The Science of Endurance and Human Limits

While earlier theories linked endurance to body temperature regulation, the 2019 Science Advances study found that digestion, not temperature, is the primary limiter, challenging prior assumptions.

Beyond 2.5 times the resting metabolic rate, the body cannot absorb enough calories, leading to the breakdown of fat and muscle stores, causing weight loss and eventual unsustainability.

Robert Scott’s 1911/12 Antarctic sledding expedition, involving 159 days of man-hauling sleds, expended about 1 million kcal per person, far surpassing modern events like the Tour de France.

Proposed by Tim Noakes, the central governor theory suggests the brain regulates exertion to prevent catastrophic failure, imposing fatigue before physical limits are reached, as discussed in Endure by Alex Hutchinson.

Humans evolved to sweat efficiently, allowing prolonged exercise in heat by dissipating body heat, unlike most mammals, giving us greater endurance than all land mammals except the Alaskan Husky.

Sports science research highlights motivational self-talk as a key strategy for coping with pain, helping athletes maintain focus and block negative thoughts during grueling events.

The Science Advances study showed that pregnancy sustains a metabolic rate of about 2.2 times the resting metabolic rate, close to the 2.5 times limit of endurance athletes, suggesting shared physiological constraints.

The 2015 Race Across the USA, a 3,000-mile run from California to Washington, D.C., involved athletes running six marathons a week for five months, revealing an L-shaped curve in energy expenditure leveling at 2.5 times resting metabolic rate.

Research by Herman Pontzer and John Speakman suggests the digestive system, particularly the gut’s ability to process and absorb calories, sets a hard limit on sustained endurance, rather than heart, lungs, or muscles.

A 2019 study in Science Advances found that humans can sustain energy expenditure at 2.5 times their resting metabolic rate (about 4,000 calories daily for an average person) during prolonged endurance events, limited by the digestive system’s ability to absorb calories.