Limits to human endurance

RESEARCHERS from Duke University have determined that irrespective of the activity, human energy expenditure cannot be sustained above 2.5 times the rate of metabolism, or calorie burning, at rest. Caitlin Thurber and colleagues used data on athletes who competed in global endurance events. The findings are published in a recent issue of “Science Advances”.

The report suggests that in terms of human evolution, all human activities share the same metabolic limits, which means natural selection that enhanced one endurance ability, such as running, may have consequently benefited many others, such as brain size expansion and gestation length.

“This defines the realm of what’s possible for humans,” said Herman Pontzer, a co-author in the study. Beyond the ceiling of 2.5 times a person’s resting metabolic rate, the body starts to break down its own tissues to make up for the calorie deficit.

The researchers incorporated original data from the Race Across the USA (RAUSA), the longest event to date from which metabolic measurements have been recorded. In this gruelling event, athletes run from Huntington Beach, California, to Washington, a distance of about 4,800 km, covering approximately a marathon a day, six days a week, for 14-20 weeks.

The researchers found that metabolic rates varied greatly based on the length of the endurance event, with high metabolic rates accompanied by decreases in total energy expenditure. The athletes’ energy expenditure started out relatively high, but invariably plunged and flattened out at 2.5 times their basal metabolic rate for the remainder of the event. That is to say that the body can “downshift” its metabolism to help stay within sustainable levels. However, according to the study, the same metabolic ceiling applied across all events. Energy expenditure could not be sustained for any physical activity above 2.5 times the rate of metabolism at rest, meaning that activities involving different muscle groups and organ systems are all united by the same energy intake controls.