Hunger on a hill is not a moral failing and not a matter of willpower. It is arithmetic. A body walking uphill with a pack burns energy at a rate that ordinary meals cannot match, and it can only absorb a fraction of what it burns while it is moving. Understanding the two numbers, output and intake, explains why the afternoon collapse happens, and how to prevent it with a pocket full of the right things eaten at the right rhythm.
How much a walking body burns
At rest, an adult uses roughly 60 to 80 kilocalories an hour. Walking on the flat at 4 to 5 km/h with a light pack raises that to about 250 to 350 kcal/h for a 70 kg (154 lb) person. Add a climb and the number jumps: ascending at a steady 300 to 400 m per hour costs 500 to 700 kcal/h, and a heavy pack on steep ground can push a large hiker beyond 800. A reasonable working figure for a mixed mountain day, with climbs, descents and stops, is 400 to 500 kcal per hour on the move.
Multiply by a seven-hour day and the total is 3,000 to 3,500 kcal for the walk alone, on top of the 1,500 or so that the body would have used anyway. Multi-day trekkers routinely record daily expenditures of 4,000 to 5,000 kcal. Heavier people burn more; cold weather and shivering add more still. Fitness lowers the cost of a given pace only modestly; mostly it lets you go faster for the same cost.
The stored fuel and why it runs out
The body carries two fuel tanks. Fat is effectively unlimited: even a lean person stores tens of thousands of kilocalories of it. Carbohydrate, stored as glycogen in the muscles and liver, is small: about 400 to 500 g in a trained adult, which is 1,600 to 2,000 kcal. The problem is that steep climbing at a hard effort draws heavily on the carbohydrate tank, and when it empties the brain, which runs on glucose, is the first to notice. The familiar “bonk” of the mid-afternoon, with its sudden weakness, irritability and poor decisions, is glycogen depletion. It arrives after roughly three to four hours of solid effort if nothing is eaten.
At an easy, conversational pace the body burns a higher proportion of fat and spares glycogen, which is one reason a steady pace outlasts a fast one. But no pace is slow enough to avoid the need to eat on a long day.
What the gut can actually absorb
Here is the constraint that most eating advice ignores. While exercising, the intestine can absorb glucose at about 60 g per hour, which is roughly 240 kcal. Endurance research has shown that adding fructose, which uses a different transport route across the gut wall, raises the ceiling to around 90 g per hour, or 360 kcal, for people who train their gut to tolerate it. Fat and protein are absorbed far more slowly and mostly contribute after the walk is over. In other words, the maximum you can usefully take on board while moving is well below the 450 kcal an hour you are burning.
The conclusion is not despair; it is priorities. Eat carbohydrate while walking, early and steadily, to keep the small tank topped up. Let fat from the body cover the rest of the bill, and pay the remaining deficit at dinner and breakfast, when the gut has time. A one-day deficit of 1,500 kcal is normal and harmless. On a week-long trek the deficit accumulates and matters, which is why long-distance hikers lose weight and eat like horses in towns.
Little and often beats the big lunch
The timing rule that follows from the absorption limit is simple: something every 45 to 60 minutes from the first hour, roughly 150 to 250 kcal at a time. Waiting for a big lunch at hour four means walking the third hour on an emptying tank and then asking the gut to process 800 kcal while the blood is needed in the legs. Many hikers feel sluggish and slightly sick after that lunch for exactly this reason.
The practical method is to keep the hour’s snack in a hip-belt pocket or a jacket pocket, not in the pack, so that eating does not require stopping. Start eating before you feel hungry; hunger on a climb is a lagging indicator. In cold weather appetite disappears even as the cost rises, so the schedule matters more, not less.
Choosing what to carry: the density question
Every food has two numbers that matter on a hill: kilocalories per 100 g, and how easy it is to eat while breathing hard. Nuts and seeds lead the first list at 550 to 650 kcal/100 g, followed by chocolate and nut butters around 500 to 600, oatcakes and hard biscuits at about 450, dried fruit at 250 to 350, and bread, wraps and cheese in the 250 to 400 range. Fresh fruit is refreshing but only 50 to 60 kcal/100 g, which means most of its weight is water you are carrying uphill. For a full day, a target of 350 to 450 g (12 to 16 oz) of food at an average of 400 kcal/100 g gives 1,400 to 1,800 kcal, which comfortably covers the absorbable share.
- For steady walking: dried fruit, oat bars, jelly sweets, a sweet drink in the bottle – fast carbohydrate the gut handles easily.
- For breaks: nuts, cheese, salami, nut butter on a wrap – dense, slower fuel that also brings salt.
- For the emergency pocket: 300 kcal that you never touch, for the day that runs three hours late.
- For the evening: a real meal with protein, 20 to 30 g, to repair muscle for tomorrow.
Salt, protein and the day after
Sweat carries sodium out of the body at a rate that varies enormously between people, and salty snacks are a cheap and pleasant way of returning some of it; crisps and salted nuts do a job on a hot day that sweets cannot. Protein matters less during the walk than after it: a portion of 20 to 30 g within a couple of hours of finishing, and again at breakfast, is what the muscle repair literature supports. Breakfast itself should be mostly carbohydrate, eaten two to three hours before a big climb if possible, or a smaller portion an hour before if not.
None of this is complicated once the numbers are known. Burn 450 an hour, absorb 250 at best, eat something every hour, carry dense food, and settle the balance at dinner. The bonk is optional.