How to Fuel a Trail Run: Nutrition for Off-Road Running

The first time I took my road marathon fuelling plan onto a proper hill route, it fell apart within ninety minutes. I had planned around distance — a gel every five miles, roughly every 40 minutes — and then spent 25 minutes climbing a single mile of steep, loose ground. By the time I hit the top I was two feeding slots behind schedule, my legs were hollow, and the descent I had been looking forward to turned into a careful, miserable shuffle.

That is the central lesson of off-road fuelling: the trail does not care about your mile splits. Learning how to fuel a trail run means switching from a distance-based plan to a time-based one, accounting for the fact that you will be out there considerably longer than the same distance on tarmac, and building a strategy you can actually execute while picking your way over roots and rock. Here is how to do it properly.

Fuel by the Clock, Not by the Mile

Your gut absorbs carbohydrate at a fixed rate per hour regardless of how fast you are covering ground. That makes time the only sensible unit for a trail plan.

The working numbers are well established in the sports nutrition literature:

• Under 60 minutes: no fuel needed for most people. Your muscle glycogen covers it.
• 60–150 minutes: aim for 30–60g of carbohydrate per hour. At the lower end for easy efforts, the upper end for hard ones.
• Over 2.5 hours: push towards 60–90g per hour. Above roughly 60g/hr you need a mix of glucose and fructose, because glucose transport across the gut wall saturates at about 1g per minute. Fructose uses a separate transporter, so a combined source lets you absorb more without it sitting in your stomach.
• Beyond 6 hours: most runners settle around 60–80g/hr and add savoury or solid food, because palate fatigue becomes the limiting factor rather than absorption.

Now apply this to real terrain. A 20km road run at 5:00/km takes 100 minutes. The same 20km on a route with 900m of climbing might take 2 hours 40. That is not one extra feed — it is closer to three. Runners consistently under-fuel trails because they plan around the number on the map instead of the number on their watch.

Before you head out, make a realistic time estimate. A rough rule that holds up well: add roughly 10 minutes per 100m of ascent to your flat-ground time, then add more for technical or boggy ground. Divide that estimate into hourly blocks, and you have your feeding schedule.

Terrain Dictates When You Eat

This is the part road-running advice never covers. Chewing, swallowing and breathing hard all compete for the same airway, and doing it while descending a rocky path at pace is genuinely risky. You will either choke slightly, break stride, or — most commonly — decide to eat later and then forget.

The fix is to plan your feeds around the profile rather than a beeping timer. Steady climbs are the ideal eating window: you are moving slower, your breathing is rhythmic, and on steeper sections you may be power-hiking anyway, which frees up your hands and your attention. I set my watch alarm for every 20 minutes and then take the next feed at the first climb or flat section after it sounds. That rarely shifts the schedule by more than five minutes and it removes the risk entirely.

Practical consequences worth adopting:

• Eat before the climb, not at the top. There is a 15–20 minute delay between swallowing carbohydrate and it reaching working muscle. Fuel at the base and the energy arrives when the gradient bites.
• Front-load slightly. Take your first feed at 30–40 minutes rather than waiting for the hour. Getting ahead early buys you slack if a technical section swallows a feeding window.
• Pre-portion everything. Fumbling with packaging on uneven ground is how food ends up in a bush. Decant into a small resealable bag or a front vest pocket you can reach one-handed.
• Never bin a wrapper on the hill. Obvious, but worth saying: a zipped rubbish pocket is standard kit.

Hydration and Sodium Off-Road

Trails give you fewer chances to drink and far fewer chances to refill. Plan for self-sufficiency between known water points, not for the best case.

Most runners lose somewhere between 400ml and 1,200ml of sweat per hour, with heat, humidity and body size driving the variation. Drinking to thirst works well for efforts under two hours. Beyond that, a target of 500–750ml per hour is a sensible default, adjusted up in heat.

Sodium concentration in sweat varies enormously between individuals — roughly 200mg to 2,000mg per litre. If you finish a long run with white salt tidemarks on your cap or a gritty face, you are at the salty end and should be taking 500–1,000mg of sodium per hour on anything over two hours. Under-replacing sodium while drinking plenty of plain water is the classic route to feeling bloated, sloshy and flat in the back half.

One trail-specific trap: cold weather blunts thirst considerably. On a raw winter hill run you can be down a litre without ever feeling like drinking. Use the timer for fluid as well as fuel.

The Descent Problem — and Why Recovery Matters More Off-Road

Climbing costs you energy. Descending costs you muscle. Running downhill loads the quadriceps eccentrically — the muscle lengthens under tension — and that produces far more mechanical damage than flat running at the same effort. It is why your legs feel wrecked after a hilly 25km in a way they never do after a flat 30km, and why delayed onset muscle soreness peaks 24 to 48 hours later.

Two things blunt it. First, keep fuelling to the finish; running the last hour empty amplifies the damage and slows glycogen restoration afterwards. Second, take recovery nutrition seriously in the hours after. Aim for roughly 1–1.2g of carbohydrate per kg of bodyweight in the first couple of hours, alongside 20–30g of protein.

Tart cherry is worth a mention here specifically because of the eccentric loading. Montmorency cherries are dense in anthocyanins, the polyphenols that give them their deep colour, and these have been studied repeatedly for their effect on oxidative stress and inflammation after muscle-damaging exercise. The trial evidence most consistently shows reduced perceived soreness and faster return of strength — exactly the window that matters after a big descent.

What We Use at FTP Endurance Fuel

We make energy pastilles rather than gels for a reason that is particularly relevant to how to fuel a trail run: you can take a partial dose. A gel is an all-or-nothing 22–25g of carbohydrate, and on rough ground that often means a sticky mess or a skipped feed. Pastilles let you take two on a short climb and three on a longer one, spreading the same hourly total across more, smaller doses — which is generally kinder to the gut.

Our Mixed Fruit Pastilles are the ones I carry for long training runs. The flavour rotation genuinely helps on multi-hour days, when palate fatigue is the real reason people stop eating. For a change of flavour, the Tart Cherry Pastilles are tart cherry flavoured and give you the same carbohydrate on the run. They come in 1kg bulk packs, which works out considerably cheaper per gram of carbohydrate than single-serve sachets and produces a fraction of the packaging. If you want real Montmorency tart cherry, it's in Power Fluids (3g per serving).

Get the principles right and the rest follows: estimate your time honestly, fuel by the clock at 60–90g of carbohydrate per hour, take it on the climbs rather than the descents, stay on top of sodium, and treat recovery as part of the session rather than an afterthought. Then rehearse it — every long training run is a chance to test the plan before it matters. If you want to trial the pastilles before committing to a bulk pack, the taster pack is the easiest place to start.