Most people who feel wrecked after a big hike or climb were not unfit. They were under-fuelled. A four-hour mountain effort can burn well over 2,000 calories, yet a banana, a handful of nuts and a single gel cover only a fraction of it. When your liver glycogen runs out, your body protects blood sugar for your brain by releasing stress hormones, and that is what turns up the next day as a heavy head and flat mood. This calculator turns a few simple details about your session into clear per-hour targets, so you can pack the right fuel before you leave.
How the numbers are worked out
Carbohydrate is set mainly by how long you are out. Up to an hour you usually need nothing. From one to two hours, around 30 g per hour keeps blood glucose steady. From two to three hours, the target rises to about 60 g per hour, which is roughly the most your gut can absorb from glucose alone. Past three hours, or at hard intensity, you can go toward 80 to 90 g per hour, but only with a glucose-and-fructose mix, because the two sugars use different transporters and let you take in more than glucose by itself.
Fluid starts from a per-hour range that rises with heat and effort and scales with your body weight, then sodium is set per litre of that fluid, higher if you are a salty sweater. The most accurate approach is to measure your own sweat rate by weighing yourself before and after a session, but these defaults are a sound place to begin. The calculator also gives a pre-load meal target for two to three hours before, and a recovery target of carbohydrate plus protein for the first hour after.
Fuelling for the mountain
The single biggest mistake is waiting until you feel empty. By then your liver glycogen is already low and the stress response has started. Start fuelling in the first 30 to 45 minutes and keep it steady, like clockwork, rather than reacting to how you feel. Keep fat and large amounts of protein small while you move, since they slow digestion and delay the carbohydrate you actually need.
Once you know the climbing your objective demands, you can build the engine to match it. A peak page in the Train for a Peak hub lists the summit-day vertical, the StairMaster calculator and treadmill incline calculator help you train that vertical indoors, and the science behind TTM explains how the whole plan adapts to the work you actually do.
Common questions
How many carbs per hour should I eat on a long hike or climb?
For one to two hours, aim for about 30 g of carbohydrate per hour. From two to three hours, move to around 60 g per hour, the ceiling your gut can absorb from glucose alone. Beyond three hours, or at race intensity, you can push toward 80 to 90 g per hour, but only with a drink or gel that combines glucose and fructose, because the two sugars use separate transporters and let you absorb more than glucose by itself. Under an hour you generally do not need to eat at all.
Why do I crash or get brain fog the day after a long workout?
The most common cause is under-fuelling. When you take in far fewer carbohydrates than you burn, your liver glycogen runs low and your body protects blood glucose for the brain by releasing stress hormones like cortisol and adrenaline. That surge, often combined with mild dehydration, shows up the next day as a heavy head, low mood and fatigue, even when your muscles feel fine. Eating 40 to 90 g of carbohydrate per hour during the effort and rehydrating fully usually removes it.
Can I just rely on fat adaptation instead of eating carbs?
Not for long or hard efforts. Even highly fat-adapted athletes still burn a meaningful share of carbohydrate, and fat is oxidised too slowly to cover high outputs on its own. Critically, your brain cannot run on fat, it needs a steady supply of glucose. Fat adaptation extends how far your stores stretch, but it does not remove the need for carbohydrate on a multi-hour mountain day.
How much should I drink per hour on a long mountain day?
A common starting range is 400 to 800 ml of fluid per hour, lower in the cold and higher in heat or at high output. The accurate way is to measure your own sweat rate by weighing yourself before and after a session: every kilogram lost is roughly one litre of fluid you did not replace. Aim to finish a long day no more than about two percent of body weight down.
How much sodium do I need in my electrolyte drink?
A practical target is about 500 to 700 mg of sodium per litre of fluid, rising toward 800 to 1000 mg per litre if you are a heavy or salty sweater with visible salt stains on your kit. Sodium is not just about cramps, it also helps you absorb both fluid and glucose in the gut, so an electrolyte mix with little or no sodium is doing only part of the job.
Should I eat nuts, bars or other fatty foods during a long effort?
Keep fat and large amounts of protein small while you are moving. They slow how quickly your stomach empties, which delays the carbohydrate you actually need from reaching your bloodstream. You already carry plenty of stored fat. During the effort, favour fast-acting carbohydrate from drinks, gels and simple foods, and save nuts, bars and richer food for before or after.
What should I eat before and after a long session?
Two to three hours before, eat a carbohydrate-rich, low-fat, low-fibre meal of roughly 1.5 to 2 g of carbohydrate per kilogram of body weight to top off liver glycogen. As soon as you finish, take in carbohydrate again, about 0.8 to 1 g per kilogram in the first hour, together with 20 to 40 g of protein, and rehydrate. A sugary drink right at the finish helps signal that the effort is over and blunts the stress response.