Elite athletes show long-term energy ceiling of 2.5x resting rate

A yearlong study of elite endurance athletes has found that even the most highly trained bodies cannot sustain calorie burns far above a consistent biological ceiling, researchers reported October 20 in the journal Current Biology. Scientists led by biological anthropologist Andrew Best tracked 14 professional ultrarunners, cyclists, and triathletes across competitions and training over 30 to 52 weeks and measured energy expenditure using doubly labeled water and training logs.

The team recorded extreme short-term energy use during multiday races, with some athletes briefly burning six to seven times their basal metabolic rate. Those peak days corresponded to several thousand to as much as 7,000 to 11,000 kilocalories, depending on the event and individual. When the same athletes’ energy use was averaged over longer stretches of training and competition, however, their daily expenditures converged around 2.4 to 2.5 times basal metabolic rate. That level corresponds to roughly 4,500 calories per day for someone whose resting energy use is 1,800 calories.

Researchers used the labeled water technique, in which participants drink water containing heavier isotopes of hydrogen and...