The Loughborough University study, published in Evolutionary Human Sciences by Cambridge University Press, found that when the human body experiences severe energy shortages, it prioritises immune defence over other vital processes such as reproduction, energy storage/body fat reserves and tissue maintenance.
The research studied 147 ultra-endurance athletes competing in extreme multi-day races around the world. This included ultramarathons in Finland, Peru, Spain and Nepal, as well as a multiweek transatlantic rowing event.
Findings showed that athletes lost substantial body mass and showed elevated physical stress symptoms during competition.
Under these conditions, biomarkers linked to immune defence increased or remained stable, while energy storage, reproductive function and tissue maintenance declined.
Dr Danny Longman, lead author from the School of Sport, Health and Exercise Sciences at Loughborough University explained: "When the human body faces extreme energy shortages, it behaves like a household on a tight budget – it pays for the essentials first. Our study shows that maintaining immune defences takes priority, even when this means reducing investment in reproduction, energy storage, and tissue repair.
“This prioritisation makes evolutionary sense: surviving an immediate threat is more important than long-term functions when resources are scarce. We found that immune markers either increased or remained stable during the races, while markers of reproduction, energy storage, and tissue repair all declined.
“This suggests the body actively maintains its defensive capabilities even under severe energetic stress, directing scarce resources toward immune function rather than other biological processes."
Researchers found that immune markers and activity increased during the races, indicating heightened biological defence against potential infection or injury.
The study also found evidence that markers of reproductive function decreased with male athletes experiencing reductions in testosterone. Markers related to female also declined.
Dr Longman added: “We found that female athletes experienced reductions in fat mass index and leptin – a hormone that signals energy availability to the brain and helps regulate reproductive hormones. When leptin levels drop too low, it can signal the body that energy reserves are insufficient to support reproduction."
Markers of muscle and cellular damage rose sharply following competition, reflecting the enormous strain placed on the body during prolonged endurance exercise.
Dr Danny Longman collaborated alongside Professor Jay Stock (Western University, Ontario, Canada) throughout the project.