Extreme endurance tests reveal immune-first survival strategy in humans

A study of 147 ultra-endurance athletes across ultramarathons and ocean rowing provides experimental evidence for life history trade-offs in humans: under extreme energy deficit, the body prioritizes immune defense while suppressing energy storage and reproductive investment, with leptin and fat mass dropping, testosterone decreasing in men, and markers of tissue maintenance rising. Cortisol spikes and weight loss accompany these shifts, a pattern consistent across sexes and events, suggesting deep evolutionary roots in energy allocation during stress. The findings have implications for athletic training and public health, but may not generalize to non-athletes or chronic stress, and recovery dynamics across populations remain to be explored.