The human body has its limits, and the number that may define the limits of human endurance is a fascinating topic to explore. While watching the Tour de France, it's easy to become desensitized to the incredible feats of endurance on display. But to truly understand the limits of human energy expenditure, we need to look beyond a single three-week block. In recent years, researchers have been building a clearer picture of where the limits of human energy expenditure might actually lie. Escape spoke with two of those researchers to learn about the speed-limiter that seems to be innate to all humans, regardless of whether you’re a Tour de France champion or you’ve never even ridden a bike.
The key to understanding the ceiling on human exertion lies in basal metabolic rate (BMR). Your BMR is the minimum amount of energy your body needs to simply keep the lights on – to pump blood around your body and maintain organ health. Your BMR accounts for between 60-75% of all the energy you expend on a daily basis and doesn’t factor in the energy required for any movement, digestion, or anything besides simply lying down while your body keeps you alive.
For context, a 30-year-old male who’s 180 cm tall and weighs 78 kg has a BMR around 1,740 calories (7,280 kJ) per day. For a 30-year-old female, at 165 cm and 68 kg, that figure is more like 1,330 calories (5,565 kJ) per day. But how does BMR apply to the limits of human endurance? Well, in recent years, researchers have become increasingly convinced that it’s possible to quantify the human ‘metabolic ceiling’ – the maximum rate of energy expenditure over a prolonged period – using BMR as a comparative tool.
Several papers in the past eight years have shown that humans aren’t able to sustain a daily average energy expenditure of more than 2.5 times their BMR, for periods of around 28 weeks or greater. For shorter efforts, exceeding 2.5 x BMR is easy for even amateur athletes, and short-term energy expenditures can reach much higher than that. Some Ironman triathletes and ultramarathon runners, for instance, seem able to burn a staggering 9-10 times their BMR (around 17,000 calories or 72,000 kJ) over the course of a single day, but the longer the effort, the lower the average daily energy expenditure a person can sustain.
Tour de France riders, for example, are known to burn around 4-5 times their BMR, daily (https://www.ovid.com/jnls/acsm-msse/abstract/10.1249/mss.0b013e31822430ed~maximal-sustained-levels-of-energy-expenditure-in-humans?ref=escapecollective.com) , over three weeks of racing. Polar explorers have shown comparable energy expenditures for more than twice as long (https://link.springer.com/article/10.1007/s004210050243?ref=escapecollective.com) (contrast the dialled nutrition of today’s Tour riders with the literal sticks of butter that men like Robert Falcon Scott and Ernest Shackleton would eat during an expedition, in order to maximise calorie intake).
But when we zoom out to longer time scales, the metabolic ceiling seems to converge on a specific point – that ceiling of 2.5 x BMR, for periods of 28 weeks or more. That’s roughly 4,350 calories (18,200 kJ) per day for the average active male, or 3,325 calories (13,920 kJ) for the average active female. Try to burn more than that on average per day indefinitely, and the body’s natural defences will step in (more on that in a moment).
What makes this particularly fascinating is that the limits of human endurance are not just about physical performance but also about the body's natural defenses. The human body has evolved to protect itself from excessive energy expenditure, and it will take measures to prevent damage. This raises a deeper question: how does the body protect itself from the extreme demands of prolonged endurance activities?
In my opinion, the answer lies in the body's ability to adapt and conserve energy. When the body detects that it is burning more energy than it can sustain, it will activate various mechanisms to slow down the rate of energy expenditure. This could include reducing the efficiency of muscle contractions, slowing down the metabolism, or even shutting down certain organs to conserve energy. These adaptations are crucial for survival and allow the body to endure prolonged periods of high-energy expenditure.
One thing that immediately stands out is the importance of nutrition in endurance activities. The body requires a steady supply of nutrients to maintain its energy levels and prevent fatigue. The extreme calorie intake of Ironman triathletes and ultramarathon runners highlights the need for proper nutrition to support prolonged endurance. What many people don't realize is that the body's ability to sustain high-energy expenditure is not just about physical capacity but also about the quality and timing of nutrition intake.
If you take a step back and think about it, the limits of human endurance are a fascinating interplay of biology, physiology, and nutrition. The body's natural defenses and adaptations are crucial for survival, and understanding these mechanisms can provide valuable insights into the world of endurance sports and human performance.
In conclusion, the number that may define the limits of human endurance is a complex and multifaceted topic. It involves understanding the body's metabolic ceiling, the role of nutrition, and the body's natural defenses. By exploring these aspects, we can gain a deeper appreciation for the incredible feats of endurance that humans are capable of achieving.
