The appeal of intermittent fasting for backpackers is real — but so are the risks. Here’s what you need to know before skipping meals in the mountains.
Food is one of the heaviest things in your pack. On a typical multi-day trip, you might carry 500–700 g of food per day — over a week, that’s 4–5 kg before you’ve added a single item of gear. So when the intermittent fasting trend crossed over into the hiking world, the logic seemed clean: eat less, carry less, move faster.
It’s a seductive idea. And like most seductive ideas on the trail, it deserves a harder look before you act on it.
THE WEIGHT CASE FOR FASTING
The maths are straightforward. Skip breakfast and carry one fewer day’s worth of food, and you’re shaving several hundred grams immediately. Compress your eating window — say, two meals instead of three — and your resupply weight drops meaningfully across a long trip. For gram-counters already dialled into base weights under 5 kg, food is often the last significant variable.
Some hikers also point to the adaptation benefits. After a period of fat adaptation, your body becomes more efficient at burning stored fat for fuel — which, in theory, reduces the rate at which you burn through your food supply. Experienced ultralight hikers who’ve spent months building this metabolic flexibility report being able to cover solid mileage on fewer calories than a carbohydrate-dependent diet would require.
So far, so appealing. Here’s where it gets complicated.
HUNGER, FATIGUE, AND THE DECISION-MAKING PROBLEM
There’s a reason military nutrition guidelines don’t include intermittent fasting protocols. Sustained physical output under cognitive load — which is exactly what a big day in the mountains demands — requires consistent fuel. When you’re hungry and tired, your brain doesn’t just feel slower. It is slower.
Research on decision-making under metabolic stress shows measurable degradation in risk assessment, route-finding, and response to unexpected situations. The technical term is impaired executive function. On the trail, it looks like this: the water crossing that should have been a clear no becomes a maybe. The weather window that’s obviously closing gets misread as fine. The navigational error that a rested, fed hiker would catch in thirty seconds takes twenty minutes to untangle.
This is not hypothetical. Trail incident reports consistently identify fatigue and poor judgement — often in the afternoon and evening, when hikers are depleted — as contributing factors in accidents. Fasting adds a second stressor on top of an already demanding physical environment. The combination is not a smart trade-off on any day where terrain, weather, or remoteness introduces genuine risk.
THRU-HIKING: PROBABLY NOT THE PLACE
If you’re on a multi-month thru-hike, fasting is even less likely to serve you well. Most thru-hikers run a chronic caloric deficit — not because they choose to, but because the energy demands of hiking 30+ km per day, six or seven days a week, consistently outpace what can practically be eaten and carried. PCT and AT hikers frequently report losing significant body mass, including lean muscle tissue, over the course of a hike. That’s the body cannibalising itself to meet energy demands.
Deliberately eating less on top of an existing deficit isn’t leaning into fat adaptation. It’s compounding an already stressful situation. It increases the risk of muscle loss, immune suppression, impaired recovery, and the kind of cumulative cognitive fog that makes the back half of a long trail significantly more dangerous than the front.
The thru-hiker’s nutrition problem is almost always eating enough, not eating less. Fasting solves the wrong problem.
WHERE IT MIGHT ACTUALLY MAKE SENSE
There is one scenario where fasting on the trail has genuine merit: rest days and zero days.
When you’re not covering miles — sitting in a trail town, pinned in your tent by weather, or deliberately resting a tired body — your energy demands drop substantially. A compressed eating window on a zero day isn’t depriving a working body of fuel. It’s simply aligning intake with lower output. You save some weight on the next resupply. You give your digestive system a lighter day. And since you’re not navigating technical terrain or making critical route decisions, the cognitive costs are minimal.
Some hikers also use the morning of a planned shorter, easier day — a flat trail day with no exposed sections and a reliable camp destination — to test a lighter breakfast or delayed first meal. Low stakes, low risk, meaningful data about how your body responds.
But this is calibration, not a system. It works because it’s applied selectively, in conditions where the downside is limited. It is not a strategy for a ridge crossing, a river ford, or any day where the terrain demands your full capacity.
THE HONEST BOTTOM LINE
The goal of reducing pack weight is sound. The principle of carrying exactly what you need — no more, no less — is one of the smarter frameworks in backpacking. Food, like shelter and insulation, is weight that earns its place. It is not dead weight to be optimised away.
Fasting on rest days? Reasonable. Fasting on big mountain days? A meaningful risk with limited upside. Fasting on a thru-hike where you’re already in deficit? Almost certainly counterproductive.
If you want to cut pack weight through your food strategy, the smarter levers are caloric density (choosing foods that deliver more energy per gram), better resupply planning (not overpacking for each section), and dialling in exactly what you eat versus what you carry out as waste. These approaches reduce weight without reducing the fuel your body and brain need to perform and stay safe.
Pack smart. Eat enough. Go further.
Train hard. Fuel right. Pack smart. Go further.
References and Further Reading
- Lieberman HR, et al. Effects of caffeine, sleep loss, and stress on cognitive performance. Psychopharmacology. 2002.
- Hengist A, et al. Intermittent fasting and exercise performance. Journal of Science and Medicine in Sport. 2021.
- Valenzuela PL, et al. Physiological Adaptations to a Long-Distance Thru-Hike. IJERPH. 2021.
- Ainsworth G. Effects of Pack Weight on Endurance of Long-Distance Hikers. Embry-Riddle Aeronautical University, Thesis.
- Schimelpfenig T, et al. Wilderness Medicine on Long-Distance Trails. Wilderness and Environmental Medicine. 2009.
- Slower Hiking. How to Cut Food Weight Without Cutting Calories. slowerhiking.com