Trail & Ultra Trail Nutrition: Fuelling for Long Days

Why ultra nutrition is a different sport

A marathon runner needs to solve nutrition for 2-5 hours. An ultra runner needs to solve it for 5, 15, sometimes 30+ hours, often while climbing, descending, changing altitude, and moving through wildly different temperatures in the same day. Gastrointestinal distress, not fitness, is the single most common reason ultra runners fail to finish. That single fact should shape how you think about fuelling: the goal isn't maximum calories, it's the most calories your gut can actually process without shutting down.

The three pillars: carbohydrate, fluid, sodium

Everything in ultra nutrition comes back to these three, and to one central tension: your cardiovascular system, your thermoregulation system, and your gut are all competing for the same blood flow and the same water. Push too hard on any one input and you start starving the others.

Carbohydrate: how much, really?

The oft-repeated 60-90g/hour range comes from the body's ability to absorb roughly 1g of carbohydrate per minute from the gut, extendable toward 1.4g/min (about 84g/hour) with a trained gut and a mix of sugar sources. But ultras run at a much lower average intensity than a marathon, commonly around 60% of VO2max, which means the honest answer for most runners is: you probably don't need anywhere near the 120g/hour numbers you'll see elite cyclists and mountain runners chasing.

Fluid and sodium: personal, not universal

Sweat sodium concentration varies enormously between individuals, from roughly 200mg to 2,000mg per litre of sweat. Two runners in identical conditions can have wildly different needs. The standard 500-800mg of sodium per litre of fluid is a reasonable starting point, but it's exactly that: a starting point, not a personalized answer.

Gut training: the step most runners skip

Most runners train their legs, heart, and lungs for months before a race, then expect their stomach to handle race-day fuelling without any preparation at all. That's the equivalent of training at an easy pace for months and then racing at your 5K pace, it doesn't work, and the gut is no different.

Gut training simply means practicing your actual race-day carbohydrate intake, at the actual rate you plan to use it, during training runs long enough to matter. If your race plan calls for 75g/hour, you need long runs where you've actually consumed 75g/hour, more than once, well before race day.

Staying in the centre square

Think of hydration and sodium status as a grid with two axes: how hydrated you are, and how much sodium is in your system. The centre square, euhydrated and normonatremic, is where you want to stay. It's also more common than most runners assume: over 30% of finishers at major 100-mile races land there by simply adjusting intake sensibly as conditions change.

The practical takeaway: think ahead, not just react. If temperature or effort is about to rise, plan to carry more fluid and gradually increase sodium before you're already behind, rather than waiting for symptoms to tell you.

Real food vs. gels

Gels are convenient, precisely dosed, and easy to carry, which is why they dominate shorter efforts. But over many hours, "palate fatigue," simply not wanting another sickly-sweet gel, becomes a real problem, and it's one of the most common reasons athletes under-eat late in long ultras.

• Early miles: gels, chews, and sports drink work well while appetite is normal and simplicity matters.
• Mid-to-late race: savoury real food (potatoes, broth, quesadillas, rice balls) often goes down easier than more sugar, and aid station food exists for exactly this reason.
• Self-sufficient sections: carry a mix, not just one type of fuel. Variety keeps things palatable when your taste for sweetness inevitably fades.

Heat, altitude, and the night hours

Rising core temperature diverts blood flow to your skin for cooling, away from your gut. That slows digestion (reduced gut motility), which means food you've already eaten sits longer than normal, undigested, while you keep adding more on top. That combination, high heat plus continued eating, is the classic setup for bloating, nausea, and vomiting late in hot races.

• In the heat: increase fluid intake, but don't force more carbohydrate than your gut can process just because you're burning more calories. If anything, back off slightly and prioritize fluid and sodium.
• At altitude: appetite genuinely suppresses at elevation. Plan for smaller, more frequent intake rather than expecting normal appetite cues to guide you.
• Through the night: appetite and motivation to eat both drop. Set alarms or fixed intervals rather than relying on hunger, which often goes quiet exactly when you need calories most.

The nutrition report card

This is a simple, genuinely useful exercise borrowed directly from ultra coaching camps: on your next long run, keep every wrapper, every empty flask, everything you consume. Afterward, lay it all out and add up carbohydrate calories, milligrams of sodium, and millilitres of fluid.

Most runners are surprised by the gap between what they think they consumed and what the wrappers actually show. Do this exercise a few times across your build and you'll know your real numbers, not your assumed ones, before race day arrives.

Do you need to be "fat-adapted"?

Low-carb, high-fat diets get a lot of attention in ultra circles, built on the idea that training your body to burn more fat spares limited glycogen stores over long efforts. The research doesn't support restricting carbohydrate to get there. Athletes eating a normal, varied diet already achieve fat oxidation rates of 0.6-0.8g/minute during steady ultra-pace efforts, in the same range that dedicated ketogenic athletes work hard to reach through restriction.

In other words, at the roughly 60% VO2max intensity most ultras are run at, your body is already burning a healthy mix of fat and carbohydrate without you doing anything special. Restricting carbohydrate doesn't meaningfully improve on that, and it does reduce your ability to produce hard efforts when the race actually demands them, on a climb, at a finish, chasing a cutoff.

Common mistakes

• Racing at a higher intake rate than you've ever trained at. The single biggest predictor of GI distress. Whatever you plan to consume on race day, you should have already consumed it, more than once, in training.
• Waiting until you feel bad to eat or drink. By the time hunger or thirst is obvious, you're already behind, and it can take hours to dig out of that hole.
• Treating your bottle as both hydration and calories. Separating fluid (your bottles) from food (your pockets) gives you independent control over each, since your needs for them change independently as heat and effort shift through the day.
• Ignoring sodium until cramping starts. Cramping is a late signal, not an early one; plan sodium intake proactively based on conditions, not reactively based on symptoms.
• Assuming more carbohydrate is always better. Past the point your gut can actually process, extra carbohydrate just becomes extra risk, not extra performance.