Eating on long runs: fuel per hour and training the gut

In an ultra, the runners who finish well aren't always the fittest. Often they're the ones who kept eating. Running for many hours uses more carbohydrate than your body can store, and a stomach that refuses food at hour six can end a race faster than tired legs. Eating while running is a skill, and like any skill it improves with practice. This guide covers how much to eat, how to train your gut to handle it, and the basics of drinking and salt.

Why you need to eat

Your muscles and liver store enough carbohydrate for roughly one and a half to two hours of steady running. After that, without food, the pace drops, effort climbs and thinking gets foggy: the famous wall. Fat stores are huge, but burning fat alone only supports a slow effort. Taking carbohydrate during a long effort keeps blood sugar steady and spares the stores, so you can keep going at the effort you planned.

How much per hour

The recommendations from sports nutrition research depend mostly on how long the effort lasts:

Why the mix? The gut absorbs glucose through one transporter, which saturates around 60 grams an hour. Fructose uses a different one. Products that combine the two (often in a ratio of about 2:1, or 1:0.8) let you absorb more in total, with less sugar sitting in your stomach.

In practice, many ultrarunners land between 50 and 80 grams an hour. Slower efforts over very long races can use a bit more fat and a bit less carbohydrate; the important thing is to keep eating steadily rather than hitting a number.

What 60 grams looks like

Read the labels: many gels hold 20 to 30 g of carbohydrate, a typical sports drink about 30 to 40 g per 500 ml, an energy bar 25 to 45 g, a banana around 25 g, a handful of boiled potatoes with salt about 20 g. Sixty grams an hour can be a gel every 30 minutes plus a little drink mix, or a bar and a gel. On long ultras, most runners need some real food too: sweet things get hard to swallow after many hours, and salty, starchy food often goes down better.

Training the gut

The gut adapts to what you feed it during exercise. Runners who regularly take carbohydrate on their long runs absorb more of it and get fewer stomach problems on race day; runners who never eat in training often struggle when they try. Treat it like any other training:

1. Start early in the season. Eat something on every run over 90 minutes, even when you don't feel you need it.
2. Build up gradually. Begin around 30 to 40 g an hour and add 10 g every week or two until you reach your race target.
3. Train it at race effort. Eating while hiking slowly is easier than while running hard; practise on long runs with race-like efforts and climbs.
4. Use race-day products. Find out what the race offers at aid stations and train with those, or plan to carry your own.
5. Note what happens. Product, amount per hour, timing, and how your stomach felt. Patterns show up quickly.

Drinking

Fluid needs vary hugely with heat, size and sweat rate, from about 300 ml an hour on a cool day to 800 ml or more in the heat. For most runners, drinking to thirst works well. The danger isn't only drinking too little: drinking much more than you sweat, especially plain water over many hours, can dilute the sodium in your blood (hyponatraemia), which is serious. Weighing yourself before and after a long run in race-like conditions shows roughly how much you lose per hour.

Salt

Sweat contains sodium, and some runners lose a lot of it (white marks on a dark cap are a clue). On long, hot efforts, taking some sodium, from drink mixes, salt capsules or salty food, helps many runners keep drinking and eating normally. Amounts vary widely between people; start modest and adjust from experience in training.

An example plan for a 100 km race

A runner expecting around 14 hours, who has trained their gut to about 70 g an hour:

• Breakfast, 3 hours before the start: a familiar, mostly carbohydrate meal (bread or oats, honey or jam, a banana), little fat and fibre.
• The first hours: a gel or half a bar every 25 to 30 minutes from the start, plus drink mix in one bottle, together about 60 to 70 g an hour.
• The middle hours: more real food at aid stations (boiled potatoes, rice, bread, soup) alongside gels; sodium from the drink mix and salty food in the heat of the day.
• The last hours: whatever still goes down. Many runners switch to liquid carbohydrate, cola or sweets late in the race.

Write the plan down by time, not by aid station: "a gel at :00 and :30 every hour" is easier to follow when tired than "eat at each aid station". And carry a little more than the plan says, in case an aid station is further than you thought.

When the stomach says no

• Slow down. At high effort the gut gets less blood. Easing the pace for 15 to 20 minutes often settles it.
• Switch texture. Liquid carbohydrate or salty food instead of another sweet gel.
• Small sips and bites. Keep something going in, even little.
• Check the heat. Cooling down (water on the head, shade) helps the stomach too.

Read next

• How many hours a week to train for a 50 km, 100 km or 100 mile race
• The taper for an ultra, week by week

Sources

1. Jeukendrup A. A step towards personalized sports nutrition: carbohydrate intake during exercise. Sports Medicine, 2014; 44(Suppl 1): S25–S33.
2. Jeukendrup AE. Training the gut for athletes. Sports Medicine, 2017; 47(Suppl 1): 101–110.