Ask most people what "eating well" looks like on an expedition and you'll get some version of the same answer: balanced meals, plenty of water, don't let yourself get run down. That advice isn't wrong at sea level. It falls apart once you add altitude, sustained cold, extreme heat, or immersion to the picture — because each of those environments pushes your appetite and your fluid balance in a specific, and sometimes opposite, direction to what feels intuitive.
The physiology, briefly
Two systems are doing the work here, and both are far less reliable under environmental stress than most people assume. Appetite is centrally regulated and heavily influenced by oxygen levels, temperature, stress hormones and sleep — not just how much energy you've burned. Fluid balance is regulated by thirst, but thirst lags well behind actual fluid loss in several of these environments, and in others it actively encourages you to overcorrect.
The practical result is that "listen to your body" is unreliable advice in every environment covered here — just unreliable in different directions. At altitude and in the cold, your body under-reports how much fuel and fluid you need. In the heat and underwater, it can either under- or over-report, and getting that wrong has genuinely dangerous failure modes on both sides.
Every environment below distorts your appetite, your fluid balance, or both — usually in the opposite direction to what feels intuitive. The goal isn't a single "expedition diet"; it's matching intake to what your body is actually doing in that specific environment, on that specific day.
Altitude: eating against your appetite
Above roughly 2,500 m, energy expenditure climbs steeply — climbers and trekkers working at altitude commonly burn 4,500–8,000 kcal a day, driven by increased ventilation, an elevated resting metabolic rate, cold exposure and the sheer physical cost of moving through steep terrain with a pack. At the same time, hypoxia is itself a direct trigger for appetite suppression. In the first two to four days at a new altitude, caloric and protein intake can each fall by 30–40%, independent of how much food is actually available. Below about 5,000 m this usually recovers with acclimatisation; above it, appetite loss can persist for the whole trip.
The combination is brutal: high demand, low intake. Most climbers on multi-week high-altitude trips end up consuming only 30–70% of what they're burning, and the result is a sustained energy deficit that shows up as 3–15% bodyweight loss over the course of an expedition — the majority of it lean muscle mass, not fat.
- Plan for 4,500–8,000 kcal/day above 2,500 m — falling short of this is a physiological reality, not a discipline failure
- Front-load calories earlier in the day, since appetite suppression is often worst by evening
- Favour energy-dense, easy-to-eat food over "healthy" bulk — fat and simple carbohydrate density matter more than fibre here
- Appetite loss is sharpest in the first 2–4 days at a new altitude and typically eases with acclimatisation below about 5,000 m
- Ventilation and fluid losses both rise with altitude, while cold, dry air adds significant insensible loss through breathing
- Cold blunts the sensation of thirst even as your actual fluid needs increase — drink on a schedule, not on demand
- Pale, high-volume urine is a reasonable rough guide; dark or low-volume urine at altitude deserves attention, not just a shrug
- Protein intake often falls further than total calories — deliberately prioritise a protein source at every meal, not just the largest one
- Lean mass loss isn't cosmetic — it impairs strength, coordination and thermoregulation on the days you most need all three
- On multi-week high-altitude trips, protecting muscle mass deserves the same deliberate planning as summit strategy
Polar: outrunning a deficit you can't win
Polar travel — skiing or hauling a sled across snow and ice for weeks at a time — produces some of the highest sustained energy expenditures recorded in human physiology. Isotope-labelled water studies from classic polar crossings measured average burns around 7,000 kcal a day, with stretches above 11,000 kcal a day during the hardest plateau sections, against an intake of roughly 5,000 kcal a day. The deficit was large enough that both expedition members lost more than 20 kg over the crossing. Modern, better-provisioned crossings target 5,500–7,000 kcal a day of intake and still finish with meaningful weight loss — the deficit is essentially built into the activity, not a sign of poor planning.
- Fat-heavy foods — butter, cheese, olive oil, nuts, chocolate — pack the most kilocalories per gram carried, which matters when every gram is hauled by you
- Small, frequent feeding beats trying to force large meals — the body genuinely cannot absorb 8,000–10,000 kcal in two or three sittings
- Even with unlimited food available, physically eating enough to match expenditure is often the limiting factor, not appetite alone
- Dry, cold air drives substantial respiratory and insensible fluid loss even without visible sweating
- Melting snow for water costs time and fuel, which makes under-hydration an easy habit to fall into — schedule drinking stops rather than relying on thirst
- Dehydration compounds cold injury risk by reducing peripheral blood flow further than the cold already does
- Frozen food is genuinely hard and sometimes risky to eat — keep high-calorie snacks in an inner pocket against body heat so they stay chewable
- Core temperature drops fastest in the minutes after you stop moving — meals and drink stops need to be quick and sheltered from wind
Desert: managing the two-sided risk of water
Desert environments present the highest evaporative sweat losses of any environment covered here — full sun, high heat and low humidity all combine to accelerate fluid loss. The instinctive response is to drink as much plain water as possible, but this is where desert nutrition gets genuinely dangerous rather than just uncomfortable. Exercise-associated hyponatremia — a drop in blood sodium from overconsuming hypotonic fluid relative to sodium losses — is a recognised and potentially serious complication of prolonged heat exposure, and current Wilderness Medical Society heat illness guidance is explicit that salty snacks and sodium-containing drinks should be freely available alongside fluid, not treated as optional extras.
- Drink on a planned schedule through the heat of the day rather than purely on thirst, but avoid the trap of forcing large volumes of plain water
- Include a sodium-containing drink or electrolyte formulation — roughly 300–500 mg sodium per drink is a reasonable working target
- Overdrinking hypotonic fluid, not underdrinking, is the more common cause of exercise-associated hyponatremia
- Appetite commonly drops in extreme heat — salty, easy-to-eat food both replaces sodium losses and is more likely to actually get eaten
- Full sun and low humidity together produce the highest sweat rates of any environment discussed here
- Front-load eating and drinking before the day's heat peaks rather than reacting once you're already behind
- Cold desert nights swing caloric needs upward too — high-altitude and stony deserts in particular can see 20–40°C diurnal temperature ranges
Marine: the dive reflex changes the rules
Water immersion, and cold water in particular, triggers a genuine physiological reflex: peripheral blood vessels constrict and blood is shunted toward the core, which the kidneys read as volume overload and respond to by increasing urine output — a phenomenon called immersion diuresis. The practical effect is that divers can become significantly dehydrated over a day of diving without ever feeling thirsty, because the normal thirst signal isn't tracking what's actually happening to their fluid balance. Dehydration matters more here than it might on land: reduced blood volume lowers breath-hold capacity and is thought to increase the likelihood of nitrogen uptake into tissue, raising decompression sickness risk. Separately, hypoglycaemia at depth can produce symptoms — confusion, poor coordination, impaired judgement — that are easily mistaken for nitrogen narcosis, which is exactly the wrong moment for that confusion to occur.
- Eat a proper meal with complex carbohydrate before diving — diving fasted increases the risk of hypoglycaemia being mistaken for narcosis at depth
- Avoid alcohol before diving — it worsens narcosis, impairs glucose regulation, and compounds dehydration
- Pre-hydrate deliberately, since post-dive thirst won't accurately reflect what immersion diuresis has already taken
- Immersion diuresis runs even in warm water and inside a wetsuit — "I feel fine" is not a reliable hydration check between dives
- Rehydrate on a schedule across a multi-dive day rather than waiting to feel thirsty
- Motion sickness and seasickness medication both commonly suppress appetite — small, frequent, easily-digestible food usually beats forcing a full meal
- Cold-water immersion drains calories quickly even on warm-water trips — treat significant time in or near the water as a cold-exposure activity for energy planning purposes
- Galley provisioning on smaller boats is genuinely limited — plan meals around what will keep and cook well over the trip length, not just what you'd choose at home
Where the four approaches actually agree
Trust the numbers over the feeling. Thirst and appetite are unreliable in every environment covered here — they under-report at altitude and in the cold, and can mislead in either direction in heat and underwater. Scheduled intake, not sensation, is the more dependable guide in all four.
Protein and electrolytes deserve the same deliberate attention as total calories. A calorie count that ignores protein preservation at altitude or sodium balance in the heat is only telling half the story.
Plan food logistics around the environment, not just the nutrition label. Food that's nutritionally ideal but frozen solid, wilting in humidity, or unappetising after a rough crossing doesn't help anyone — practical eatability under the actual conditions matters as much as the numbers.
Some weight loss is close to unavoidable on long altitude and polar expeditions. The realistic goal is minimising it and protecting lean mass, not eliminating it outright — expecting to finish a multi-week high-altitude or polar trip at a stable bodyweight is usually the wrong benchmark.
Applying a single "eat clean, drink plenty of water" rule across every environment. In altitude and polar settings, the real risk is chronic underfuelling and lean mass loss. In desert and marine settings, the more urgent risk can be electrolyte disturbance from mismanaged fluid intake — not simply dehydration.
Building the plan for your specific trip
The figures above are a starting point, not a substitute for planning against your actual itinerary. A guided altitude trek with daily resupply has very different logistics to a self-supported polar traverse. A desert crossing with vehicle support changes the calculation compared to one where everything is carried. Trip length, group size, resupply points, and any pre-existing medical conditions — including diabetes, which changes glucose management underwater in particular — all shift the detail even where the underlying physiology stays the same.
If you're planning a specific expedition and want a nutrition and hydration strategy checked against the real itinerary, terrain and conditions rather than a generic figure, that's exactly the kind of thing worth a proper conversation before you go.
Planning nutrition for a specific trip?
Generic advice only goes so far. If you'd like a nutrition and hydration plan reviewed, or a full pre-expedition medical consultation covering gear, environmental risk and medications, get in touch. Written advice starts at $75 AUD + GST, with a 60-minute video consultation at $175 AUD + GST.
Get personalised advice → Learn more