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Pushing the Limits of Human Endurance

When we work out rigorously on a regular basis, our bodies adjust to limit our ability to expend energy, a new study shows.

Our bodies seem to adjust to prolonged, repeated physical exertion and its energy demands by burning fewer — instead of more — calories over the course of the day, even if our exertions continue at the same level, according to a surprising new study of energy expenditure conducted during a 20-week running race across the United States.

The study is among the first to quantify the upper limits of human daily energy expenditure and endurance, whether someone is running across the country, competing in the Tour de France or pregnant. The study’s counterintuitive findings have implications for athletes, our understanding of human evolution, and our hopes that training for a marathon or other endurance event will help us shed weight.

To most of us, it seems obvious that when we are physically active, we burn more calories than when we are sedentary. The harder or longer we work out, the more of these calories we will expend, ad infinitum.

But a small but growing body of research suggests there are limits. A 2012 study of energy expenditure among modern hunter-gatherers, for instance, found that despite being in motion almost all day, the tribespeople burned about the same number of daily calories as those of us who sit behind desks all day. In effect, the tribespeople’s bodies seemed to have found ways to reduce their overall daily energy expenditure, even as they continued to move.

The study’s authors concluded that this finding made sense from an evolutionary standpoint. The fewer calories our forebears had to expend on days when they hunted, the less food they would need to bring down.

But the human caloric ceiling remained unknown and difficult to quantify. Finding it, the scientists reasoned, would require studying people who were exercising regularly at or near their physical limits and seeing how their metabolisms responded over time.

Then, in 2015, the right situation arose. For a one-time event called the Race Across USA, participants would cross the country on foot from California to Washington, D.C., running approximately a marathon almost every day for about 20 weeks.

A group of scientists, including some who had conducted the 2012 study of hunter-gatherers, asked to monitor the racers’ metabolisms. Six participants agreed, and the researchers measured their baseline daily energy expenditure in the week before they began racing. They used a gold-standard technique called doubly labeled water, in which hydrogen and oxygen are replaced with isotopes that trace the body’s production of carbon dioxide.

The researchers repeated the metabolic testing each day during the first week of daily marathons, and then again during the runners’ final week, about five months later. (Only three of the original volunteers remained in the race.