Researchers have discovered there's a cap on the number of calories the human body can burn in one day, 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 Cell Biology.

Fourteen ultra-endurance athletes were monitored over the course of a year. Participants consumed water 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.

During multi-day races the athletes burned six to seven times their basal metabolic rate – the minimum amount of calories the body needs for life-sustaining functions – around 7,000 to 8,000 calories. However, it was discovered that this type of calorie burn can’t be sustained in the longer term. Over the course of 30 or 52 weeks, the rate that the athletes burned calories was closer to 2.5 times their basal metabolic rate.

Best says: “If you go over the ceiling for short periods that’s fine. You can make up for it later, but long term it’s unsustainable because your body will start to break down its tissue and you’ll shrink.

“However it takes running about 11 miles on average a day for a year to achieve 2.5 BMR and most people would get injured 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.