The changes in the runners’ energy expenditures were striking. In their first week of repeated marathons, the runners burned about 6,200 calories a day on average, a steep increase over their typical energy expenditure from the week before — and about what would be expected, based on their new level of activity.

But 20 weeks later, although they were running just as much and at about the same pace, the racers had lost little body weight, and were expending about 600 fewer calories each day on average than they did in the first week.

By the end of the event, the researchers calculated that the runners were expending about two and a half times their resting metabolic rate each day, a notable decline from the early days of the event, when they were burning at least three and a half times their resting rate.

To better understand their finding’s significance, the researchers combed through the few past studies of energy expenditure using doubly labeled water. Those involved participants in other long, grueling physical endeavors, including the Tour de France bike stage race, Arctic expeditions, ultramarathons, marathons and even pregnancy.

The researchers found that in any event that lasted longer than about 12 hours, participants’ energy expenditure tended to increase substantially and then, over time, decline, until it plateaued at somewhere near two and a half times their everyday, resting metabolic rate.

The researchers also examined past studies of overeating, in which people gorged on food to see how much weight they gained and how quickly, and found that most of them added pounds at a rate that suggested they could absorb about two and a half times their basic caloric needs. That is, participants might swallow more calories, but their bodies could not process anything beyond that limit.

Our bodies seem somehow to have become capable of recognizing when we are in danger of breaching the barrier beyond which we cannot easily replenish lost energy — about two and a half times our basic metabolic rate, according to Herman Pontzer, an associate professor of evolutionary anthropology at Duke University, who oversaw the new study with John Speakman and others. If we repeatedly approach that barrier, by, for instance, running marathons day after day, we apparently reduce our daily energy burn.