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Can you burn endless calories or is there a metabolic limit? Here’s the research

This research highlights the metabolic, biological ceiling to human endurance

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Is there a limit to the number of calories you can burn? What about when you’re taking part in endurance exercise like a triathlon? Researchers explored just that in an interesting study that looked at how much energy humans can burn over longer periods, such as during certain endurance sports or laborious work that’s physically demanding on the body. Is there a metabolic ceiling and a calorie-burning limit? How much energy can the human body burn and sustain over time?

The research

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In a study published in the journal Current Biology, researchers measured how many calories 14 elite ultra-endurance athletes burned compared to their normal resting energy requirements, known as basal metabolic rate (BMR). The athletes were ultra-runners, triathletes, and cyclists. The goal was to test this ‘metabolic ceiling’ as athletes competed in extreme events lasting from one day to nearly two weeks, during both high- and low-intensity training. The researchers tracked how many calories the athletes burned and compared it to their BMR.

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People and animals can maintain a certain level of energy use for long periods, known as the basal metabolic rate (BMR). This is the number of calories your body has to burn just to stay alive and to continue your bodily processes like breathing and pumping blood.

The results

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The results showed the following:

  • There appears to be a metabolic ceiling that caps the amount of energy a human body can use over time. Our bodies can only digest so much food and process so much energy in one sitting. Regardless of how athletic the individuals were, their bodies hit an upper limit.
  • Over longer periods of weeks to months, the athletes could sustain burning around 2.5 times their resting energy rate (the number of calories their body needs to burn at rest to stay alive and perform basic bodily functions like breathing). The most elite athletes weren’t able to sustain energy use beyond that for those longer durations, though a few athletes very briefly reached slightly higher levels up to about 2.7 times their BMR.
  • Over a day or two, athletes could briefly burn around 10 times their BMR, as shown in other studies.
  • However, as the activity duration extended to weeks or months, energy use dropped off before leveling off at around 2.5 times their BMR.
  • When they took part in multi-day races, some of these athletes burned 6 to 7 times their BMR, which amounts to 7,000 to 8,000 calories per day. Still, over longer periods of 30 and 52 weeks, the calorie-burning rates returned to the predicted metabolic ceiling of 2.5 times the BMR. You can go over the ceiling briefly for short periods, but this study shows it isn’t sustainable in the long term and that the body can begin to break down tissue.

The takeaway

Man running on street by a body of water.
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This research highlights the metabolic, biological ceiling to human endurance and a limit on calorie-burning, even for elite ultra-endurance athletes. In other words, human metabolism has its limit. With endurance exercise, over weeks or months, this study shows that even the fittest endurance athletes reach a steady-state energy-burning limit around 2.5 times their resting metabolic rate.

Steph Green
Steph Green
Steph Green (Steph Zee) is a singer-songwriter and professional writer with over 12 years of experience in healthcare and… Read Full Bio
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