This is general safety information, not medical advice. Consult a doctor before travelling to altitude, before starting a new training programme, or before taking any medication. In an emergency, call the local emergency number — 112 anywhere in the EU, 911 in the US and Canada.

Food and water attract more folklore than any other mountain subject, and less of it survives contact with the sources. Everything below comes from the Institute of Medicine, the UIAA Medical Commission, the ACSM, the US water reference intakes, the Wilderness Medical Society and peer-reviewed field measurement. Nothing here is a diet, a plan or a dose. Where a famous number failed verification, we print the failure instead.

What a mountain day actually costs

The honest answer is a range, and the range is the useful part. The Institute of Medicine reports measured field values: a winter training course at "4,919 ± 190 kcal/d for the entire 11-d study"; Mount Rainier at 2,500–3,100 m at "3.2 ± 0.2 × BMR (4,558 ± 566 kcal/d)"; and, in the first four days of intense activity, "7,131 ± 225 kcal/d." That report is from 1996 and its populations military — order-of-magnitude, not a target. A 2025 doubly-labelled-water study of Arctic winter ski-touring puts the ceiling higher: "20.8 ± 4.7 MJ/day" for women and "31.1 ± 7.5 MJ/day" for men, roughly 4,970 and 7,430 kcal/day.

For an ordinary day the 2024 Adult Compendium of Physical Activities is more use. Taking 1 MET as "3.5 mL/kg/min": hiking cross-country 6.0 METs; walking normally through fields and hillsides, no load, 5.3; hiking with a daypack 7.8; climbing hills unloaded at 6–10% grade 7.0, at 11–20% grade 8.8; with a 20+ lb load at 5–20% grade 10.0. Gradient and load move the cost of a day far more than distance does.

Sources: IOM — Nutritional Needs In Cold And In High-Altitude Environments, ch. 20; PMC — Alaska Mountain Wilderness Ski Classic, Front Physiol 2025; PMC — 2024 Adult Compendium, J Sport Health Sci; Compendium of Physical Activities — walking tables.

The deficit you cannot eat your way out of

On a long trip at altitude the arithmetic stops balancing. Doubly-labelled water on five climbers found them in "negative energy balance (-5.7 +/- 1.9 MJ/day) in both situations, during the preparation in the Alps and on Mt. Everest," losing 2.2 ± 1.5 kg of which "on average two-thirds" was fat. Twenty-one days at 6,542 m produced "negative energy balance throughout the observation period", fat accounting for "74 +/- 15% of the loss of body mass".

The obvious response is to eat more, and there is a trial on that. Double-blind, placebo-controlled, 41 participants, a 21-day Himalayan expedition: the carbohydrate arm received "an additional 15,058 +/- 6211 kcal… Nevertheless, the functionally important residual mass decreased in both groups by 6%… with no effect of allocation." The conclusion — "Factors other than negative energy balance must contribute to body-composition changes with chronic hypoxia."

Eat enough carbohydrate: that is ordinary sports nutrition, and the IOM still advises a diet "poor in salt and rich in carbohydrates" at altitude. But the stronger belief, that carbohydrate loading prevents wasting, is contradicted by that trial — and the UIAA hedges too, since the assumption that carbohydrate-rich foods are preferred at the highest altitudes "is not consistent in studies".

Appetite, meanwhile, fails before willpower does, at a predictable threshold. The UIAA: "Appetite and taste perception are both suppressed at high altitude… This 'mountain anorexia' effect can result in a significant loss of body weight beginning at altitudes around 3600m for some, and at around 5000m for most (i.e. weight loss 1-2 kg/week)", with "caloric intake… reduced by a third when at around >5000m." Its companion figure — basal metabolic rate up "by 10-20% or more" — needs a hedge: the IOM presents conflicting data and concludes that "no consensus exists as to the magnitude of the effect."

The same document draws the distinction that matters in the field. Mountain anorexia is not altitude illness: "This weight loss is independent to any symptoms of acute mountain sickness (AMS) where the affected person will feel hungry, but has no desire to eat or drink due to nausea." But appetite loss and nausea are on the AMS list too, which is why not eating is a symptom to interrogate rather than tolerate — "All symptoms at altitude must be considered as altitude related until proven otherwise" is the first golden rule.

Sources: Europe PMC — Westerterp et al., J Appl Physiol 1992; Europe PMC — energy balance at 6,542 m, J Appl Physiol 1994; Europe PMC — Am J Clin Nutr 2009 randomised trial; IOM ch. 20; UIAA MedCom Consensus No. 4 — Nutrition (PDF); UIAA MedCom Consensus No. 2 — AMS/HAPE/HACE (PDF).

Hydration: the goal is not the maximum

The ACSM position stand sets an objective most trailhead advice does not: "The goal of drinking during exercise is to prevent excessive (>2% body weight loss from water deficit) dehydration and excessive changes in electrolyte balance" — and because sweat rates and sweat electrolyte content vary so much between people, "customized fluid replacement programs are recommended." Avoiding a large deficit; not maximising intake.

The US reference intakes put adequate total water intake at "3.7 L… per day" for young men and "2.7 L per day" for young women, and treat the ordinary mechanism as sufficient: "fluid intake, driven by the combination of thirst and the consumption of beverages at meals, allows maintenance of hydration status." Losses at altitude are higher and have been measured — the UIAA reports "water losses of 3.0±0.5 L/day in sedentary subjects, and more in climbers at 3.3± 0.6 L/day" on Everest.

An honesty note our sources force on us. The popular explanation for those losses — that you lose litres of water simply by breathing cold, dry air — is one we cannot put a number to. The IOM's own chapter says the magnitude "is not clear." The totals are documented; the breathing share is not, and we will not print a figure no source supports.

Sources: Europe PMC — ACSM position stand, Med Sci Sports Exerc 2007; NASEM — Dietary Reference Intakes for Water, ch. 4; UIAA MedCom Consensus No. 4 (PDF); IOM ch. 18.

The danger nobody warns you about is drinking too much

Exercise-associated hyponatraemia (EAH) "is defined by a serum or plasma sodium concentration below the normal reference range of 135 mmol·L-1 that occurs during or up to 24 h after prolonged physical activity" — in settings, the Wilderness Medical Society adds, "in which medical care is limited and often not available". The mechanism is "overconsumption of hypotonic fluids"; the symptoms are "nausea, vomiting, confusion, headache and seizures"; and it can end in "pulmonary and cerebral edema, respiratory failure and death".

It gets mistaken for something else: "Most exercise-associated hyponatremia cases at the Grand Canyon… are often misdiagnosed as exertional heat illness". A six-year study found EAH, dehydration and heat exhaustion occurring "at nearly the same rate", and in the fatal case presented blood sodium was 123 mmol/L. Whether EAH is also mistaken for AMS has been raised in the literature and is mechanically plausible — the symptom lists overlap — but the one paper on point carries no readable abstract, so we report a question raised, not a finding.

Altitude does not appear to make it worse: at the Leadville 100 (2,800–3,840 m) "Twenty percent of the post-race participants had EAH", women significantly more often than men (40% vs 16%; p=0.039), and the authors concluded that "High altitude… does not appear to enhance the incidence of EAH." The prevention statement is one sentence: "The most individualized hydration strategy… is to drink fluids when thirsty." The UIAA agrees — "Don't overdrink in single sessions."

A conflict we are not going to hide. The NPS Ten Essentials page tells hikers "you should drink water often and before you feel thirsty." We print both, because the tension is where the risk lives: forcing fluid on a schedule, past thirst, through a long day is the behaviour the hyponatraemia literature is about. If a clinician has told you otherwise, follow the clinician.

Sources: Europe PMC — WMS EAH guidelines, 2019 update; PMC — Front Med 2017; PMC — J Clin Med 2014; PMC — Grand Canyon EAH, CJASN 2024; PMC — Leadville 100, Phys Act Nutr 2021; Europe PMC — EAH masquerading as AMS, Clin J Sport Med 2008 (record only); UIAA MedCom Consensus No. 4 (PDF); NPS — 10 Essentials.

Packing food people will actually eat

The binding constraint is not calories on paper: "There is no point carrying up food that is not going to be eaten", says the UIAA. Take energy-dense foods and easily prepared carbohydrates "for self-selection at meals or that can be put in pockets for easy access while climbing"; carry spices, because taste perception is blunted; and expect waste — ration studies report that "food items that are not liked are discarded (typically 10-20%, but up to 40%)".

The NPS states the Extra Food essential — one of the ten systems — plainly: "Pack an extra day's supply of food, preferably no-cook items… like salty and easy to digest snacks." On cadence, the most concrete recommendation we found comes from the sudden-cardiac-death prevention literature: "Repeated rests with energy and fluid intake, i.e., every 30 to 60 min, during skiing and hiking activities." And one item belongs in a doctor's office rather than a shop — "Have your iron status checked by your doctor before setting out… Women and vegetarians are at special risk."

Nothing here is a meal plan, and it should not become one — the ACSM recommends customised approaches, and the UIAA sends you to a clinician for the one blood value it names. What the evidence supports is short and unglamorous: a hard day costs more than it looks, weeks at altitude run a deficit eating does not close, appetite fails before willpower does, and thirst is a better guide than a schedule.

Sources: UIAA MedCom Consensus No. 4 (PDF); NPS — 10 Essentials; PMC — Burtscher et al., Int J Environ Res Public Health 2021.

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