Although it may seem like some athletes have super-human endurance, a new study published on 5 June in Science Advances suggests everyone reaches the same metabolic limit or maximum possible level of exertion that can be sustained long-term (1).
Just how far can the human body push itself? Well, to determine the limit of what’s possible for humans, the international team of researchers assessed the energy expenditure of athletes — six runners in total, including five men and one woman — during an intense and gruelling event.
More specifically, they measured the number of calories burned daily by a group of participants who ran six marathons per week for five months as part of the 3,000-mile (more than 4800-km) Race Across the USA in 2015, spanning from California to Washington, D.C.
And to measure the calories, the replaced the hydrogen and oxygen in drinking water with harmless isotopes, deuterium and oxygen-18, and then examined how these chemicals were flushed out in the sweat, urine, and breath of the athletes. This allowed them to determine how much carbon dioxide (CO2) an athlete produces, which can be directly related to how many calories are burned. Samples collected throughout the race — during the first and final leg.
A possible limit on human endurance
After collecting samples across the 20-week period, Thurber and her colleagues plotted the data over time and analyzed the results along with previously obtained metabolic data from other endurance events, including triathlons, ultramarathons, long-distance cycling races such as the Tour de France, and Arctic expeditions.
Whereas the energy expenditure of athletes started out fairly high, it eventually levelled off at 2.5 times their basal metabolic rate — the amount of energy expended at rest — for the remainder of the event. Moreover, after 20 weeks of back-to-back marathons, the athletes burned about 600 fewer calories.
Essentially, the researchers discovered that energy expenditure levels off after around 20 days, at which point humans can only burn calories at a rate that is 2.5 times faster than their resting metabolic rate, no matter what the event. And that includes even the world’s fastest ultra-marathoners — which means, eating more calories won’t always help you run harder, faster or longer.
Scientists previously suggested that human endurance may be linked to body temperature. But these new findings have thrown a bit of a spanner into the works. Nonetheless, digestion is certainly a limiting factor in endurance events — athletes must be able to process food and absorb calories and nutrients for fuel.
Once the body hit its endurance limit — the point at which it starts burning calories more quickly than they can be absorbed from food and converted it into energy — it must use fat reserves for energy instead. So, fat reserves are therefore crucial before a long race — which is something many endurance athletes already know too well.
Another interesting finding of the study was that the energy expenditure of the athletes was only slightly higher than that of pregnant women. And the same limit applies in both cases.
The scientists still don’t know exactly how this energy is being used during longer endurance activities. But now there is evidence to back already widely suspected: there is a limit on human endurance.
(1) Thurber, C. et al. Extreme events reveal an alimentary limit on sustained maximal human energy expenditure. Science Advances (2019). DOI: 10.1126/sciadv.aaw0341