calorie sweeteners are also found in some foods and beverages that athletes consume. Stevia is one such natural sweetener and is often touted as being safer than artificial sweeteners. Stevia sweeteners usually come from the Stevia rebaudiana Bertoni plant that is native to South America and, like aspartame, they are about 200 to 300 times sweeter than sugar.

Sugar alcohols—otherwise known as polyols—are an additional type of sweetener used to replace real sugar in certain foods. Many polyols are considered natural because they are found in plant foods such as fruits and vegetables. In contrast to the other sweeteners mentioned so far, most polyols provide some energy, albeit less than sugar. Most contain about 1.5 to 3.0 kilocalories per gram, whereas sugar supplies 4 kilocalories per gram. When it comes to sweetness levels, polyols tend to be somewhat less potent than sugar.

Context and Dosage Are Key

It’s important to acknowledge there are numerous aspects of physiology that influence overall gut health. Energy-free/low-calorie sweeteners could impact anything from gut hormone secretion to intestinal motility to the microbiome (i.e., the population of microbes living in your gut). That means we should avoid making overly simplistic conclusions about whether these sugar substitutes are good or bad for your gut’s overall fitness.

With that caveat aside, let’s look at some of the relevant research on these sweeteners. In animals, feeding artificial sweeteners like sucralose and aspartame sometimes (e.g., gastric inhibitory peptide, peptide YY), which impact hunger and intestinal motility. In addition, a study that fed mice sucralose, aspartame, or saccharin for 11 weeks found alterations in the gut microbiome, which caused . This isn’t an isolated finding, as other studies have found compositional with artificial sweetener consumption.

Still, mice aren’t humans, and many of these animal experiments used dosages (relative to body weight) that were way higher than the amounts most people consume on a daily basis. As a result, it is incredibly hard to say how well some of these findings apply to people who consume modest amounts of manmade sweeteners. For example, although some animal research suggests that artificial sweeteners could negatively affect hunger, body weight, and glucose metabolism, most experiments in humans have not found this to be true. In fact, a meta-analysis of clinical trials that was published this year found consuming non-caloric/low-calorie sweeteners in place of sugar led to Likewise, another review of human experiments found that replacing sugar-sweetened beverages with artificially-sweetened versions prompted .

On the other hand, eating loads of artificial sweetener could potentially lead to problems metabolizing carbohydrate in the body, but these effects probably only occur at very high doses. In one study, volunteers ate the equivalent 10 Sweet’N Low packets’ worth of saccharin per day for a week, yet only four of seven volunteers at this extreme dose. The point is, dosage matters.

Overall, if sugar in your diet is replaced with a reasonable amount of artificial sweetener, it is unlikely that your metabolic health will be negatively impacted in a major way. And although artificial sweeteners can certainly alter the gut microbiome, we don’t yet know what the long-term implications of those changes are.

Are Sugar Substitutes a Source of Gut Symptoms in Athletes?

The surest thing we know about this topic is that eating loads of polyols is akin to dropping a bomb in your gut. About half to three-quarters of ingested polyols get absorbed, meaning that a portion of them hang around in the gut’s lumen. The fact that polyols are incompletely absorbed is precisely why they contain less energy than sugar. While a gram or two of unabsorbed polyols isn’t a big deal, larger amounts sitting in the gut can wreak havoc. In , about half of people who ate chocolate bars containing 32 grams of a polyol called sorbitol suffered from diarrhea, flatus, stomach aches, and bloating. Likewise, in a study of , stomach aches and watery stool were more frequent in comparison to when they ate candies with regular sugar.

These are just a couple of the many studies that have shown polyols can be disagreeable to the gut if 20 grams or more are eaten in a single sitting. From personal experience, I can still vividly recall being transformed into a human flatus factory after I ate an entire bag of sugar-free gummy bears as a kid. It turns out the gummy bears had maltitol as a sugar replacer. (Apparently, I’m not the only one who has had enteric distress from sugar-free gummies; you can check out some horror stories .) On a mechanistic level, unabsorbed polyols draw water out of your body and into your gut lumen, resulting in loose, watery stools. Bacteria in the latter half of your gut also ferment these unabsorbed polyols, provoking gas and bloating.

Clearly, eating a good dose of polyols isn’t a wise choice in proximity to exercise. Very few of the classic sports drinks marketed to endurance athletes contain polyols, so you probably need not worry about them when drinking those beverages. Other products like diet and meal replacement bars, however, are much more likely to be chock-full of polyols. One potential red flag to look for is a product that markets itself as being “sugar free” but that still has a semi-sweet flavor like chocolate, vanilla, or strawberry. Look at the ingredients list to see whether any polyols are listed. For some products, you can figure out the exact quantity by looking for sugar alcohols under the total carbohydrate section of the Nutrition Facts panel.

As for Stevia, there is little reason to think that the amounts found in most products would cause notable gut issues. In an experiment involving eating cookies (how fun!), adding in comparison to standard Stevia-free cookies. Likewise, there is little evidence from animal or human experiments that consuming modest amounts of artificial sweeteners like , gas, or stool abnormalities. There are always exceptions, though, and a small subset of athletes may want to avoid artificially sweetened foodstuffs before and during exercise if they know from their experience that they are triggered by them.

Summary

Eating excessive amounts of artificial sweeteners like aspartame, saccharin, and sucralose can have undesirable effects on the body, but the dosages required for this to occur are quite high. In other words, downing a six-pack of diet soda or a dozen Sweet’N Low packets every day is much more likely to lead to problems than a once-a-day diet soda habit. Scientists can’t provide a 100 percent guarantee that there will be no long-term adverse effects from eating modest amounts of artificial sweeteners, but the probability is pretty low for most people that they will cause lasting harm.

When it comes to natural sweeteners, polyols are the type you should be most mindful of as it relates to preventing gut issues. Because they are incompletely absorbed in the gut, these sugar alcohols can cause stomach pain, loose stools, gas, and bloating, especially if 20 or more grams are eaten at one time.

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Fueling during exercise is a delicate dance. Eat too little, and you risk heading straight for a bonk. Eat too much, and your gut may revolt, sending you to the roadside (or trailside) privy. Indeed, countless runners—from recreational to elite—have at one time or another pushed their fueling just a bit too hard and paid the price for it in the form of gut distress.

The whole goal of fueling during prolonged exercise is to maintain a steady supply of carbohydrate to burn. The fuel tanks that house carbohydrate in your body are rather small and can become depleted within a few hours, depending on the intensity of exercise. This is precisely why most dietitians tell their athletes to ingest carbohydrate during maximal effort exercise that lasts more than 60 to 90 minutes.

In contrast to carbohydrate stores (called glycogen), fat reserves are comparatively unlimited. Even a runner who weights 125 pounds with 10% body fat has roughly 50,000 kilocalories’ worth of fat energy squirreled away. To put that in context, consider that most runners expend during a standard 26.2-mile marathon. In other words, there is virtually no risk of your fat fuel tanks running dry during endurance races, including single-stage ultras.

Because fat stores offer an essentially limitless supply of energy, there is a belief among some athletes and practitioners that training the body to burn more fat and less carbohydrate is a win-win scenario. Naturally relying more on fat for fuel would help preserve glycogen and also reduce the need to chow down carbohydrate during exercise, which, in turn, could lessen the odds of stomach troubles. The goal of this article is to examine this fat adaptation theory from a scientific perspective and discuss some of its implications for fueling, performance, and gut function.

Fuel Use during Exercise

The body can oxidize a variety of organic compounds to produce energy, but it has a predilection for carbohydrate (mainly glucose) and fat. When lounging on the couch, the majority of energy produced (say two-thirds) comes from the breakdown of fat, although there is quite a bit of . As people go from resting to jogging to full-out running, the percentage of energy derived from burning fat steadily declines. What this means is that the body inherently prefers to use carbohydrate at higher exercise intensities and that fat burning has a maximal limit, especially during vigorous exercise (>75% of VO₂ max).

The human machine is quite adaptable, though, and it turns out that eating loads of fat increases fat utilization during exercise. In one illustrative study, a high-fat diet (60% of energy intake) eaten for just two days caused nearly at a moderate exercise intensity in comparison to a diet that was only 22% fat. More recently, a study of elite ultrarunners who had been following high-fat, ketogenic diets for at least a half year found that in comparison to similar-caliber ultrarunners who had been following high-carbohydrate diets. These studies are just two of the many that show that by eating lots of fat, the body can adapt to burn more fat during moderate-intensity exercise.

Dietary Fat and Performance

So, given that eating fat profoundly shifts the body’s ability to burn fat, why isn’t every endurance runner on the planet eating a high-fat diet? Although there are several reasons (e.g., taste, food preferences, tradition, etc.), an important one to consider is how high-fat diets impact your capacity to burn carbohydrate. Years ago, the observation that eating a high-fat diet accelerates fat burning during exercise was thought of as a “carbohydrate sparing effect.” This was a positive adaptation in the minds of many, as it meant that an athlete would have more carbohydrate stores remaining near the end of a prolonged exercise bout.

Over the past decade and a half, though, scientists have uncovered something interesting, which is that high-fat diets aren’t really carbohydrate sparing. Instead, they actually interfere with an athlete’s ability to use carbohydrate, particularly during intense exercise. In basic terms, this means that when a runner wants to kick it into high gear, their engine may sputter like a car with only three of its four cylinders working. This is because eating lots of fat needed to metabolize carbohydrate into useable energy during exercise. Indeed, two recent experiments ( and ) led by renowned Australian-based sports scientist Louise Burke confirm that high-fat diets impair performance during races that last about 40–50 minutes. These observations may help explain why elite distance runners often gravitate toward eating high-carbohydrate diets; in one survey of 10 Ethiopian runners, the .

While high-fat diets probably aren’t an optimal choice for competitions lasting up to a few hours, there is less clarity when it comes to ultras. Ultrarunners just simply can’t sustain high intensities during their races, and most of them lumber away somewhere between 40% and 60% of their VO₂ max during single-stage events. Consequently, the contribution of fat to energy production is higher during these races than those lasting less than a few hours. To date, research on whether has been mixed, though the studies typically don’t show any detriments to eating more fat. There are anecdotes of athletes successfully following as well as high-carb diets in the realm of ultramarathoning, and when you combine these anecdotes with the equivocal scientific evidence, the take-home point seems to be that performance on fat-laden diets will vary between athletes. Ultimately, a trial-and-error approach is needed to figure out whether a high-fat diet is a good option for an individual athlete.

Fat Adaptation as a Gut Remedy

are incredibly common during ultrarunning, and on an intuitive level, it makes sense that some digestive trouble is inevitable when your run 50+ miles straight. As one example, 96% of runners at the 2013 Western States 100-mile Endurance Run reported some type of gut symptom, and . Furthermore, these problems became increasingly prevalent during the latter half of the race, a time when an athlete’s internal carbohydrate stores typically dwindle. For an athlete who naturally burns more carbohydrate and less fat (due to following a carb-rich diet), running out of carbohydrate reserves can be disastrous in this scenario. If they happen to experience gut problems during the race, they will inevitably slow down because they are less capable of relying on their fat stores for energy and they won’t be able to consume much carbohydrate as their gut revolts. In contrast, a fat-adapted runner is naturally less reliant on carbohydrate to begin with and may not need to fret as much about potential stomach troubles from overeating carbohydrate.

These supposed benefits of high-fat diets on gut comfort during ultrarunning are entirely speculative. In other words, there aren’t studies showing that fat-adapted athletes have less gut distress during competition because they avoid consuming large amounts of carbohydrate. Still, for an athlete who has a sensitive stomach, a high-fat diet is one of several approaches they could take to minimize their risk of gut woes. The longer the race, the more likely it is that a high-fat diet may offer some benefits. As an example, it’s more likely that a high-fat diet would enhance performance during a 100-mile race than a 31-mile (50 km) race.

Best of Both Worlds?

So far, I’ve referenced two polar approaches when it comes to pre-competition nutrition: high-fat versus high-carbohydrate diets. What if, however, there were a mixed strategy that offered the best of both worlds? It turns out that several research groups have tried to achieve this sort of balancing act by having athletes follow a high-fat diet for a week or two followed by a day or two of crushing carbohydrate before competition. In this type of scenario, the goal is to upregulate fat burning while simultaneously maximizing muscle stores of carbohydrate. As a bonus, this strategy might also allow the gut to better tolerate carbohydrate ingestion during the race itself, should an athlete consume a good amount of it (say 45–90 grams per hour). The to an extent, and after just a few days on a high-carbohydrate diet, digestion and absorption of carbohydrate can become more efficient.

Studies that have done this sort of high-fat diet followed by carbohydrate restoration have shown interesting—albeit sometimes inconclusive—performance results. In one instance, six days of a high-fat diet followed by one day of eating loads of carbohydrate was compared to a standard high-carbohydrate diet. Ultimately, the during a 1-hour cycling test that was carried out after four hours of cycling at 65% of VO₂ max. However, these effects didn’t quite reach what’s called statistical significance, meaning it’s possible the difference was due to chance. Another study used a similar protocol and .

Unfortunately, it’s incredibly challenging to recruit competitive runners for weeks-long studies where their diets and training are completely controlled. On top of that, the exercise tests often last 3 to 5 hours, and the volunteers repeat these tests a couple of times. (Imagine the fun of staring at a laboratory wall for 5 hours while on a bike or treadmill.) Indeed, the two studies I just outlined only had seven and five subjects, respectively! These tiny samples make it extremely difficult to distinguish between true performance benefits and day-to-day noise in performance measurements.

Take-Homes

Fat is a key fuel for sustaining any form of prolonged exercise, and it’s not hard to understand why some runners are drawn to the appeal of high-fat diets. Before making the switch, it’s important to consider what sort of activity you want to excel at. If you’re trying to PR in the half-marathon, for example, eating a fat-laden diet is not your best choice. In contrast, if you’re running trail races that last 3+ hours, then a high-fat diet is a more viable choice. In theory, a high-fat diet could reduce gut issues that arise from trying to ingest loads of gels and sports drinks during competition.

On a practical level, most studies looking at the performance effects of high-fat diets used intakes of about 60%–70% of energy as fat. In the table below you can see a menu that provides roughly 61% of dietary energy as fat. Remember, if you do decide to go high-fat, it might be wise to re-up your carbohydrate intake for 1 to 3 days before competition so that your gut is ready for any carb-rich foods you eat along the way to the finish line.

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I can still hazily recall the initial few months after our son was born in 2018. The adrenaline from having our first child (and keeping him alive) got me through the first several days without feeling completely knackered, but my lack of nocturnal restoration eventually took its toll.

One of the most bothersome aspects of my sleep deprivation was a heightened sensitivity to physical pains and discomforts. My primary complaints were two that I’d wager are familiar to more than a few readers: lower back pain and an upset stomach, including indigestion. Thankfully, the void of sleep induced by rearing a newborn is a transient affair, and my issues largely self-resolved once our son started sleeping through the night.

My experience is by no means an anomaly. A lack of sleep, as it turns out, makes all sorts of aches, pains, and pangs more noticeable. And as a cumulating body of research tells us, insufficient sleep plays a major role in gut discomforts. In this article, we’ll look at some of the underlying physiological explanations for this phenomenon, discuss its relevance for athletes, and examine some basic tips to help minimize gut problems that stem from a lack of quality sleep.

Your Body Has a Built-In Clock and So Does Your Gut

Humans—much like all other animals—have biological clocks that are responsible for producing daily oscillations (a.k.a., circadian rhythms) in physical and behavioral processes. These internal, innate clocks are widespread throughout the body and its organs, including the gut. A structure in your brain, known as the suprachiasmatic nucleus, is the master regulator of your body’s biological clocks, and it’s heavily influenced by light-dark cues from your eyes. Beyond light-dark changes, other external cues like temperature fluctuations, metabolic activity (e.g., exercise), and eating influence your biological clocks.

It’s been known for at least several decades that the digestive tract exhibits circadian patterns of function. Take for example , which peaks during the day with a large drop-off overnight. Similarly, found that the solid components of a small meal left the stomach more slowly at 8 p.m. in comparison to 8 a.m. Another investigation found that the small intestine’s migrating motor complex (a sweeping contractile activity that helps clear partially digested foodstuffs) was than overnight.

These studies went to great lengths to document what many of us suspect intuitively, which is that the gut is typically much more active during the day and goes into a relative hibernation mode during the evening and while we sleep. One implication of this information is that gut problems could be worse when athletes compete and train in the evening, particularly if they ingest large amounts of food and fluid immediately before or during exercise.

Sleep Disruption and Gut Issues

Sleep problems—particularly when they last for weeks on end—can mess up your gut’s internal clock, making it more likely your digestive processes will get of whack. In one survey of Minnesota residents, for example, sleep disturbances were associated with (IBS). In a similar analysis, waking up in the middle of the night at least four times per month was correlated with , nausea, diarrhea, and loose stools.

It’s probably obvious to many of you that we can’t solely blame sleep disruptions for the gut troubles observed in these studies. For one, there are likely other confounding health and lifestyle factors that could be simultaneously contributing to the gut and sleep issues. Secondly, it’s also possible that the link between the gut problems and sleep is in the other direction, i.e., gut symptoms interfere with sleep. Indeed, if you are regularly suffering from gut pain, it wouldn’t be surprising if your sleep was disturbed because of it.

To really show that a lack of sleep directly impacts gut symptoms, one needs to experimentally restrict sleep. There are, in fact, studies that have done this sort of thing. One such experiment found that among 10 people with gastroesophageal reflux disease, a single night of poor sleep (less than 3 hours) when their esophagi were exposed to hydrochloric acid. More as it relates to general pain perception, the results of about a dozen studies seemingly confirm that inducing severe sleep loss for a single night or several days and lowers their pain tolerance. In total, the evidence is strong that sleep loss—especially when it’s severe or prolonged—can make all sorts of unpleasant sensations worse.

So, what are the underlying biological mechanisms linking poor sleep to gut woes and other types of pain? Of course there are many complexities to consider, but one good explanation is increased inflammation in the body, which is known to exacerbate many forms of pain. Dysregulation in the release and binding of pain-modifying chemicals in the brain (serotonin, dopamine, opioids) is also thought to play a key role.

Ways to Improve Your Sleep

First and foremost, if you are consistently having sleep problems that are causing health issues or that diminish your quality of life (including your ), then it would be prudent to speak to a healthcare provider before trying these strategies. Disorders like insomnia and sleep apnea are serious matters that warrant careful medical evaluation. The strategies I discuss here are things that you may consider discussing with your doctor, and he or she can provide better insight and context as to whether these tactics are a good idea for you personally.

• Melatonin supplementation: Melatonin is a hormone your body produces to trigger drowsiness and regulate sleep-wake cycles. Melatonin secretion ramps up in the evening and peaks in the middle of the night, and its production sharply declines with age, which may partially explain the commonness of sleep disturbances in older people. Due to its involvement in regulating sleep, melatonin supplementation has been tested as a sleep aid in numerous studies. Given the wide range of formulations and dosages used in these trials, as well as the assortment of clinical sleep conditions that have been studied, simple conclusions about melatonin’s effectiveness should be viewed with skepticism. That said, one finding that often emerges is that melatonin , basically the amount of time it takes to go from being awake to being asleep. Interestingly, melatonin use has also in a few small experiments. The most frequently used dosages are between 1 and 10 milligrams, taken 30-60 minutes before bedtime. Because the quality of many dietary supplements is sub-par, it’s worth considering only buying products that have been tested by third-party organizations like NSF Certified for Sport and US Pharmacopeia. Other considerations about melatonin supplementation can be found at the .
• Practice mindfulness: Although it may sound like a hippy-dippy activity reserved for hipsters, yogis, and monks, mindfulness has been slowly making its way into the mainstream. More and more athletes, including pros, have started turning to mindfulness and other forms of mediation to manage their competitive anxieties and gain a mental edge. Mindfulness is basically the practice of focusing on the present moment, keeping in mind not to judge your thoughts and feelings. Your focus can be on bodily sensations, thoughts, or things in your environment. When it comes to sleep, practicing mindfulness may lessen ruminative thoughts about the past as well as anxieties of the future, thereby facilitating the onset and sustainment of sleep. The small number of controlled studies that have evaluated mindfulness as a sleep intervention generally show that it works like stress reduction and equally as well as other evidence-based sleep enhancers. As an added bonus, mindfulness can help ease digestive symptoms among people with gut disorders like IBS and dyspepsia. With the advent of apps like Headspace and Calm, there are guided meditation options available for just about any type of person.
• Sleep hygiene: The term sleep hygiene is believed to have been introduced in 1939 by pioneering sleep researcher Dr. Nathaniel Kleitman. Although it may sound like a strategy for dealing with “dirty sleep,” sleep hygiene simply refers to a comprehensive lifestyle and behavioral approach to improving one’s slumber. Some of the more common components of sleep hygiene programs include keeping daytime naps to less than 30 minutes, shunning stimulants like caffeine for at least 8 hours before bed, and exercising daily while also avoiding vigorous activity close to bedtime. On the environmental side of things, controlling temperature, light, and noise are priorities. Many people sleep better in a cool room (e.g., 65℉), but you would need to do some self-experimentation to find your optimal temperature. While it may be difficult to do in our smartphone-driven world, avoiding screen time within 30 minutes of when your head hits the pillow is also a good idea. And consider these other . Overall, the key is setting up a consistent routine that you are able to stick with.

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