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Despite the almost superhuman feats of athletic prowess displayed in marathons, Iron person contests, and the Tour De France, a new study from Duke University just published in the journal Scientific Advances suggests that that the human body has physical limits defined by our digestive system’s abilities to absorb calories.  A collaborative group from the USA, Scotland, and China studied runners completing 6 marathons a week for 5 months as they raced across the United States.

The data indicates a limit to peak energy expenditure at 2.5 times a person’s basal metabolic rate.  Once the energy demands surpass this wall, the body begins to break down its own muscle and fat tissues to provide fuel, and performance wanes.  

In all contests whether running, cycling, or dragging sleds across the arctic, energy expenditure follows an L-shaped curve with the plateau defined by the gut.  The calorie expenditure max is about 3500/day for women and 4500/day for men. 

Despite this data, athletes continue to push the envelope with their natural abilities and sometimes by consuming dangerous drugs and food supplements.  Just know that, as always, you can’t fool mother nature.

Caitlin Thurber, Lara R. Dugas, Cara Ocobock, Bryce Carlson, John R. Speakman, Herman Pontzer. Extreme events reveal an alimentary limit on sustained maximal human energy expenditure. Science Advances, 2019; 5 (6) DOI: 10.1126/sciadv.aaw0341

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