Fueling a 100K Ultra: When Calorie Targets Meet Reality
How to build a 100K fueling plan that survives contact with a real race: carbohydrate targets per hour, why GAP changes your burn rate, and how to plan around aid stations instead of hoping.
Most 100K runners do not bonk because they trained too little. They bonk because their fueling plan was a vague intention - “eat something at each aid station” - that collapsed the moment their stomach turned at hour seven. A plan that survives a 100K is specific, rehearsed, and built around two numbers: how much you can absorb and how much you actually burn.
Start with absorption, not expenditure
Your limiting factor is rarely how many calories you need - it is how many your gut can take in while you run. For most trained runners that ceiling is 60-90 g of carbohydrate per hour, and reaching the top of that range requires a glucose-plus-fructose mix and months of gut training. Pick a realistic hourly carbohydrate target first, then build the race around delivering it.
The nutrition calculator turns your body weight, race duration, and target intake into an hour-by-hour carbohydrate and fluid plan you can actually carry.
Why GAP changes your burn rate
Energy expenditure on a 100K is not constant. A long climb burns far more per kilometre than a runnable descent, even at the same heart rate, because you are doing more vertical work. This is the same insight behind grade-adjusted pace: the mountain, not the distance, drives the cost.
The practical consequence: your fueling should not be evenly spread by distance. The big climbs are where your glycogen drains fastest, so they are where you most need carbohydrate already on board - which means eating before the climb, not during the suffering.
Plan around aid stations, do not hope at them
Translate your hourly target into a per-segment plan:
- Estimate your time between each aid station (use your GAP-based pacing).
- Multiply by your hourly carbohydrate target to get the grams you must carry for that segment.
- Decide what you top up at each aid station versus what you carry from the start.
A drop bag full of food you cannot stomach at hour ten is useless. Rehearse the exact products on long runs, and have a backup plan for when sweet gels stop working - real food, savoury options, or a switch to drink-mix calories.
The failure modes to design against
- Starting behind. If you under-fuel the first three hours while you feel great, you spend the rest of the race chasing a deficit you cannot repay.
- Flavour fatigue. Carry variety; a single flavour for 14 hours is how strong stomachs quit.
- Fluid and sodium drift. Carbohydrate planning fails if you are dehydrated or low on sodium, because absorption stalls. Plan fluid and electrolytes alongside calories, not as an afterthought.
Fuel the climbs, respect your absorption ceiling, and rehearse the plan until it is boring. On a 100K, a boring nutrition plan is the one that gets you to the finish.