Bonking — the sudden loss of energy that turns a strong hiker into a shuffling wreck — is caused by glycogen depletion, the same mechanism that ends running races. Your muscles and liver together store approximately 1,500 to 2,000 kcal of glycogen; at a moderate hiking pace with a loaded pack, you burn through this in 3 to 5 hours without replacement. The fix is not willpower — it is eating 150 to 250 kcal every 45 to 60 minutes before you feel hungry.
What Is Bonking and Why Does It Happen on Trail?
Exercise physiologists call it hypoglycaemia during exercise — your blood glucose drops faster than your liver can release glycogen reserves, your muscles switch to fat oxidation (which is slower and cannot sustain high-intensity effort), and cognitive function drops alongside physical output. Hikers typically recognise bonking by three symptoms: extreme fatigue with no preceding warning, loss of concentration and difficulty with route decisions, and shaky legs on descents. A 2020 study from the University of Birmingham found that athletes who ate carbohydrate every 45 minutes maintained performance output 23% higher in the final hour of exercise than those who fuelled only at the start. Hiking is lower intensity than running but the duration is far longer — a 10-hour mountain day creates even greater cumulative glycogen demand than most running events.
How Many Calories Do You Actually Need Per Hour Hiking?
A 75 kg hiker on moderate terrain with a 10 kg pack burns approximately 450 to 600 kcal/hr. Over a full 8-hour day that is 3,600 to 4,800 kcal — far beyond what most hikers actually carry. The goal is not to replace every calorie burned (your body has fat reserves for that) but to keep blood glucose high enough to maintain muscle and cognitive function. Sports dietitians recommend consuming 150 to 300 kcal per hour of sustained hiking, prioritising easily digestible carbohydrates. Protein and fat slow gastric emptying and are best saved for breaks and camp meals. Refer to the full-day calorie guide for detailed calculations by body weight, terrain and pack weight.
What to Eat and When: The Fuelling Schedule
The most common mistake is eating a large breakfast and then waiting until lunch to eat again. By the time you sit down to lunch at hour 4, you are already in glycogen deficit and it takes 30 to 45 minutes for food to restore blood glucose — meaning the damage is done. A better strategy is the continuous fuelling approach: eat 150 to 200 kcal every 45 minutes from the first hour, regardless of hunger. Set a phone or watch timer. Foods that work best are high in simple carbohydrates with a medium to high glycaemic index — these convert to blood glucose fastest. Dates, rice cakes, energy chews and gels all work well. Reserve nuts, nut butter and cheese for stops, as the fat content makes them slower to convert.
| Food | kcal / 100 g | Glycaemic Speed | Best Use |
|---|---|---|---|
| Medjool dates | 277 | Fast (GI 55–65) | On-the-move snacking |
| Energy gels (Maurten / SiS) | 240–280 | Very fast | Hard uphills, emergencies |
| Banana | 89 | Medium (GI 45–60) | First 2 hours (fresh carry) |
| Tortilla with peanut butter | ~320 | Slow-medium | Planned breaks only |
| Mixed nuts | 607 | Very slow | Camp meals, sustained energy |
The Role of Blood Sugar, Caffeine and Sodium
Caffeine at 3 to 6 mg/kg body weight (200 to 400 mg for a 70 kg hiker) delays fatigue by blocking adenosine receptors and has a well-documented 2 to 3% improvement in endurance performance, according to a 2021 meta-analysis in the British Journal of Sports Medicine. For a long hike, taking 100 mg of caffeine (one espresso or a caffeine chew) at the 3-hour mark can meaningfully extend the useful performance window. Sodium is as important as carbohydrate for energy maintenance — sodium depletion causes cramping and reduces the rate at which carbohydrate is absorbed from the gut. Add 300 to 500 mg of sodium per litre of water in hot conditions or on days over 8 hours. See the hiking electrolytes guide for a practical electrolyte mixing guide. For packing your snack supply efficiently, the Sea to Summit Ultra-Sil Sack 4L is a 22 g stuff sack that keeps snacks organised and accessible at the top of your pack without adding meaningful weight.
Hot Breakfasts vs Cold Starts: Does It Matter?
A cooked breakfast increases your starting glycogen level significantly compared to a cold cereal bar. The Snow Peak LiteMax Titanium Stove at 56 g and the UCO Spork add minimal weight but allow a hot porridge or instant noodle breakfast that sets a higher glycogen baseline for the day. Research from the American College of Sports Medicine suggests that a 600 to 800 kcal carbohydrate-rich breakfast 2 to 3 hours before a long hike extends the time to glycogen depletion by 45 to 60 minutes compared to a 300 kcal fasted start. On full mountain days, this difference matters. For broader snacking strategy, the best hiking snacks for energy guide lists calorie density and glycaemic profile for 20 common trail foods.
Frequently Asked Questions
What does bonking feel like on a hike?
Bonking typically presents as sudden overwhelming fatigue that does not improve with a short rest, accompanied by shaky legs, difficulty concentrating on the trail ahead and sometimes mild nausea. Unlike normal tiredness which builds gradually, bonking arrives quickly and feels disproportionate to how hard you have been hiking. Eating 40 to 60 g of fast carbohydrate usually reverses symptoms within 15 to 20 minutes.
How often should I eat on a long hike to avoid bonking?
Eat 150 to 250 kcal every 45 to 60 minutes, starting from the first hour on trail. Do not wait until you feel hungry — hunger signals often lag behind actual glycogen depletion by 20 to 30 minutes. Set a timer on your watch if you tend to forget. On hikes over 6 hours, eating more frequently in the first half of the day is more effective than front-loading a big lunch.
Are energy gels good for hiking?
Energy gels work well for hard uphill sections where you cannot stop to eat solid food. For sustained moderate-pace hiking, however, real food with a medium GI (dates, rice cakes, bananas) provides more sustained glucose release than the very high GI spike from pure gel. Use gels strategically at high-intensity moments rather than as your primary fuelling strategy across a full day.
Can you bonk at moderate hiking pace?
Yes. The idea that bonking only happens to runners is incorrect. Any sustained aerobic effort lasting more than 3 hours without adequate carbohydrate replacement will eventually deplete liver glycogen. The slower pace of hiking just means depletion takes longer — typically 4 to 6 hours rather than 60 to 90 minutes as in running — but the mechanism and outcome are identical.
Does eating fat prevent bonking?
Fat cannot prevent bonking because it is oxidised too slowly to maintain blood glucose during sustained exercise. Fat is an excellent fuel for low-intensity hiking and for long-duration calorie density, but it does not substitute for carbohydrate in preventing acute glycogen depletion. The most effective approach is carbohydrate for sustained energy and fat for calorie density on multi-day trips.