New research published in evolutionary human science This provides evidence that when the human body is subjected to extreme physical stress, it tends to prioritize immune defenses over other fundamental biological functions such as reproduction and tissue repair. These findings suggest that our evolutionary biology has programmed us to allocate limited energy to immediate survival needs during periods of extreme fatigue. This research will help scientists better understand the deep roots of human adaptability and energy management.
Scientists conducted this study to test a core idea in biology known as life history theory. This theory proposes that all living organisms have a limited energy pool, which must be divided between competing biological needs. These needs typically include defense, body maintenance, energy storage, and reproduction.
During periods of severe energy deficiency, the body must make biological trade-offs to survive. The general idea suggests that limited resources are diverted to the body systems that provide the greatest immediate survival benefit.
Although biologists have documented these biological tradeoffs in animals such as insects and birds, there is little clear evidence in humans. Observational studies in humans are often unable to demonstrate these trade-offs. This is because individual differences in health and diet mask how the body shifts its resources.
To get around this problem, scientists turned to ultra-endurance athletes. Events like multi-day ultramarathons push human physiology to its absolute limits, creating a temporary but severe energy deficit. This extreme physical state provides a window into how the human body naturally re-prioritizes energy when pushed to the brink.
“This research was motivated by both scientific curiosity and personal experience. As an ultra-endurance athlete myself (setting multiple official Guinness World Records and fastest known times in ocean rowing and ultra swimming), I have experienced first-hand the physical and psychological effects of this type of competition,” said study author Danny Longman, Senior Lecturer in Human Evolutionary Physiology at Loughborough University and co-leader of the Human Evolutionary Ecophysiology Laboratory.
“Life history theory predicts that when energy is scarce, our bodies make strategic ‘decisions’ about where to allocate limited resources, prioritizing functions essential for immediate survival over those that can be deferred. This idea is central to evolutionary biology and well documented in other species, but clear experimental evidence that these trade-offs actually occur in humans is lacking. The challenge is ethical: we cannot experimentally starve people to test the theory.”
“Ultra-endurance athletes provided a unique solution: they were voluntarily exposed to extreme energetic stress under controlled and measurable conditions. This allowed us to observe what happens when the human body is pushed to its limits and must make real-time allocation decisions between competing physiological demands such as immune defense, reproduction, energy storage, and tissue maintenance.”
For the study, the scientists recruited 147 ultra-endurance athletes, 107 men and 40 women. These participants competed in one of five grueling multi-day events set in highly demanding environments. Four of the events were ultramarathon foot races held in Finland, Peru, Spain, and Nepal. The fifth event was a multi-week ocean rowing competition across the Atlantic Ocean.
The scientists collected physical measurements, saliva, and blood samples from the athletes one to two days before each competition began. A second set of samples was then collected either immediately after the athlete crossed the finish line or the day after. This allowed the researchers to track changes in various biological markers during the course of extreme physical exercise.
First, the researchers measured general energetic stress by tracking changes in body weight and cortisol. Cortisol is the main stress hormone released by the body to help mobilize energy. The scientists also tracked specific biological markers related to four key areas: defense, storage, reproduction, and maintenance.
To monitor protection, they measured levels of interleukin-6, a protein that indicates immune system activation and inflammation. They also tested the athlete’s serum’s ability to kill bacteria and destroy damaged red blood cells in a laboratory setting.
To assess energy stores, the scientists calculated the participants’ fat mass index and measured their leptin levels. Leptin is a hormone produced by adipose tissue that signals the brain about the state of the body’s energy stores.
For reproductive investment, the researchers tracked testosterone in both men and women, and estradiol, the main female hormone, in women. Finally, they monitored biological maintenance by examining markers of tissue damage and oxidative stress. Oxidative stress occurs when there is an imbalance between harmful free radicals and the antioxidants that neutralize them.
To track this maintenance of the body, they analyzed myoglobin, a protein released into the blood when muscles are injured. They looked at this alongside other markers of cell damage and cartilage destruction.
The data show evidence that participation in these extreme events caused significant energetic stress. The athletes experienced significant decreases in weight and fat mass, along with a sharp rise in the stress hormone cortisol.
In the face of this extreme energy deficit, athletes’ bodies made clear biological trade-offs. Biomarkers associated with immune defense generally increased or remained stable, especially in male athletes. This suggests that the body is devoting dwindling energy reserves to activating the immune system and preparing it to fight potential infections.
At the same time, systems related to storage, reproduction, and maintenance were significantly curtailed. Both men and women showed significant reductions in leptin and fat mass, indicating that stored energy was burned more rapidly. Markers of reproductive investment were also reduced, and male athletes showed a significant reduction in testosterone.
The bodies also seemed to sacrifice physical maintenance during the event. Levels of myoglobin and other markers of cellular stress spiked. This indicates that the body had accumulated structural damage to the muscles and cartilage, but did not have the reserves needed to properly repair those tissues at the time.
“We were really surprised that the patterns were consistent across such diverse conditions, including male and female athletes, runners and ocean rowers, and races in the Arctic and Amazon jungles,” Longman told SciPost. “Despite these dramatically different challenges, we saw the same basic pattern: reproductive hormones and energy reserves were reduced, while immune function was broadly protected or enhanced.”
According to Longman, the strength and consistency of this effect suggests that these biological tradeoffs are deeply embedded in human physiology.
“Our bodies are incredibly adaptable, but that ability to adapt comes with trade-offs. When energy is depleted due to illness, food insecurity, extreme exercise, or other stressors, the body doesn’t just randomly shut down. It makes strategic choices to prioritize immediate survival functions, such as immune defenses, and temporarily downregulate less urgent systems, such as reproduction and some repair processes.”
“This has practical implications: It helps explain why athletes who train hard often get injured, why chronic stress affects fertility, and why people fighting infections lose weight and feel fatigued. Understanding these trade-offs can inform better approaches to public health, athletic training, and managing periods of physiological stress.”
As with all research, there are some caveats. Because ultra-endurance athletes are highly trained individuals, their physical conditioning may protect them from some of the negative effects of severe energy loss. The average person may have different biological responses under similar stress.
“We are not suggesting that ultra-endurance exercise is ‘natural’ or that our ancestors engaged in similar activities on a daily basis,” Longman said. “Rather, we are using these extreme conditions as a tool to uncover trade-offs that would otherwise be difficult to detect. It is also important to understand that many of the biomarkers we measured serve multiple physiological roles. Therefore, although we have classified them as life history functions, the reality is more complex.”
“Finally, these are short-term responses measured over days to weeks. It remains to be seen whether different patterns emerge during chronic and long-term energetic stress, or how these trade-offs change in different populations and life stages.”
To develop this further, the scientists plan to investigate how these biological selections change across different demographics and recovery periods.
“Important questions remain about how these patterns of trade-offs change over the human lifespan and in different populations,” Longman said. “Do children, the elderly, pregnant women, or people with different ancestral backgrounds exhibit the same priorities? What happens during recovery, and in what order do these functions recover once energy is available again? Can we identify individual differences that predict who is most vulnerable to these trade-offs under stress?”
“From a practical perspective, the key question is: How can we collaborate with sports scientists to leverage this knowledge to optimize training programs that minimize injury risk and health impacts? Understanding these trade-offs can inform how we approach our clinical practice with patients experiencing the physiological stress of illness and treatment. More broadly, this research contributes to evolutionary public health by using our understanding of human evolutionary biology to design better health interventions. If we could understand how the body naturally allocates resources under stress, could we work with those patterns rather than against them?”
Ultimately, this project highlights the benefits of merging different scientific disciplines to address contemporary health problems. “This research represents a true collaboration between evolutionary biology, sports science and public health,” Professor Longman said. “This shows that an evolutionary perspective is not just about our ancient past, but provides practical insights into modern health challenges.”
“I’m particularly excited about the potential applications, including helping athletes train more safely, understanding why food insecurity has a wide-ranging impact on health, and informing clinical approaches for patients experiencing the physiological stress of illness and treatment. I would also like to thank the athletes who participated, who endured blood draws and tests during the most physically demanding moments of their lives to contribute to science. Their dedication made this research possible.”
“Importantly, I would also like to highlight that this paper is the result of a long-term collaboration led by Professor Jay Stock (Western University, Canada) and Professor Jonathan Wells (UCL Great Ormond Street Institute of Child Health, London). Their expertise in human evolutionary biology and life history theory has helped shape this work.”
The study, “Experimental Evidence for Life History Tradeoffs During Ultra-Endurance Physical Activity,” was authored by Daniel P. Longman, Alison Murray, Emily L. Brown, Courtney Lewis, Richard M. Millis, Tomasz J. Nowak, Krizia Ivana T. Udoquim, Michael P. Muehlenbein, Jonathan CK Wells, and Jay T. Stock.
