How to Fuel a 50K or 100K Race

Why fueling an ultra is different from a marathon

In a marathon, many runners can get away with gels and not much else. In an ultra, the same approach will eventually fail you. The longer the distance, the more your body needs real, digestible food, and the more your gut becomes a limiting factor if you haven't trained it.

I learned this the hard way in my earlier ultras. I treated a 50km like a long marathon, carried a handful of gels, and spent the final 15km running on empty with a stomach that had stopped cooperating entirely. What I didn't understand at the time was that the gut shuts down under sustained stress if you haven't trained it to stay open. Once it does, it doesn't matter how many gels you're carrying. You can't absorb them. Learning to fuel properly, starting early, using real food, and training the gut in training runs rather than on race day, changed everything for me and is now one of the things I spend the most time on with the athletes I coach.

How many calories do you actually need?

The general recommendation for ultra running is to aim to replace roughly 200-300 calories per hour, though individual needs vary. The goal is not to replace everything you burn (that's impossible) but to stay ahead of the deficit enough to maintain energy and avoid a complete breakdown.

I don't give athletes rigid calorie targets because chasing numbers in a race tends to cause more anxiety than it solves. What I do give them is a structure: eat something every 30-45 minutes, and always eat at aid stations regardless of whether you feel hungry. Hunger is a lagging indicator in ultra running. By the time you feel it, you're already behind. I also factor in conditions. A hot race at altitude demands a different approach to a cool, flat course. We work out a plan together in training and refine it based on what the athlete can actually stomach at effort.

Real food vs sports nutrition

There is no single right answer. Some athletes thrive on gels and chews. Others can't stomach them after hour four. The key is to know what works for you before race day, not discover it mid-race.

The case for real food

Real food becomes more important the longer you're out there. After three or four hours, many athletes start to find gels nauseating, and the psychological lift of eating actual food, a banana, some rice cakes, a piece of potato with salt, is real and significant. Personally, I rely heavily on rice cakes, peanut butter sandwiches, and whatever looks good at aid stations on longer efforts. Real food is slower to digest but it keeps you fuller for longer and tends to be more palatable deep into a race when flavour fatigue with sweet, synthetic products kicks in.

The case for sports nutrition

Gels and chews have a clear role, particularly in the first half of a race when your gut is still functioning well and you need fast, portable fuel. They're easy to carry, easy to dose, and don't require much thought. I use them in races and recommend them to athletes, especially during sections where stopping to eat real food isn't practical, like a long climb or a technical descent. The key is not to rely on them exclusively for anything over five or six hours.

Aid station strategy

Aid stations are a gift and a trap at the same time. The gift is real food, fluids, and a mental reset. The trap is that it's very easy to sit down and lose ten minutes when two would have been enough. My advice: know what you're stopping for before you arrive. Walk in, fill your vest, grab the food you planned to eat, and keep moving. If you need to sit down, set a time limit and stick to it. Coke is almost always at aid stations and I recommend taking it whenever it's available, particularly in the second half. The sugar, caffeine, and fizz combination is genuinely effective when you're in a low patch.

Gut training: practising your race nutrition

Your gut is trainable. Athletes who eat and drink during training teach their bodies to absorb fuel efficiently while running. Those who don't often suffer for it on race day.

From early in a training block I ask athletes to start eating and drinking during their long runs, even when they don't feel like it. The goal isn't the calories at that point, it's teaching the gut to stay open under effort. We build from there, gradually increasing what they're taking in and getting more specific as race day approaches. Athletes who struggled with GI issues, bloating, nausea, or an inability to eat past a certain point in training almost always improved significantly once they committed to gut training consistently. The gut adapts. It just needs the stimulus.

Hydration and electrolytes

Dehydration is obvious. But overhydration (hyponatremia) is a real and underappreciated risk in ultras, particularly on hot days when athletes drink too much plain water. Electrolyte management matters as much as fluid intake.

My basic guidance is to drink to thirst and always have electrolytes alongside plain water, especially in races over three or four hours. On hot days or at altitude I'll ask athletes to be more proactive with sodium. A good electrolyte tab or salt capsule every hour in warm conditions can make a significant difference. For a 50K the approach is relatively simple: carry a 500-750ml soft flask, drink when thirsty, take electrolytes with it. For a 100K, particularly one that extends into the night when you're less likely to feel thirsty, I ask athletes to set a reminder to drink regularly regardless of how they're feeling.

Caffeine strategy

Caffeine is one of the most well-researched performance enhancers in endurance sport. In ultras, timing is everything. Use it too early and you blunt the effect when you need it most.

For a 50K I'll usually suggest saving caffeine for the second half, from around 30km onwards, where it can help sharpen focus and push through fatigue. For a 100K, particularly one that goes into the night, caffeine becomes a more deliberate tool. I ask athletes to reduce their daily caffeine intake in the week before a race to reset their tolerance, then use it strategically in the race itself: a caffeinated gel or a strong coffee at an aid station around the point where the night is deepest and fatigue is peaking. The goal is to have something in reserve when you genuinely need it, not to chase a stimulant high that fades before the finish.

A sample race day fueling plan

Every athlete is different and individual plans should be built around your specific tolerances, the race conditions, and what you've practised in training. That said, here is a practical starting framework:

For a 50K (approximately 5-8 hours): Eat something in the 90 minutes before the start. In the race, aim for 200-250 calories per hour: a mix of gels in the first half and real food from aid stations in the second. Drink to thirst with electrolytes throughout. Take caffeine from 30km onwards if needed. Carry two to three gels in your vest plus a small amount of real food for the sections between aid stations.

For a 100K (approximately 10-16 hours): Same approach to the 50K but scaled up significantly. Plan your aid station stops in advance and know exactly what you'll take at each one. Have a dedicated real food strategy from the halfway point. Build in a caffeine plan for the back half, particularly if the race extends into darkness. Accept that appetite will drop and eat anyway. Salty food becomes more appealing than sweet food late in the race, so plan for this.

The most important thing in both cases is not leaving nutrition to chance. Write it down, rehearse it in training, and trust the plan even when you don't feel like eating.