How much fuel do you need during a marathon?

Two gels and a cup of sports drink at every other aid station. That is what fuelling looks like for many runners in their first marathon, and over four hours it comes to perhaps 15 to 20 grams of carbohydrate per hour. The recommendation for a race that long is 60 to 90.

Many runners decide their fuelling at the first aid station, although it can be worked out gram by gram beforehand. When a marathon falls apart after 30 km, fuel is often the explanation, and how to read a race like that afterwards is in the post on races that didn't go to plan.

How much you need depends on how long you run

The position statement that the American College of Sports Medicine and the North American dietetics associations published together in 2016 splits it by time. Under about 45 minutes you need no fuel during the race, and up to about an hour and a quarter small amounts are enough, even just rinsing your mouth with sports drink. So a 5K runs on breakfast. Between one and two and a half hours the recommendation is 30 to 60 grams of carbohydrate per hour, and for longer races up to 90.

For most runners the half marathon lands in the 30 to 60 gram range. Almost everyone who runs a marathon is out for more than two and a half hours, so for the marathon it is up to 90.

Why the limit sits at 60

Your gut absorbs glucose through one transporter, and it saturates at around 60 grams per hour. Fructose is absorbed through a different one. In a series of studies Asker Jeukendrup and colleagues showed that a mix of the two lets the body use 90 grams per hour or more, while glucose alone stays around 60.

In practice that means reading the packet. If it only lists maltodextrin or glucose, that product takes you up to about 60 grams an hour. To go higher you need one that also contains fructose or ordinary sugar (sucrose), which is half fructose. Many sports gels state the ratio, for example 2:1.

Work out your grams per hour

A gel contains anything from around 20 to 40 grams of carbohydrate depending on the brand, and a typical sports drink about 60 grams per litre. "A gel every half hour" therefore means 40 grams an hour with one brand and 80 with another.

Take a 3:45 marathon and aim for 60 grams an hour. That comes to around 225 grams for the whole race, for example seven 25 gram gels and the rest from the sports drink at the aid stations. Seven gels need to go somewhere, so decide beforehand whether they sit in a belt or in your pockets.

Check what the organisers hand out, and at which kilometres. If you plan to drink from the aid stations, drink the same brand on your long runs.

Your gut can be trained

Between 30 and 50 percent of endurance athletes get stomach problems when training or racing, according to a 2014 review. Much of it can be trained away. Ricardo Costa and colleagues had runners take carbohydrate during their sessions for two weeks and saw both fewer stomach problems and better results in a running test afterwards, compared with a group given a placebo.

Start on your long runs with what you tolerate today, perhaps 30 grams an hour, and increase a little at a time over a few weeks. Use the products you plan to use in the race, at race pace.

Drink when you are thirsty

Fluid is easier than fuel, because your body tells you. The international consensus conference on low blood sodium in endurance athletes concluded in 2015 that every known death from low sodium during exercise was caused by the athlete drinking too much. The advice is to drink to thirst.

Don't skip the aid stations on a warm day, but don't force down a cup at every station because you read that you should. If you take your fuel from sports drink, fluid and fuel become the same thing, and then you have to count both.

Carb loading

For races that take more than about 90 minutes, Louise Burke and colleagues recommend 10 to 12 grams of carbohydrate per kilo of body weight per day for the last 36 to 48 hours. For a 70 kilo runner that is 700 to 840 grams a day, which is much more than most people eat on a normal day. A big plate of pasta gives maybe 100 grams.

It is easier if you cut down on fibre, fat and protein on those days and fill up with white bread, rice, juice and sports drink. Your weight will usually go up a kilo or two, and that is water bound to the carbohydrate in your muscles, which is exactly what you want to bring with you. How to train through the last few weeks is in the post on tapering.

Before a 10K or a 5K you don't need to load. Normal food is enough.

Breakfast on race day

The same 2016 position statement gives 1 to 4 grams of carbohydrate per kilo one to four hours before races longer than an hour. For someone who eats three hours before and weighs 70 kilos, 2 grams per kilo is enough, so 140 grams: two large slices of white bread with honey, a banana and two glasses of juice come close.

Less fat, protein and fibre than usual, and ideally something you have eaten before a long run that went well.

Caffeine

Caffeine is one of the few supplements where the research clearly shows it helps in long races. The International Society of Sports Nutrition wrote in 2021 that 3 to 6 milligrams per kilo improves endurance by 2 to 4 percent across dozens of studies, most often taken an hour before. For 70 kilos that is 210 to 420 milligrams, roughly two to four cups of brewed coffee.

Caffeine also gives some runners stomach pain or sends them to the toilet, and that is not something you want to find out at 15 km. Try it before a long run first, in the amount you plan to take.

Fika in Coach Fartlek

Coach Fartlek is our own app, so expect bias. Elite includes Fika, the team's nutritionist. She is an AI like the coach and not a dietitian, and she tells you herself when something belongs with a health professional: stomach trouble that lasts for weeks, signs that you are eating too little, signs of a troubled relationship with food, or symptoms of low sodium after drinking a lot. You reach her directly in the chat, and the coach hands over to her when you ask about food.

She reads your plan but never changes it. On the day your training week starts she writes a weekly note based on the week's sessions and what you usually eat. If the week has a long run, it gets its own advice on carbohydrate, fluid and sodium per hour, calculated from how long the run is expected to take, and a sentence on why the amount goes up or stays the same from last week. She builds up from what you have told her your stomach handled, and holds when she doesn't know, within the same limits as above: up to 60 grams without fructose and up to 90 with.

If a race is within a week, the note gets a race-day plan covering breakfast, the start and fuel per hour. She only includes carb loading for races expected to take more than 90 minutes, and only if she knows your weight. She uses your weight for arithmetic only, like the loading per kilo. She never comments on it and never advises anyone to lose weight.

If you photograph the label on your gel, she saves the product with carbohydrate, sodium and caffeine per serving, and then gives the amounts in your own products alongside the grams. She only suggests caffeine in amounts you have already tried, and never for the first time on race day.

The trial week includes the coach but not Fika, and on Pro food questions go to the coach. The app needs a watch connected through Intervals.icu and covers 5K to marathon. The tiers are on the pricing page.

Sources

• Thomas, Erdman & Burke (2016): Nutrition and athletic performance
• Jeukendrup (2010): Carbohydrate and exercise performance: the role of multiple transportable carbohydrates
• de Oliveira, Burini & Jeukendrup (2014): Gastrointestinal complaints during exercise
• Costa et al. (2017): Gut-training: the impact of two weeks repetitive gut-challenge during exercise
• Hew-Butler et al. (2015): Statement of the third international exercise-associated hyponatremia consensus development conference
• Burke et al. (2011): Carbohydrates for training and competition
• Guest et al. (2021): International society of sports nutrition position stand: caffeine and exercise performance