Over a long mountain day your body burns carbohydrate, water, and sodium at predictable rates. Hitting roughly 60 to 90 g of carbohydrate per hour, 400 to 800 ml of fluid per hour, and 500 to 1000 mg of sodium per litre keeps the engine running and the brain sharp. Almost every story of bonking on a summit day is a fueling story, not a fitness story. Use the Endurance Fueling Calculator to turn these ranges into your own per-hour numbers from weight, duration, and conditions.
Why mountaineers underfuel more than other endurance athletes
Marathoners eat for two and a half hours of effort in mild conditions. Mountain athletes eat for eight or twelve hours in cold, at altitude, with appetite suppressed and hands gloved. Three forces conspire against fueling on a mountain day. Cold blunts thirst, so you do not feel like drinking even as you sweat through layers on the way up. Altitude suppresses appetite: energy intake drops measurably above roughly 3,500m (11,500 ft), often by 40 percent or more on extended stays (Westerterp et al., 2000), and the suppression deepens with elevation. And the long approach feels easy, so people skip the early calories on the rationale that they are not working hard yet, then pay for it on the steep middle and the descent.
The cost rarely shows up in the legs. It shows up in decision-making on the descent, when liver glycogen is empty, blood sugar is low, and the brain is rationing glucose under stress hormones (Jeukendrup, 2014). The two pillars below set the per-hour numbers; the third pillar is timing.
Carbohydrate is the engine, not optional
Your brain runs on glucose and your working muscles burn a steady share of carbohydrate alongside fat at any intensity above an easy walk. Liver glycogen, the buffer that keeps blood glucose steady, empties in roughly 90 to 120 minutes of moderate effort if you do not eat. From that point onwards the body protects blood glucose by releasing cortisol and adrenaline, which is felt as a heavy head, low mood, and the first signs of the "bonk."
The per-hour targets are well established in the endurance literature (Jeukendrup, 2014; Burke et al., 2018). Under an hour, you usually need nothing. From one to two hours, around 30 g of carbohydrate per hour keeps blood glucose steady. From two to three hours, the target rises to about 60 g per hour, which is roughly the ceiling your gut can absorb from glucose alone. Past three hours, or at higher intensity, you can go toward 80 to 90 g per hour, but only with a multiple-transportable carbohydrate source: a glucose-and-fructose blend (typically a 2:1 ratio), because the two sugars use different intestinal transporters and let you take in more than glucose by itself can carry across. This is why modern endurance drink mixes are not pure dextrose.
Fluid is less than you think, but constant
The old advice to "drink until you cannot drink any more" caused hyponatremia deaths and has been abandoned. Modern guidance starts from a per-hour fluid range and tunes it from individual sweat rate and conditions. A typical starting band is 400 to 800 ml per hour, lower in the cold and higher in heat or at hard effort, then scaled to body weight. The accurate way to personalise this is to measure your own sweat rate: weigh yourself before and after a hard one-hour training session in similar conditions. Every kilogram lost is roughly one litre of fluid you did not replace. The target is to finish a long mountain day no more than about two percent of body weight down (Sawka et al., 2007).
In the cold, thirst is a lagging indicator. By the time you feel thirsty on a winter ridge, you are already meaningfully dehydrated (Kenefick et al., 2018). The defense is to drink to a schedule, not to thirst, and to insulate bottles or use a thermos so the water stays drinkable when you need it.
Sodium is the variable everyone forgets
Sodium is usually framed as a cramp story, but the more important role is gut absorption. Sodium co-transports with both fluid and glucose across the intestinal wall: a drink with sodium is absorbed faster than plain water, and a drink with sodium plus glucose is absorbed faster still. This is the principle behind oral rehydration solutions and behind every well-formulated sports drink.
A practical target is 500 to 700 mg of sodium per litre of fluid as a baseline, rising toward 800 to 1000 mg per litre if you are a heavy or salty sweater, the kind whose kit shows visible white salt stains after a hard day. Most "electrolyte" tablets sold for hiking are well under 500 mg per litre when made up at the recommended concentration, which is why plain water with a token mineral mix often does not feel like enough on a long, hot day.
Train your gut, then run the calculator
Gut tolerance for in-effort carbohydrate is trainable, like any other system (Costa et al., 2017). If you have never eaten 60 g per hour during a session, do not try it for the first time on a summit day. Start at 30 g per hour on long training days, build to 60 g over a few weeks, and only push toward 80 to 90 g per hour after several sessions at 60 have felt clean. Test every food and drink in training. The first rule of summit-day fueling is "nothing new on race day."
Once you have your weight, your typical sweat rate, and a target duration for the day, the Endurance Fueling Calculator turns the ranges in this article into your own per-hour numbers, including the pre-load meal target and the recovery target for the first hour after.
The athletes who hold judgement together on the descent are not the toughest. They are the ones who fueled the engine while it was still running, on the schedule they set before the alarm went off.
Build the engine, then feed it
Train to Mountain builds a personalised plan around your peak that grows aerobic durability and the gut tolerance long days demand. Fueling is the layer on top of the engine: the plan tells you what to train, the calculator tells you how to fuel that training and the day itself. See the mountaineering training pillar for the full method, or test your readiness with the Summit Readiness Calculator.
Common questions
Does altitude actually suppress appetite?
Yes, and the effect is well documented. Energy intake drops sharply above roughly 3,500m (11,500 ft) even when food is freely available, and the drop deepens with elevation. Westerterp and colleagues (2000) measured energy deficits of 40 percent and more on extended high-altitude stays. The mechanism is partly hormonal, partly the sensation of mild nausea that hypoxia produces. The functional consequence is that on a summit day at altitude you have to plan your fueling, because you will not feel like eating at exactly the moment you most need to.
How is fueling a single-push summit day different from a multi-day expedition?
On a single push you fuel for the work in front of you and recover at the bottom. The per-hour numbers in the calculator are designed for this case. On a multi-day expedition you have a second problem: chronic energy deficit accumulates across days, especially at altitude. Climbers routinely lose 5 to 10 percent of body weight on multi-week trips. The defense is to push calorie-dense, palatable food at camp (not just summit-day fueling) and to monitor body weight as one of your acclimatisation signals.
Should I cut carbs in the base period to train my body to burn fat?
Low-carb training can raise fat oxidation rates at low intensities, but it does not remove your need for carbohydrate when intensity rises or the day stretches past three hours. Critically, your brain cannot oxidise fat: it needs glucose, and judgement is what gets you down the mountain safely. Most current endurance nutrition guidance (Burke et al., 2018) recommends periodised fueling rather than chronic restriction: lower carbohydrate availability for some easy aerobic sessions, full availability for hard sessions and for any session that simulates summit-day duration.
Why do I feel sick when I try to eat at altitude or in heat?
Both altitude and heat shunt blood away from the gut to the skin and working muscles, which slows gastric emptying and digestion. Solid food, fat, and concentrated sugars sit longer and feel heavy. The fix is to switch the form, not the amount: shift toward liquid carbohydrate (drink mix at 6 to 8 percent concentration), gels with water chasers, and small, frequent, easy-to-chew foods like dates, candied ginger, or rice cakes. Train this strategy in heat and at altitude before you need it on a real day.
How do I fuel when I cannot carry a lot of liquid?
Use higher-concentration drink mix in a smaller bottle (8 to 10 percent carbohydrate by weight, around 80 to 100 g per litre) and chase it with water from cached or melted-snow sources. Carry sodium electrolyte tablets to add to the cached water. Carbohydrate gels are calorie-dense and travel well but need water to absorb cleanly, so do not run them without fluid. On routes where water is genuinely scarce, drop overall carbohydrate target slightly and accept that you will rebuild glycogen aggressively at the bottom, not during.