Theoretical Limits of Human Energy Use

The theoretical limits of human energy use have intrigued scientists for many years, especially when it comes to athletes who constantly push these biological boundaries. A new study that tracked marathon runners over a year sheds light on this issue.

Findings of the Recent Study

Published in the journal “Current Biology,” the study revealed that marathon runners can burn up to 11,000 calories a day during competitions. However, they cannot sustain this level for long without consequences. The findings suggest a biological limit even for the world’s top athletes that cannot be surpassed.

Andrew Best, a co-author of the study and a biological anthropologist at the Massachusetts College of Liberal Arts, expressed surprise at the results, noting that some runners struggle to approach this limit.

The Metabolic Ceiling for Humans

The study builds on previous research aimed at determining the metabolic ceiling of the human body, which is the maximum rate of calorie burning the body can endure. Earlier research from the 1980s and 1990s suggested this limit is four to five times the basal metabolic rate (BMR), which is the energy required to maintain the body at rest.

However, later studies from shorter races showed that athletes could reach 9.4 times BMR in 11-hour Ironman races and 8.5 times BMR in 25-hour marathon races.

Effects of Physical Exertion on the Body

The study indicates that maintaining exercise near the metabolic ceiling comes at a cost. Amanda McGrosky, an evolutionary anthropologist, points out that the body compensates by slowing digestion, weakening immune responses, and temporarily shrinking brain tissue.

There is also some evidence that people invest less energy in sexual arousal and reproduction after races.

Potential Factors for Raising the Metabolic Ceiling

Experts caution that the study sample was small, raising questions about whether the proposed metabolic ceiling represents a true limit for humans. Athletes might benefit from understanding how close they are to this limit during training and competitions.

Some hypothesize that the metabolic ceiling essentially reflects a limit in digestion and nutrient absorption that fuels the body’s muscles.

Conclusion

While the study provides important insights into the biological limits of human energy use, questions remain about whether this is an absolute limit for humans or if it can be surpassed in the future. With advancements in sports nutrition, future athletes may be able to consume and burn more calories than currently possible.