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The most famous joke in music involves a lost tourist in Manhattan who asks a passing musician how to get to Carnegie Hall. The reply: “Practice, practice, practice.” This quip popped to mind when I was reading a new study about the caloric requirements of 100-mile mountain ultramarathons. You’ll burn something like 16,000 calories during one of these races, which is an amazing number—but it takes more than calories to reach the finish line.
The study, which appears in the International Journal of Sports Physiology and Performance, dissects the performance of two participants in the Wasatch Front Endurance Run, a 100-miler in Utah with a cumulative total of almost 25,000 feet of climbing and descending and a highest point above 10,000 feet. The subjects were both men, 45 and 31 years old, and both had previously completed several 100-milers. A research team led by Andrew Creer of Utah Valley University fed the subjects “doubly labeled” water, which contains isotopes of hydrogen and oxygen that scientists can use to figure out exactly how many calories you’re burning and how much water your body is using.
This isn’t the first time researchers have used doubly labeled water to study ultrarunners. A previous paper by Brent Ruby of the University of Montana—who is also a co-author on the new paper—pooled data from ten runners at the Western States 100-miler and found that they burned an average of 16,130 calories while running for 26.8 hours. The twist in the new paper is that the researchers continued following the runners for seven days after the race to study how their bodies responded to the enormous physiological stress and caloric deficit they had incurred.
During the Race
Both of the Wasatch runners took 32.8 hours to finish the race. Their calorie expenditure was strikingly similar: 15,723 and 15,888 calories, even though one of them weighed 164 pounds and the other weighed 131 pounds. The bigger runner managed to take in an estimated 8,767 calories during the race, while the smaller one took in 7,429. In both cases, that means they managed to replace only about half the calories they burned, leaving an energy deficit of about 8,000 calories.
Replacing half your calories is fairly typical for ultrarunners. These runners were getting between 40 and 50 grams per hour of carbohydrate, which is lower than sports nutrition recommendations of up to 90 grams per hour—and much lower than the reported fueling rates of 120 grams per hour and beyond that some elite cyclists and ultrarunners have been experimenting with in recent years. But it’s consistent with the upper limits of what most non-pros can tolerate unless they’ve been deliberately training their digestive system to handle more.
The doubly labeled water method also gives an estimate of “water turnover,” which reflects how much water has been replaced in your body. During the race, the estimated turnover for the two runners was 14.6 and 15.5 liters, respectively, which is roughly 500 fluid ounces. Those numbers aren’t universal, since they depend on environmental conditions (the temperatures during the Wasatch race ranged from about 40 to 85 degrees Fahrenheit) and individual factors, like sweat rate. But they give a rough idea of how much you might expect to drink during a race like this.
The runners lost 3.3 and 4.8 pounds, respectively, between the start and finish of the race, which suggests only mild dehydration. Some of the weight loss is likely from the carbohydrate and fat stores they burned rather than fluid losses. It’s hard to get a good estimate of exactly how much fluid they drank during the race: one runner estimated 15 liters, which makes sense; the other estimated 21 liters, which seems like an overestimate given the water turnover data. But overall it looks like they managed their hydration pretty well.
The Aftermath
There are two main things happening the day after a 100-mile race. One is that you’ve got a massive energy deficit to make up; the other is that you’ve trashed your body and need to repair it. Even marathons induce a lot of muscle damage thanks to the repeated footstrike impacts. Ultramarathons make that worse, and downhill running—25,000 feet of it, in this case—exacts a particularly high toll. You might also end up with some swelling, which increases fluid turnover.
Over the 24 hours following their race, the Wasatch runners burned 4,953 and 4,276 calories respectively, roughly triple their basal metabolic rates—even though they were presumably moving as little as possible. Fitness magazines sometimes talk about the “afterburn” effect following hard workouts, and it’s clearly a real thing if your workout lasts 33 hours. Still, their weights were back to normal within 24 hours, or in fact slightly higher than their pre-race values, which suggests that another common fitness trope—compensatory eating—was in full force.
Over the seven days following the race, calorie-burning drifted back down to normal levels, with seven-day averages of 3,245 calories per day for one runner and 2,721 for the other. They did no training during this period. Interestingly, water turnover during the post-race week averaged 6.0 liters per day in one runner and 3.4 liters per day in the other, illustrating the substantial the person-to-person differences that can show up in hydration habits. There was no indication that the second runner was wasting away or suffering from dehydration.
Case studies like this don’t necessarily tell us what’s optimal. These runners were impressive but not elite: the winning time last year was just over 19 hours. Creer and his colleagues speculate that you’d probably want to take in more calories if you’re targeting elite-level performance. Still, it’s interesting to see detailed numbers about what it takes to cover this kind of distance. And the data on the energy demands of recovery is particularly interesting. It’s reminiscent of a puzzling detail that cropped up in a study I wrote about recently on protein needs for endurance athletes: you apparently need more on rest days than you do on training days. Training is hard work; but from your body’s perspective, recovery is also hard work, so make sure you’re giving it enough fuel for the job.
For more Sweat Science, join me on Threads and Facebook, sign up for the email newsletter, and check out my new book The Explorer’s Gene: Why We Seek Big Challenges, New Flavors, and the Blank Spots on the Map.
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