Trail Running Nutrition: How Many Carbs Per Hour Do You Need?

Nutrition failures end more trail races than injuries do — and almost all of them are preventable with a plan made before the start line, not decisions made at kilometer 40 with an empty tank. This guide covers the actual numbers: how many carbs per hour you need, how that scales with your body weight, how much water and sodium to pair with it, and why most runners who “bonk” are simply under-fueling, not doing anything more complicated than that. Nutrition is one part of a full trail running training plan — it needs to be trained, not improvised on race day.

Why Glycogen Runs Out Faster Than Most Runners Think

Your body’s glycogen stores — the fuel your muscles pull from during sustained effort — run out after roughly 60 to 90 minutes of continuous exercise. This is true for every trail runner, at every fitness level, on every race longer than about 90 minutes. It’s not a sign of poor fitness; it’s basic exercise physiology that applies whether your finish time is 3 hours or 15.

The practical implication: fueling isn’t something you start once you feel hungry or tired. By the time hunger or fatigue shows up, you’re already running on a deficit you can’t fully recover from mid-race. Eating on a schedule, not by feel, is the single highest-leverage nutrition habit in the sport.

How Many Carbs Per Hour You Actually Need

The widely supported range for endurance efforts over roughly 2–3 hours is 60–90 grams of carbohydrate per hour, confirmed across sports nutrition research (see the review in Nutrition for Ultramarathon Running: Trail, Track, and Road, PubMe and echoed consistently across coaching resources in the sport.

That range scales with body size. A practical formula: estimate your hourly calorie burn at roughly 700 kcal/hour for a 70kg runner, scaled proportionally to your own weight, then take about 40% of that from carbohydrate sources (carbohydrate providing 4 calories per gram):

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calories/hour ≈ 700 × (your weight in kg / 70)

carbs/hour ≈ min(90, calories/hour × 0.4 / 4)

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The 90g/hour figure isn’t arbitrary — it’s the generally accepted ceiling for carbohydrate absorption even when using a mixed glucose-and-fructose source (which allows for faster gut absorption than a single sugar type alone). Going above it doesn’t reliably deliver more usable fuel; it mostly increases the risk of GI distress.

In gel terms, since that’s how most runners actually fuel: one standard energy gel is roughly 20–22g of carbohydrate. At a 70g/hour target, that’s about 3 gels an hour — one roughly every 20 minutes, not one “whenever it feels like time.”

The Most Common Mistake: Under-Fueling, Not Over-Fueling

Despite the well-documented 60–90g/hour target, real-world data on ultra runners consistently shows most people fall well short of it — one commonly cited figure from race-day nutrition tracking puts the average closer to 37 grams of carbohydrate per hour, roughly half of the low end of the recommended range. That gap is a large part of why “hitting the wall” happens to experienced runners, not just beginners — it’s usually a fueling math problem, not a fitness problem.

Water and Sodium — Getting the Ratios Right

Hydration needs are lower than most runners assume, and they don’t scale linearly with effort the way carbohydrate needs do. A reasonable baseline is 500 ml of water per hour, rising to roughly 600 ml/hour for efforts past the 6-hour mark. Sodium should scale proportionally with your water intake — roughly 600mg of sodium for every 750ml of fluid consumed.

This matters because muscle cramps are usually a sodium problem, not a hydration problem. Drinking more water without replacing electrolytes can actually worsen cramping by further diluting sodium concentration in the blood (a real medical condition called hyponatremia in severe cases). The fix isn’t more water — it’s electrolytes on a schedule, roughly every 30–45 minutes, regardless of how thirsty you feel.

Carb-Loading Before Race Day

The 24–48 hours before a long race matter more than most runners’ pre-race routines reflect. A reasonable carbohydrate loading target is 10–12 grams of carbohydrate per kilogram of body weight in that window — for a 70kg runner, that’s roughly 700–840g of carbohydrate spread across a day and a half, meaningfully more than a typical diet. Race morning is too late to start this process; glycogen stores take time to build.

Training Your Gut Like You Train Your Legs

The gut adapts to nutritional load the same way muscles adapt to physical load — through repeated, specific practice. Most runners never rehearse race-day nutrition until race day itself, then are surprised when their stomach shuts down at kilometer 40 on a product or timing they’ve never actually tested.

The fix is straightforward: starting in the specific-endurance phase of training (roughly 8–12 weeks before an ultra-distance race) and continuing through peak training weeks, use every long run as a nutrition rehearsal — the exact products, in the exact timing and order, that you plan to use on race day. There should be no surprises left for the actual event.