How to fuel an ultra
Fuelling is the discipline most runners underestimate and the one that most often ends their race. This guide covers the practical framework — how much to eat, when, and why your route matters more than any generic calculator.
The central challenge
In most sports, fuelling is simple: eat before, drink during, recover after. In ultras, fuelling is continuous, terrain-dependent, and constrained by a gut that gets less cooperative as the hours pass.
The core tension is this: your body burns somewhere between 500 and 900 kcal per hour depending on pace, gradient, and body weight. Your gut can absorb at most 60–90g of carbohydrate per hour — and that ceiling drops as the race goes on. You cannot fuel by calories alone. You need a strategy built around what your gut can handle.
That strategy also changes with the terrain under your feet. Eating a gel while climbing a 20% gradient is difficult. Eating a bar on a technical rocky descent is reckless. Real fuelling is execution-aware — it accounts for where you can actually stop to eat and where you need to just keep moving.
How many carbs per hour?
The honest answer: it depends on how long you're racing and how well-trained your gut is. Here are practical working ranges, based on current endurance fuelling guidance:
| Race duration | Recommended range |
|---|---|
| Under 6 hours | 55–90 g/hr |
| 6–10 hours | 50–80 g/hr |
| 10–16 hours | 45–70 g/hr |
| 16–24 hours | 40–65 g/hr |
| 24–36 hours | 35–60 g/hr |
| 36 hours+ | 30–55 g/hr |
These are recommended ranges, not precise requirements. Your working target — the number you plan your schedule around — will sit within this range based on your experience level, gut tolerance, and race priority.
A common mistake is picking a carb target from a short race and applying it to a long one. A 70g/hr target that works perfectly in a 6-hour event will often cause gut distress in hour 14 of a 100-miler. The gut becomes more stressed as race duration increases, and the practical upper limit decreases. Plan for the duration you're actually racing.
Why terrain changes execution
You can have the right carb target and still eat at the wrong moments. Terrain is the execution layer — it determines what you can practically consume and when.
Steep and sustained climbs
Breathing is hard. Chewing is impractical. This is the best time for gels — fast to open, easy to consume, no chewing required. Fluid intake also increases because you're sweating harder. Drink mix in your bottle provides continuous carb delivery without demanding attention.
Flat and runnable sections
Effort is lower, breathing is easier, and this is where eating is most practical. Bars, chews, and real food work well here. These sections are where you can top up carbs more deliberately — not just squeeze a gel on the move.
Technical descents
Hands may be needed for balance. Concentration is on footfall. This is not the time to be opening a wrapper. A good fuelling plan schedules nothing on technical descents and instead ensures you arrive at them topped up from the section before.
The key insight is that fuelling needs to happen before difficult terrain, not during it. An experienced runner fuels proactively at the base of a big climb — not while gasping halfway up. A route-aware plan builds this into the schedule automatically.
How race duration changes your strategy
A 6-hour 50km and a 24-hour 100-miler are completely different physiological events, even if they both involve running. The fuelling strategy should reflect this.
In shorter events, gels and chews can carry the entire day. Gut tolerance is high, effort is high, and carb absorption is at its best. In longer events, food format needs to evolve. Real food becomes important not just for palatability but for gut rest — something that doesn't require the same absorption pathway as high-concentration carb products.
In very long races (20+ hours), drink mix often becomes the backbone of carb delivery because it requires no decision-making, no stopping, and no chewing. Discrete eating events — a gel here, a chew there — fill the gap. The ratio of continuous to discrete fuelling shifts as the race gets longer.
Late-race fuelling is also harder psychologically. Sweetness fatigue is real. Many runners report that gels become unpleasant after 10 hours. Planning for this means scheduling less sweet options in the second half, varying formats deliberately, and not relying on a single product to carry the entire race.
Training runs vs race day fuelling
Race day fuelling should not be the first time you try your fuelling strategy. That sounds obvious, but many runners approach race day with a plan they've never tested under load.
Training runs have a different goal than race fuelling. You're not trying to sustain maximum effort for 18 hours — you're teaching your gut to handle carb intake while running, building the habit of eating on schedule, and finding out which products you can actually tolerate when you're tired.
On training runs shorter than 5 hours, a reduced carb target (8–10g/hr lower than race-day targets) is often appropriate. The intensity is lower and the duration shorter. What matters more is the habit: eating at the right intervals, carrying the right products, opening gel wrappers with cold hands.
A fuelling practice session — a run specifically designed to simulate race-day fuelling — is different again. Here you use your full race-day carb targets to stress-test your gut, practise your rhythm, and validate your product choices. Ultra Fuel Planner supports all three session types: race day, training run, and fuelling practice.
Why route-aware planning matters
A generic fuelling calculator gives you a number: 60g of carbs per hour. It tells you nothing about when, what, or how much to carry between each checkpoint.
Your actual race has structure: a 4km climb starting at km 22, a technical descent into the valley, a long flat section to the second aid station, then a brutal final ascent. The fuelling rhythm for those sections is completely different — and a schedule that treats every hour identically isn't really a plan.
Route-aware planning uses your GPX file to understand where the climbs are, where the technical sections are, and how long each section between aid stations actually takes at your pace. It then builds a carry plan: what to pack, how much fluid to carry, how many gels to have on hand before the big climb.
This is also what makes race fuelling different from training fuelling in practice. Your training routes probably don't have the same terrain profile as your race. A plan built from your race GPX is the closest you can get to a rehearsal without actually running it.