When we work out rigorously, our bodies limit our ability to expend energy

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. 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. 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.

Source: www.nytimes.com