After the 2022 Tour of Flanders, circulated on social media showing a detailed nutrition plan carefully taped to race winner Mathieu van der Poel’s handlebars. Some rough math indicated that the Dutchman had scarfed down well over 100 grams of carbohydrate per hour while winning the 169-mile race in just over six hours—a remarkable digestive feat, given that conventional sports nutrition guidelines say that our ability to absorb carbs tops out at around 90 grams per hour.
To outsiders, van der Poel’s mega-carb intake seemed like an isolated novelty. But in the year and a half since then, very high doses of carbohydrate have become an emerging trend among elite endurance athletes. New science suggests that it really is possible to consume as much as 120 grams of carbohydrate per hour, and some pros are reportedly even higher than that. The unanswered question, for now, is whether it makes you faster.
The latest study to address the issue was in the Journal of Applied Physiology by a research group led by Robert Jacobs of the University of Colorado Colorado Springs. Jacobs and his colleagues use real-world data to simulate the range of physiological characteristics seen in elite marathon runners and calculate how much carbohydrate they would need to successfully run a sub-two-hour marathon.
If you’re fit, tapered, and properly carb-loaded, you already have a substantial supply of carbohydrates when you start a marathon, primarily stored as glycogen in your liver and muscles. The exact amounts depend on a variety of factors including the size of your liver and legs, but on average the researchers estimate that elite male marathoners start with 690 grams of glycogen and female marathoners start with 499 grams.
At first glance, this seems promising, because they calculate that a two-hour marathon should require 612 grams of carbohydrate for elite men and (because women are smaller on average) 528 grams for women. The problem is that you can’t empty the tank completely, for a variety of reasons. One is that running doesn’t use every single muscle in your legs equally, so the underused muscle fibers will still have some stored glycogen when you hit the wall. Overall, you can use up about 62 percent of your stored carbohydrate during a two-hour marathon, leaving a substantial shortfall that you need to make up by drinking or eating more carbs.
To make the numbers work for a two-hour-marathon, Jacobs and his colleagues calculate that an average elite male marathoner would need to consume 93 grams of carbohydrate per hour, while an average female would need 108 grams per hour—significantly higher than the 90-gram max in current sports nutrition guidelines.
This is an interesting analysis, though it’s worth acknowledging that it puts the cart (what we’d like to run) before the horse (what we’re actually capable of). We could run a similar calculation of how many carbs we would need to run back-to-back marathons in four hours, and that calculation would produce an astronomically high number, but the number would have no real-world meaning. The limits of endurance depend on far more than carbohydrate supply.
Still, Jacobs offers three arguments for why the two-hour-marathon numbers are worth taking seriously. One is that recent lab studies have shown that humans are, in fact, capable of burning more than 90 grams of exogenous (meaning from drinks or food rather than internal storage) carbohydrates per hour. For example, by Tim Podlogar, an exercise physiologist at the University of Birmingham and nutritionist for the Bora-Hansgrohe pro cycling team, fed cyclists 120 grams of carbohydrate per hour and found that they were capable of burning just over 90 of those grams per hour. (The rest will be either excreted or stored for later use.) Reaching this level of carb burning involved drinking a mix of glucose and fructose in a ratio of 1:0.8, a shift from the usual ratio of 1:0.5 in many current sports drinks and gels.
The second argument is that the digestive system can adapt. Sure, if you try feeding a bunch of volunteers 120 grams of carbohydrate per hour, their stomachs will rebel and you’ll conclude that it’s “impossible” to absorb that much. But try training your gut for a few months, like the Mathieu van der Poels of the world have done, and who knows what you’re capable of?
The third argument is anecdotal: lots of very fast athletes in sports such as cycling, triathlon, and running are reportedly exceeding 90 grams per hour. Velo’s Jim Cotton had back in October about the “carbohydrate revolution” in cycling. “There’s been a massive change in energy intakes in the last five or six years,” Ineos Grenadiers nutritionist Aitor Viribay Morales told Cotton. “It’s one of the biggest reasons why cyclists are producing such high power, for so long, and how they are reproducing it day to day.”
Let’s agree, then, that moving the goalposts from 90 to 120 grams per minute is plausible. But there are reasons to be cautious before leaping on the bandwagon. For one thing, elite endurance athletes can burn carbohydrates way more quickly, and for a much longer time, than the rest of us. One reason that earlier studies concluded that humans couldn’t burn more than 90 grams per hour is probably that the subjects were merely well-trained rather than elite.
Even if you can absorb 120 grams per hour, it might not make you faster. In Podlogar’s study, cyclists burned more exogenous carbs when they consumed 120 rather than 90 grams per hour, but that didn’t reduce their rate of endogenous carb-burning—that is, they were still depleting the glycogen stores in their muscles just as quickly. Other studies have observed the same effect, and a few studies have even found that consuming higher levels of carbohydrate actually suppresses fat-burning so effectively that you end up burning your internal carbohydrate stores more rapidly than you otherwise would, which is the precisely the opposite of what you’d hope.
As it happens, Medicine & Science in Sports & Exercise has just published an academic debate—they call it “Contrasting Perspectives”—on whether ketogenic diets are beneficial for athletic performance. Arguing is Tim Noakes; arguing are Louise Burke and Jamie Whitfield of Australian Catholic University, who led a series of studies on ketogenic diets in elite racewalkers. I wrote about some of Burke and Whitfield’s results here, and I’m not going to rehash that whole discussion. My general take remains the same as it did then: keto for endurance is a neat idea in theory, but when you actually test performance it doesn’t live up to the hype.
What’s interesting is that despite the title—“Ketogenic Diets Are Beneficial for Athletic Performance”—Noakes’s pro-keto case doesn’t spend much time arguing that ketogenic diets enhance performance. Instead, he focuses on a slightly different argument: that contrary to conventional wisdom, consuming more carbs during exercise doesn’t make you faster. In brief, he argues that the only benefit of ingesting carbohydrates during exercise is that it keeps your blood sugar high enough to fuel your brain, which would otherwise make you feel profoundly tired. That would explain why many studies have seen performance benefits from sports drinks, even though Podlogar and others have found that super-high carb doses don’t actually make internal carb stores last longer. To keep blood sugar up, , would only require 20 grams of carbohydrate per hour, regardless of whether you normally eat a low-fat or high-fat diet.
So should we be pushing our carb intake up to 120 and beyond, or dialing it back to 20? The honest answer, at this point, is that both ideas are speculative. The world of elite endurance sport is voting with its feet (or, perhaps, going with its gut) in favor of carb-mania. My guess is that they’re right, assuming your primary interest is winning races in Olympic sports. But we should keep in mind that no one has proven that 120 grams per hour makes you faster than 90, and even the case for 90 over 60 isn’t watertight. The only way to settle the argument is with more data from well-designed experiments—or in the meantime, like Mathieu van der Poel, by winning races.
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