The Physiology of a 24-Hour Mountain Bike Race

This article originally appeared on Outside

When people say, "You can't outrun a bad diet," I smile and nod. It's perfectly true, in a holistic sense. Exercise and diet are two different things, with separate effects on your health and performance. You can't automatically compensate for deficiencies in one area by being extra good at the other, any more than donating to charity makes it OK to embezzle at work.

Still, there's a little voice in my head that asks: "Well, how far were you planning to run?" It may be true that you'd have to jog three or four miles to burn the calories in a single McDonald's Happy Meal, but some people run a lot farther than four miles. The question of how many calories a person can burn in a given day is an interesting scientific one, with some researchers arguing that our ability to get food through the digestive tract is actually the fundamental limitation on feats of sustained human endurance.

That's why a new study in the International Journal of Sports Physiology and Performance, from a team led by Brent Ruby of the University of Montana along with colleagues from the University of Wisconsin, caught my eye. Ruby and his colleagues collected a bunch of data from a competitor in the Mudslinger Oregon 24, a mountain bike race in Oregon that involves racking up as many 11-mile loops as you can in 24 hours. The most interesting detail: they fed their subject a dose of very expensive doubly labeled water--a mix that contains water molecules with either an uncommon isotope of hydrogen or an uncommon isotope of oxygen--that enables them to calculate with very high accuracy how many calories he burned over the course of his 24-hour ordeal.

The subject, a 41-year-old male, managed to complete a very impressive total of 238 miles. The course, for context, includes "dominantly soft-dirt, single-track, and loose-gravel fire roads with occasional large rocks that required periodic rider dismount." In the process, he also climbed a total of over 25,000 feet, not too far from Everesting. He started out with an average heart rate of 162 beats per minute for the first three laps, but had dropped to 113 for the last three laps, with an overall average of 134. He wasn't messing around.

His total energy expenditure during the event was 9,775 calories. In round numbers, that's roughly quadruple the typical energy expenditure of an adult male who's living a normal life rather than biking all day and night. It's a lot--but it's not unprecedented. Earlier studies have tended to focus on extreme examples like polar explorers and Grand Tour riders. A 2019 study, for example, found that seven riders in the Giro d'Italia averaged 7,719 calories per day for 24 straight days.

An earlier paper by Ruby and his colleagues found that Ironman triathletes burned just over 9,000 calories, and that ten Western States 100-mile finishers averaged a total expenditure of 16,310 calories while finishing in 26.8 hours. That works out to a little over 14,000 calories per 24-hour period. Again, that's just an average: some of the subjects were several thousand calories higher. Those who run 100-mile races are not exactly typical shmoes, but Ruby is careful to point out that these weren't the people winning the race. The title of that paper is "Extreme endurance and the metabolic range of sustained activity is uniquely available for every human not just the elite few."

Of course, you can't go on burning that much fuel indefinitely. That's the point Herman Pontzer and John Speakman make about the "alimentary limit" on sustained endurance: you simply can't eat (and digest) that much. In this case, Ruby's 24-hour cyclist managed to shovel down 5,616 calories, a little over half of what he burned. Most of that was in the form of carbohydrate, which he managed to consume at a rate of 58 grams per hour. That's impressively close to the traditional guidance for marathoners of 60 grams per hour (though more recent research has pushed the target up to 90 or even 120 grams per hour). Given how much trouble most people have stomaching that amount of carbohydrate, it's hard to imagine the cyclist doubling his intake over an entire day.

One other detail enabled by the doubly labeled water measurement: his total water turnover was 17.2 liters (about 4.5 gallons). He drank 13.3 liters (3.5 gallons), and lost about seven pounds from start to finish. That weight loss came entirely in the first half of the race, which was during the daytime in temperatures that climbed as high as 92 degrees Fahrenheit with 84 percent humidity. During the night, it dropped to 37 degrees with 17 percent humidity, so it's not surprising his fluid loss stabilized.

We don't get any information about how the cyclist made his fuel and hydration decisions, but it's interesting how closely his rate of fluid consumption (the darker line in the graph below) matches the air temperature (the lighter line) over the course of the race. It's what you'd expect to see if someone was drinking according to thirst:

When it was hot, he drank a lot; when it was cool and he was presumably sweating less, he drank less. Given that some of the seven pounds he lost would be the result of burning fuel stores rather than losing fluids, it's likely that he got his hydration pretty close to perfect.

Ruby is probably most famous--or infamous--for a study he ran back in 2015 that compared McDonald's fast food with products from Gatorade, Cliff, Power Bar, and other sports nutrition companies. They all worked equally well for restocking glycogen after a 90-minute bike ride and fueling a subsequent 20K time trial. Calories, in this view, are calories (or at least carbs are carbs). As I said at the top, I don't think that's true in the long term: there's more to health than burning as many calories as you consume. Still, it's not a bad place to start--and Ruby's data offers a reminder that, with sufficient fitness and motivation, we're capable of burning a staggering number of calories.

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