“The body degrades its tissues”: athletes, scientists have discovered a hidden limit in human endurance

Andrew Best, an anthropologist at the Massachusetts College of Liberal Arts and himself an endurance athlete, coordinated a study that revealed the existence of an insurmountable metabolic threshold. Even the best-trained athletes fail to burn calories above a certain rate over the long term.

The team followed fourteen ultra-runners, cyclists and triathletes during their competitions and training phases. The method used was based on the ingestion of water enriched with deuterium and oxygen-18, two isotopes whose urinary elimination makes it possible to calculate the production of carbon dioxide and therefore the total caloric expenditure.

The results show that during multi-day events, some athletes temporarily reach energy expenditure levels equivalent to six or seven times their basal metabolism, or approximately 7,000 to 8,000 calories daily. Yet when scientists calculated the average over periods of thirty or fifty-two weeks, energy consumption consistently fell to around 2.4 times basal metabolism.

This phenomenon is explained by an unconscious energy reallocation. When the body massively mobilizes its resources for physical effort, it reduces other areas of expenditure. “Your brain has a considerable influence on your propensity to move, fidget, and your desire to take a nap”specifies the researcher. “All that fatigue we feel preserves calories.”

Best nevertheless tempers the practical significance of this work for ordinary people: “For most of us, we will never reach this metabolic ceiling. You need to run on average about eighteen miles per day for a year to reach 2.5 times the basal metabolism. The majority of people, myself included, would injure themselves before any energy limit comes into play.”

This work, funded by Duke University and the Massachusetts College of Liberal Arts, now opens the way for new research into the impact of this metabolic ceiling on other biological processes.