Extreme athletes just helped scientists unlock a deep evolutionary secret about human survival

A new study published in Evolutionary Human Sciences provides evidence that when the human body undergoes extreme physical stress, it tends to prioritize immune defense over other basic biological functions like reproduction and tissue repair. These findings suggest that our evolutionary biology programmed us to allocate limited energy toward immediate survival needs during periods of intense exhaustion. This research helps scientists better understand the deep roots of human adaptability and energy management.

Scientists conducted this research to test a core idea in biology known as life history theory. This theory proposes that all organisms have a limited pool of energy, which they must divide among competing biological needs. These needs generally include defense, physical maintenance, energy storage, and reproduction.

During periods of severe energy shortage, the body must make biological trade-offs to survive. The prevailing idea suggests that limited resources are diverted to bodily systems that offer the greatest immediate survival benefit.

While biologists have documented these biological trade-offs in animals like insects and birds, clear evidence in humans has been scarce. Observational studies in humans often fail to show these trade-offs because individual differences in health and diet mask how the body internally shifts its resources.

To bypass this problem, scientists turned to ultra-endurance athletes. Events like multiday ultramarathons push human physiology to its absolute limits, creating a temporary but intense energy deficit. This extreme physical state provides a window to observe how the human body naturally reprioritizes its energy when pushed to the brink.

“This research was motivated both by scientific curiosity and personal experience. As an ultra-endurance athlete myself (I’ve set multiple official Guinness World Records and fastest known times for ocean rowing and ultra swimming), I’ve experienced firsthand the physical and psychological impact of this type of event,” said study author Danny Longman, senior lecturer in human evolutionary physiology at Loughborough University and co-leader of the Human Evolutionary EcoPhysiology Lab.

“Life history theory predicts that when energy becomes scarce, our bodies make strategic ‘decisions’ about where to allocate limited resources – prioritizing functions critical for immediate survival over those that can be deferred. While this idea is central to evolutionary biology and has been well-demonstrated in other species, we’ve lacked clear experimental evidence that these trade-offs actually occur in humans. The challenge is ethical – we can’t experimentally starve people to test the theory.”

“Ultra-endurance athletes provided a unique solution: they voluntarily undergo extreme energetic stress in controlled, 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 like immune defense, reproduction, energy storage and tissue maintenance.”

For their study, the scientists recruited 147 ultra-endurance athletes, consisting of 107 men and 40 women. These participants were competing in one of five grueling, multiday events set in highly demanding environments. Four of the events were ultramarathon footraces taking place 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 their respective events began. They then gathered a second set of samples immediately or a day after the athletes crossed the finish line. This allowed the researchers to track changes in a wide variety of biological markers over the course of the extreme physical exertion.

First, the researchers measured general energetic stress by tracking changes in body mass and cortisol. Cortisol is a primary stress hormone that the body releases to help mobilize energy. The scientists also tracked specific biological markers related to the four key areas of defense, storage, reproduction, and maintenance.

To monitor defense, they measured levels of interleukin-6, a protein that indicates immune system activation and inflammation. They also tested the ability of the athletes’ blood serum to kill bacteria and destroy damaged red blood cells in a laboratory setting.

To evaluate energy storage, the scientists calculated the participants’ fat mass index and measured their leptin levels. Leptin is a hormone produced by fat tissue that signals the status of the body’s energy reserves to the brain.

For reproductive investment, the team tracked testosterone in both men and women, as well as estradiol, a primary female sex hormone, in the women. Finally, they monitored biological maintenance by looking at markers of tissue damage and oxidative stress. Oxidative stress happens when there is an imbalance between harmful free radicals and the antioxidants that neutralize them.

To track this physical maintenance, they analyzed myoglobin, a protein released into the blood when muscle is damaged. They looked at this alongside other markers that indicate cellular damage and the breakdown of cartilage.

The data provides evidence that participation in these extreme events caused massive energetic stress. Athletes experienced a significant drop in body weight and fat mass, alongside sharp spikes in the stress hormone cortisol.

Faced with this extreme energy deficit, the athletes’ bodies made clear biological trade-offs. Biomarkers related to immune defense generally increased or remained stable, particularly in the male athletes. This suggests the body dedicated its dwindling energy reserves to keeping the immune system active and ready to fight off potential infections.

At the same time, systems related to storage, reproduction, and maintenance were heavily suppressed. Both men and women saw massive drops in leptin and fat mass, showing that stored energy was rapidly burned. Markers of reproductive investment also declined, with male athletes showing significant drops in testosterone.

The body also appeared to sacrifice physical maintenance during the events. Levels of myoglobin and other markers of cellular stress skyrocketed. This indicates that the body was accumulating structural damage to muscles and cartilage but lacked the spare energy required to properly repair those tissues at the time.

“I was genuinely surprised by how consistent the pattern was across such diverse conditions – male and female athletes, runners and ocean rowers, races in the Arctic and the Amazon jungle,” Longman told PsyPost. “Despite these dramatically different challenges, we saw the same fundamental pattern: immune function was broadly protected or enhanced while reproductive hormones and energy stores declined.”

According to Longman, the strength and consistency of this effect suggests these biological trade-offs are deeply embedded in human physiology.

“Our bodies are remarkably adaptive, but that adaptability involves trade-offs. When energy is scarce – whether from illness, food insecurity, extreme exercise, or other stressors – your body doesn’t shut down randomly. It makes strategic choices, prioritizing immediate survival functions like immune defense while temporarily downregulating less urgent systems like reproduction and some repair processes.”

“This has practical implications: it helps explain why athletes training too hard get injured more often, why chronic stress may affect fertility and why people fighting infections may 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. Ultra-endurance athletes are highly trained individuals, meaning their physical conditioning might protect them from some of the negative consequences of severe energy loss. Average individuals might show different biological responses under similar stress.

“We’re not suggesting that ultra-endurance exercise is ‘natural’ or that our ancestors routinely engaged in similar activities,” Longman noted. “Rather, we’re using these extreme conditions as a tool to reveal trade-offs that would otherwise be difficult to detect. It’s also important to understand that many of the biomarkers we measured serve multiple physiological roles, so while we’ve categorized them into life history functions, the reality is more complex.”

“Finally, these are short-term responses measured over days to weeks – we don’t yet know whether different patterns might emerge during chronic, long-term energetic stress, or how these trade-offs might vary across different populations and life stages.”

To expand on this, the scientists plan to examine how these biological choices shift across different demographics and recovery periods.

“Important questions remain about how these trade-off patterns may vary across the human lifespan and in different populations,” Longman said. “Do children, older adults, pregnant women, or people from different ancestral backgrounds show the same prioritization hierarchy? What happens during recovery – once energy becomes available again, in what order do these functions bounce back? Can we identify individual differences that predict who is most vulnerable to these trade-offs under stress?”

“From a practical standpoint, key questions include: How can we work with sports scientists to use this knowledge to optimize training programs that minimize injury risk and health consequences? How might understanding these trade-offs inform clinical approaches for patients experiencing physiological stress from illness or treatment? More broadly, this research contributes to evolutionary public health – using our understanding of human evolutionary biology to design better health interventions. If we understand how the body naturally allocates resources under stress, can we work with those patterns rather than against them?”

Ultimately, the project highlights the benefits of merging different scientific fields to tackle modern health issues. “This study represents a genuine collaboration between evolutionary biology, sports science and public health,” Longman said. “It demonstrates that evolutionary perspectives aren’t just about our ancient past – they offer practical insights into modern health challenges.”

“I’m particularly excited about the potential applications: helping athletes train more safely, understanding why food insecurity has such wide-ranging health effects and informing clinical approaches to patients experiencing physiological stress from illness or treatment. I also want to acknowledge the athletes who participated – they endured blood draws and testing at the most physically demanding moments of their lives to contribute to science. Their commitment made this research possible.”

“Importantly, I’d also like to stress 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), whose expertise in human evolutionary biology and life history theory has been instrumental in shaping this work.”

The study, “Experimental evidence of life history trade-offs 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. Udquim, Michael P. Muehlenbein, Jonathan C. K. Wells, and Jay T. Stock.