The Kili Bonk: Calorie Depletion on Kilimanjaro
Recognising glycogen depletion, the signs, and what our guides do when a climber hits the wall.
Summit night on Kilimanjaro begins at midnight. By 4 or 5 in the morning, some climbers are moving with purpose and clarity. Others are stumbling, nauseous, and cannot understand why their legs have stopped working — when they felt strong an hour earlier. The difference is often not fitness. It is fuel.
The bonk — glycogen depletion — is one of the most underreported reasons climbers turn back from Stella Point or Uhuru Peak. Not altitude sickness. Not injury. Simply: the body ran out of carbohydrate to burn, and the brain started shutting down non-essential functions to stay alive. This is what it looks like on Kilimanjaro, how to recognise it, and exactly what our guides do when it happens.
What Is the Kili Bonk?
Glycogen is the storage form of carbohydrate — your muscles and liver hold roughly 1,500–2,000 kcal of it at any time. During sustained effort, your body breaks it down to glucose, which enters the bloodstream and fuels working muscles and, critically, the brain.
At sea level, a 70 kg person burns approximately 400–500 kcal per hour during vigorous hiking. At 5,000m on Kilimanjaro, that rate climbs to 600–800 kcal per hour. Your summit push from Barafu Camp to Uhuru Peak takes 6–10 hours. The arithmetic is simple: you are burning through your entire glycogen reserve before you reach the crater rim.
Three factors make Kilimanjaro uniquely likely to trigger glycogen depletion:
- Cold suppresses digestion: Blood is redirected from the gut to peripheral muscles and skin for thermoregulation. The digestive system effectively shuts down as a fuel-processing organ at altitude.
- Altitude suppresses appetite: Leptin levels increase at altitude, signalling fullness. Climbers do not feel hungry even when their bodies are running at 3,000–4,500 kcal daily deficits.
- The effort is continuous: Unlike a marathon or cycling event where you can refuel, summit night is 6–10 hours of unbroken effort with no real opportunity to stop and eat a proper meal.
The Signs You Are Running Out of Fuel
The bonk does not announce itself. It comes on gradually, then suddenly. Climbers often do not recognise it in themselves — which is why your guide will be watching for these signs throughout summit night.
Subjective — how you feel
- •Sudden lightheadedness, especially when standing still
- •Fine tremor in the hands — shaky fingers
- •Excessive yawning despite adequate sleep
- •Mood drop: irritability, withdrawal, or sudden silence
- •Feeling "spaced out" or unable to focus on simple tasks
- •Legs feeling heavy in a way that does not match the gradient
Objective — what guides observe
- •Loss of coordination — stumbling where the terrain is not difficult
- •Slurred or slow speech
- •Tunnel vision at the periphery of visual field
- •Inability to maintain pole-pole pace despite encouragement
- •Sudden reluctance to continue despite no stated complaints
- •Elevated heart rate disproportionate to effort
Bonking vs. Altitude Sickness
Both can occur simultaneously, which is why recognition matters. The key distinction: bonking improves within 20 minutes of carbohydrate intake and rest. If the climber eats a glucose tablet, sits for 5 minutes, and improves — that was the bonk. Altitude sickness (AMS/HACE/HAPE) does not improve with food alone. It requires descent.
Why Carbohydrate Loading Fails on Kili
The standard endurance athlete protocol — 3 days of high-carbohydrate eating before an event — is sound for a marathon or century ride. On Kilimanjaro, it runs into three specific problems.
Cold burns through glycogen faster than temperate conditions
At -15°C, your body is spending significant energy simply maintaining core temperature. This is thermoregulation — not exercise — and it consumes glycogen at a rate that loading cannot fully offset. Your extra stores are partially earmarked for staying warm, not just climbing.
Fat adaptation claims do not apply at altitude
Some endurance athletes follow high-fat, low-carb diets and claim efficient fat oxidation during activity. At altitude, this is counterproductive. Fat metabolism requires more oxygen per unit of energy than carbohydrate — the opposite of what you need when oxygen is scarce. Altitude favours carbohydrate as a fuel precisely because it is oxygen-efficient.
Better acclimatisation means higher exertion
This is the cruel paradox of Kilimanjaro. A well-acclimatised climber — one who has taken the longer 8-day itinerary, who is sleeping better, who has fewer AMS symptoms — is typically moving faster and burning more calories than an unacclimatised climber on the same terrain. The fitter you feel, the more fuel you are consuming.
What Climbers Can Do Before Summit Night
Prevention is straightforward. It requires discipline, not willpower.
Three days before summit — glycogen supercompensation
Carbohydrate 60–70% of total intake. No hard training.
Reduce training load to zero. Increase pasta, rice, bread, potatoes, and bananas. This is not permission to eat junk food — complex carbohydrates from wholefood sources top up glycogen stores in muscle and liver without the gastrointestinal consequences of processed food at altitude.
On the mountain — scheduled snacking, not hunger-driven eating
Every 45–60 minutes on the trail. High-GI. 150–250 kcal per snack.
By the time you feel hungry at altitude, you are already in caloric deficit. Set a timer. Energy bars, glucose tablets, dried fruit, chocolate. Not in response to sensation — on a schedule. At camp, eat everything served at dinner. This is your one reliable full meal of the day.
Summit night — glucose only above 5,000m
Glucose tablets. Electrolyte solution. Warm honey tea. Nothing heavy.
Your digestive system has effectively stopped processing solid food above 5,000m. Attempting to eat a protein bar or energy gel with complex ingredients will leave you with nausea and no energy. Glucose tablets enter the bloodstream in 5–10 minutes. This is the only form of fuel that works at the crater rim.
What Guides Do When a Climber Hits the Wall
Our guides are trained to recognise and respond to the bonk as a distinct physiological event from altitude sickness — though the two can coexist. The protocol is specific, and it is drilled in guide training.
1. Immediate stop
The climber is sat down — not lying down, which makes breathing harder at altitude. Wind shelter if available. Guide gives a calm, clear instruction: "Sit. We are stopping for five minutes." No negotiation.
2. Rapid carbohydrate delivery
Guide administers glucose tablets (3–4) or honey sachets dissolved in warm water. Small sips of electrolyte solution. The goal is 30–50g of fast-acting carbohydrate immediately.
3. Warm drink
Warm sweet tea or water with honey. The warmth helps blood flow to the gut, which facilitates absorption. Cold water at high altitude is harder to absorb.
4. Reassessment at 15 minutes
If tremor improves, lightheadedness resolves, and the climber reports feeling "better," they may continue with increased snack frequency — glucose tablet every 45 minutes rather than every 90. If no improvement, the guide begins descent protocol.
5. Descent if unresolved
Inability to stand, confusion, slurred speech, or symptoms that do not resolve with carbohydrate intake — the guide calls for immediate assisted descent. No summit attempt is worth a medical emergency. Descending 200–300m often resolves symptoms entirely.
Why this differs from budget operators
Budget operators often run 1 guide to 10–12 climbers. At that ratio, a guide cannot monitor individual climbers for early bonk signs through 6–8 hours of summit night. Our ratio is 1:7 maximum. Every guide on our team has the carbohydrate protocol memorised and has managed at least one bonk incident per season. They are watching for it before you feel it.
Recovery After the Bonk
Most bonk recoveries are complete within 24 hours — assuming the climber descends and rests properly. The body restores glycogen through food and rest within 24–48 hours. There is no lasting metabolic damage from a single bonk episode if managed correctly.
What to do in the 48 hours after:
- Eat on schedule, not on hunger: Appetite will return before glycogen stores are fully restored. Continue carbohydrate-dominant eating even if you do not feel hungry — 3,000+ kcal per day, spread across 5–6 eating occasions.
- Prioritise sleep: Glycogen restoration is fastest during sleep. Prioritise 8–9 hours per night. Descending to lower altitude (Moshi or Arusha) immediately after the climb helps sleep quality significantly.
- Watch for prolonged fatigue beyond day 3 post-climb: If you are still experiencing significant fatigue, brain fog, or inability to exercise at day 3–4 post-climb, see a physician. This is not normal recovery and may indicate ongoing altitude-related inflammation or another issue.
- Post-climb refeeding: The day after descent, eat high-glycaemic carbohydrates: white rice, bread, potatoes, fruit juice, honey. These restore liver glycogen fastest. You do not need to restrict calories on the day after descent — your body will use everything.
Fuel for the Summit.
Ask our team about the pre-departure nutrition briefing included in every booking. We cover summit night eating, snack schedules, and what to bring from home.
Frequently Asked Questions
What is the Kili bonk on Kilimanjaro?
The Kili bonk is glycogen depletion — when your body runs out of its primary fuel source (stored carbohydrate) during a climb. On Kilimanjaro it is triggered by the combination of 6–10 hours of continuous effort at altitude, suppressed appetite reducing intake, and a basal metabolic rate 15–25% above baseline at 3,500m+. The result is sudden physical and cognitive deterioration that can come on within minutes.
How does altitude suppress appetite on Kilimanjaro?
Altitude suppresses appetite through two mechanisms. First, the hormone leptin increases at altitude, signalling fullness. Second, cold itself suppresses digestion — blood is redirected from the gut to peripheral muscles and skin for thermoregulation. The result is that climbers genuinely do not feel hungry even when their bodies are burning through fuel at 3,000–4,500 kcal per day.
What should I eat during a Kilimanjaro climb to prevent bonking?
Carbohydrate-dominant eating with scheduled snacking every 45–60 minutes on the trail — not in response to hunger. On summit night, simple sugars only: glucose tablets, electrolyte solutions, honey in warm tea. Attempting to eat heavy food above 5,000m will leave you with nausea and no usable energy. Three days before summit, increase carbohydrate intake to 60–70% of total calories to maximise glycogen stores.
How do I know if I am bonking on Kilimanjaro?
Early signs: trembling hands, lightheadedness, sudden yawning, mood drop or irritability, excessive fatigue that does not match your pace. Later signs: loss of coordination, slurred speech, tunnel vision at the margins, inability to maintain pace. Distinguish from altitude sickness: bonking improves within 20 minutes of carbohydrate intake and rest. Altitude sickness does not improve with food alone.
What do guides do when a climber hits the wall on Kilimanjaro?
The protocol is: immediate 5-minute rest stop, high-GI food (glucose tablets, honey, sweet tea), warm drink, and reassessment. If symptoms resolve, the climber continues with increased snack frequency. If symptoms do not resolve or worsen — confusion, loss of coordination, inability to stand — the guide initiates descent immediately. No summit attempt is worth a medical emergency.