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Professor Craig Sale, Editor-in-Chief, The Journal of Nutritional Physiology
“It is often in the most challenging real-world settings where the limits of human function are truly exposed.” Professor Craig Sale explains how extreme environments can act as natural stress tests, providing a unique and powerful lens through which to examine human physiology.
Professor Craig Sale
When human performance is pushed to the limits, whether under heavy load, in harsh climates, during survival situations, or during prolonged exertion, the physiological systems we often take for granted become visibly vulnerable. Yet, the body can be remarkably resilient, and these extreme environments can reveal some essential truths, in a way that no laboratory ever could. In this way, extreme environments provide a unique and powerful lens through which to examine human physiology.
While laboratory-based research allows for tight experimental control, it is often in the most challenging real-world settings where the limits of human function are truly exposed. I have had the good fortune of speaking with, observing and studying humans who perform in highly challenging situations, and I have long been struck by how environments that challenge the body can also clarify its priorities. From a nutritional physiology perspective, these environments can act as natural stress tests, revealing both the robustness and fragility of the systems that underpin energy balance, metabolism, health and performance.
Redefining upper limits
When working with those who operate at the edges of human performance, the lesson is consistent: as conditions become more extreme, nutrition becomes less about marginal gains and more about maintaining the fundamental capacity to function. At the edge of human performance, what and when we eat underpin not just our strength or endurance, but our decision‑making, safety, and adaptation. In recent conversations with athletes undertaking ultra-endurance adventures in extreme conditions, it is interesting to hear how what is ‘possible’ from a dietary or nutritional standpoint, often goes well beyond what traditional laboratory identified guidelines might suggest as being ‘upper limits’. Importantly, this challenges what we know and believe about nutritional physiology and, of course, in challenging environments, these changes are not mere abstractions; they are visible in real time.
One of the most consistent insights from extreme environments is the importance of energy availability. Under conditions of high physical demand, whether that be traversing a desert, climbing at altitude, or enduring prolonged cold exposure, energy expenditure increases substantially. What might be striking, however, is not simply the magnitude of this increase, but the difficulty that many individuals will have in matching it with adequate intake. Appetite suppression, logistical constraints, and gastrointestinal issues all have the potential to contribute to a sustained energy deficit, further highlighting the fact that human energy availability is not regulated solely by physiological demand, but is heavily influenced by environmental and behavioural factors. The consequences of getting this wrong also become apparent, whereby, over time, reductions in body mass, including the loss of lean tissue, as well as other physiological perturbations relating to the conservation of energy, can occur more rapidly. Extreme environments, therefore, provide a clear demonstration of how tightly energy availability is linked to overall physiological health, although, of course, it is by no means the only relevant factor.
Biochemical nuances
Environmental stress might also shift the body’s reliance upon different fuel sources in ways that are not always evident under controlled conditions; observations that clearly reinforce the concept that optimal macronutrient strategies are context dependent. It can be even more nuanced than this, since extreme environments also highlight the limitations of our capacity to utilise nutrients effectively. Gastrointestinal function is frequently compromised under stress, with reduced appetite, nausea, and altered gut motility all commonly reported.
This has significant implications for nutrient intake and absorption, which also has implications for micronutrient status and its ability to support adaptation and resilience, demonstrating that nutritional recommendations must consider not only what the body requires, but also what it can tolerate. In this sense, extreme environments remind us that nutrition is as much about practicality and behaviour as it is about biochemistry. A further example relates to hydration status; in hot environments, high sweat rates lead to substantial fluid and electrolyte losses, challenging our ability to maintain plasma volume and thermoregulation. Even here though, the situation is not straightforward; overconsumption of fluid without adequate electrolyte replacement can lead to problems. Even in the cold, hydration issues can emerge, including blunted thirst responses leading to voluntary dehydration.
How does the body respond to demanding conditions
Beyond individual nutrients, extreme environments underscore the importance of preparation and adaptation, although this is not always possible. In many cases, situations arise spontaneously and often we have no prior data to work with, particularly when an event is rare or even a one off. In any case, responses to dietary interventions are highly variable, influenced by genetics, training status, prior exposure, and behavioural factors. Extreme environments magnify this variability, making it clear that there is no universal solution.
Studying nutrition in extreme environments forces us to confront the gap between theory and practice. In controlled settings, it is relatively straightforward to prescribe optimal energy and nutrient intakes. In the extremes, however, these ideals are often unattainable. Food must be portable, palatable, and practical; feeding opportunities may be limited; and individuals must contend with fatigue, stress, and environmental discomfort. As such, the most effective nutritional strategies are often those that strike a balance between scientific principles and feasibility.
In this way, extreme environments do more than challenge human physiology, they refine our understanding of it. They reveal the importance of flexibility, both metabolic and behavioural, and highlight the central role of nutrition in supporting adaptation and performance under stress. As interest in extreme endurance events, exploration, and occupational performance continues to grow, so too does the relevance of these insights.
By examining how the body responds to the most demanding conditions, we can develop more effective, evidence-based strategies that extend beyond the extremes, informing nutritional practice in more conventional settings. Ultimately, extreme environments remind us that nutritional physiology is not a static discipline, but one that must continually adapt to the contexts in which it is applied.
Watch the discussion, ‘How to swim 1000 miles: The biology of resilience & recovery‘ to hear more about physiology and function when pushing performance to the body’s limits.
Find out more about The Journal of Nutritional Physiology and the journal’s latest call for papers.
