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Should Endurance Runners Adopt a High-Fat Diet?


If you've ever run a marathon, you know fueling your body the right way before, during, and after the race is incredibly important. Now consider doing an ultramarathon (anything longer than 26.2 miles). You need some serious wheels—and the right fuel to keep them rolling. (P.S. Endurance running does more than burn cals—it has a bonus benefit for your bones too.)

A new study published in the Journal of Food and Nutrition Research took a look at the dietary habits of ultramarathoners completing 100-mile races to see if there's a key to making it through these grueling events. The results? If you're an ultramarathoner, you might want to reach for the avocado, bacon, and olive oil, because high-fat diets were correlated with the fastest finishing times. But—in a surprising contradiction—those who followed a self-reported low-carb or paleo diet plan were actually the most likely to drop out without finishing the race.

The study required 47 ultramarathon runners to complete a dietary and training questionnaire between six weeks and one week out from a 100-mile race. They were asked to jot down their food intake for three nonconsecutive days, and to chose one of the following to categorize their diet: Traditional American (approximately 50 percent carbohydrate, 15 percent protein, 35 percent fat), Vegan (no animal products), Vegetarian (no meat), Paleo (based on Paleolithic eating habits which were typically high in meat and vegetables, and restrictive of grains and dairy), Other, or None. They were also asked to label their carbohydrate intake as high, low, or none.

The study found that, overall, this selection of runners consumed a diet near the low end of the traditionally recommended carbohydrate intake for endurance athletes. But they exceeded the recommendations for both protein and fat. And despite the traditional belief that a high-carb diet is best for endurance runners, those in the low-carb group actually finished significantly faster than the high-carb group. This could be linked to the fact that the research showed a connection between a higher habitual dietary fat intake to faster finishing times. (Decreasing the number of calories from carbs means you're probably increasing the number of calories from fat.)

Before you decide that this means low-carb and high-fat eating is the secret to a faster ultramarathon finish, consider this: Runners who reported that they followed a Paleo diet or a low-carb diet plan were also more likely to leave the race with a "did not finish" (DNF). A whopping 57.1 percent of Paleo runners and 50 percent of low-carb runners in the study didn't finish their race. (Don't forget that dietitians aren't crazy about low-carb diets in general.) Meanwhile, 100 percent of the high-carb runners finished. And nope, training was not a factor in these results. All the runners, regardless of their diet choices, ran about 50 to 55 miles a week.

The researchers suspect that the group of runners with higher dietary fat intake might also have had greater caloric intake, and the extra calories (see: fuel) may be what led to those faster finishing times. Runners on high-carb diets might not be getting the calories they need to power through to the end.

Ultimately, more research is needed to determine how to fuel for an ultramarathon for optimum performance. These bigger, longer, beastlier races are in a class of their own. In the meantime, whether you're running a marathon, half marathon, 5K, or just around the block, making sure you're eating both enough, and enough of everything (protein/fat/carbs/fiber) will make all the difference between reaching your goals and needing to sit this one out.


Lauren Mazzo

Lauren Mazzo is a digital assistant editor for Shape and Fitness. She's an Ithaca College alumna, a Rochester, NY, native and an NYC transplant.  More →

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