The number of calories the human body can burn is limited over time, with significant increases only possible in short bursts, according to a new study

Researchers have discovered that there’s a cap on the average number of calories the human body can burn, that even most extreme athletes can’t surpass.

The research, Ultra-endurance athletes and their metabolic ceiling was led by anthropologist, Andrew Best, at the Massachusetts College of Liberal Arts and published in the journal, Cell Biology.

Fourteen ultra-endurance athletes were monitored over the course of a year for the study. Participants consumed water that had been enriched with deuterium and oxygen-18 and by following how quickly these isotopes left the body through urine, the scientists were able to determine how much carbon dioxide they had expired and then estimate their calorie expenditure.

In the longer term, burning calories at a high level is unsustainable, because the body will start to break down

The most extreme calorie needs
During multi-day races the athletes increased their calorie burn, recording levels that were six to seven times their basal metabolic rate – the minimum amount of calories the body needs for life-sustaining functions – using around 7,000 to 8,000 calories. However, it was discovered that this type of calorie burn cannot be sustained in the longer term.

Examining their results over the course of a longer period –30 to 52 weeks – the rate that the athletes burned calories averaged closer to 2.5 times their basal metabolic rate, with this understood to be a sustainable baseline.

Best says: “If you go over the ceiling for short periods for things such as endurance events, that’s fine. You can make up for it later, but in the longer term burning calories at this level is unsustainable because the body will start to break down its tissue and the body will shrink.

Wider implications
The implications for everyday athletes were explained by Best, who said: “Someone would have to run about 11 miles on average every day for a year to achieve a basal metabolic rate of 2.5 and most people would get injured doing this before any sort of energetic limit came into play.”

The researchers also found that as athletes neared this ceiling they subconsciously limited energy usage elsewhere by reducing activities such as walking or fidgeting.