🏃 There is an insurmountable limit to human endurance, even among athletes

A boundary is emerging for the most seasoned endurance athletes. Beyond apparent performances and records, their bodies seem to obey a fundamental biological law that regulates long-term energy expenditure. This highlights the fundamental physiological processes that govern energy management in the human body.

The study published in Current Biology reveals the existence of a metabolic ceiling that even the most trained athletes cannot sustainably exceed. This research conducted on ultra-endurance specialists demonstrates that the human body has its own limits, independent of willpower or training. The researchers used particularly precise measurement methods to reach these conclusions.

An innovative measurement method

The concept of "basal metabolic rate," which corresponds to the minimum energy expended by the body at complete rest, served as a fundamental reference for this study. The researchers sought to determine to what extent elite athletes could sustainably exceed, and by how much, this baseline level. Their experimental protocol involved 14 high-level athletes specialized in extreme disciplines, followed for nearly a year.

The methodology used relied on doubly labeled water, a metabolic measurement technique considered the most reliable. Participants consumed water enriched with stable isotopes of deuterium and oxygen-18. Regular analysis of their urine allowed tracking the fate of these markers and accurately calculating their carbon dioxide production and total caloric expenditure.

The results showed that during the most intense competitions, some athletes reached energy consumption levels equivalent to 6 or 7 times their basal metabolism. However, when these measurements were extended over 30 to 52 weeks, the average energy expenditure consistently fell back to around 2.4 times the basal rate. This observation confirms the existence of an insurmountable metabolic ceiling over time.

The body's adaptation mechanisms

The body deploys strategies to maintain its energy balance over the long term. The research highlighted how the body compensates for periods of caloric overconsumption by subtly reducing other metabolic activities. The brain would play a central role in this regulation by modulating unconscious expenditures.

Scientists observed that athletes spontaneously reduced their ancillary movements and increased their rest time outside of training sessions. These behavioral adjustments, often imperceptible, actively participate in preserving the overall energy balance. The body thus optimizes its resources according to their availability.

This intelligent energy management would explain why weight loss often reaches a limit despite increased physical activity. The body would always find a way to reduce certain expenditures to compensate for additional efforts. This discovery could have important implications for understanding dietary regimens and training programs.

The evolutionary perspective provides additional insight into these observations. The metabolic ceiling could represent a biological adaptation aimed at preserving vital functions such as immunity or reproduction. In situations of prolonged metabolic stress, the body would prioritize immediate survival at the expense of other less urgent functions.

To go further: What is basal metabolism?

Basal metabolism represents the minimum energy that the body consumes at complete rest to ensure its vital functions. It includes breathing, blood circulation, and thermal regulation. This basic energy expenditure varies according to age, sex, and body composition.

In an average adult, basal metabolism represents about 60 to 70% of total energy expenditure. It is generally higher in men than in women and gradually decreases with age. Muscle mass significantly influences this basal metabolism.

Accurate measurement of basal metabolism requires strict conditions: physical and mental rest, neutral ambient temperature, and fasting for several hours. For these assessments, scientists primarily use two methods: laboratory measurement in specially designed rooms, or analysis by doubly labeled water which allows monitoring in real conditions.