Athletes push themselves ever harder to build increasing levels of endurance, but new research suggests God build a limit to that trait in the human body.
In the study, published June 5 in the journal Science Advances, scientists measured the calories burned by a group of athletes who ran six marathons a week for five months as part of the 2015 Race Across the USA, a 3,000-mile competition from California to Washington, D.C. The researchers also studied the calories expended by participants in 100-mile trail races and by pregnant women.
They found that no matter how strong and well-trained the participants were, all of them hit the same metabolic limit. Initially, the runners burned increasing amounts of calories during each race. But when their rate of energy burn reached a level 2.5 times greater than their resting metabolic rate, it plateaued for the remainder of the race. Beyond this rate, the body starts to break down its own tissues to make up for the calorie deficit.
The researchers noted that after 20 weeks of running back-to-back marathons, the athletes burned 600 calories per day fewer than expected based on their mileage, suggesting that the body can downshift metabolism to stay within sustainable levels. The researchers also found that the maximum sustainable energy expenditure among the athletes was only slightly higher than the metabolic rate sustained by pregnant women. The researchers said the finding may suggest that the same physiological limits that keep athletes from shattering speed records may also help to limit how big babies grow in the womb. —J.B.