Marathon Fueling: How Many Carbs per Hour and How to Practice

Fueling means taking in carbohydrate while you run so that your stored fuel lasts the distance. Your body holds a limited amount of carbohydrate as glycogen in muscle and liver, and a marathon at race effort draws it down faster than a half marathon or a long training run. The research supports about 30 to 60 grams of carbohydrate per hour for runs of one to two and a half hours, and up to about 90 grams per hour beyond that, provided the higher amount comes from a mix of sugars and has been practiced. A 25 g gel every 25 to 30 minutes gets a runner to the middle of that range.

This guide covers the carbohydrate side. Fluid, sodium, and caffeine are separate plans.

Why fueling matters in a marathon

Carbohydrate is the fuel your muscles prefer at marathon effort, and the body stores only a limited amount of it. Over a race of two hours or more, those stores run low, and running pace becomes hard to hold regardless of fitness. Taking in carbohydrate during the run spares the stores and supplies fuel directly.

Burke, Hawley, Wong, and Jeukendrup (2011) reviewed the evidence in their sports-nutrition consensus paper and describe the benefit of carbohydrate during exercise as growing with duration. For efforts of about an hour, small amounts help. For efforts of one to two and a half hours, 30 to 60 g per hour is the supported range. Beyond two and a half hours, up to 90 g per hour has been shown to help, with the type of carbohydrate becoming important.

How many grams per hour

Jeukendrup (2014) reviewed what had changed in the understanding of carbohydrate during exercise and offered duration-based guidance.

The 60 g ceiling for a single sugar exists because glucose absorption from the gut is limited. Adding fructose, which uses a different transporter, raises the total the gut can absorb, which is why products aimed at long races list both. Jeukendrup also notes that these are upper limits that trained, practiced athletes reach, and that individual tolerance varies widely.

For most first and second marathons, 45 to 60 g per hour is the practical target. Raise it toward 90 g only if you have run long sessions at that intake without stomach trouble.

Turning grams into gels and drinks

Read the carbohydrate grams off the labels of the products you will use. Typical values:

• A gel: 20 to 30 g, some larger
• A sports drink at the usual 6 to 8 percent concentration: 30 to 40 g per 16 oz (500 mL) bottle
• A chew or block: 8 to 10 g each

The arithmetic is expected hours times grams per hour, minus what your drinks provide, divided by grams per gel, rounded up. A four-hour runner at 60 g per hour needs 240 g. If a sports drink covers 30 g an hour, that is 120 g from drinks and 120 g from gels, which is five 25 g gels. The marathon fueling calculator does this and spaces the gels through the run.

Whole gels round the total up, so a plan will often land a little above the target. That is fine. What matters is that the plan matches what you practiced.

When to take it

Start early and take it on a schedule.

1. First gel within the first 30 to 45 minutes. Absorption takes time, and starting late means running the middle of the race in deficit.
2. Then every 20 to 30 minutes, or at the intervals your calculator plan spaces them.
3. Take gels with a few sips of water. A concentrated sugar load without fluid is the most common cause of stomach upset.
4. Nothing new in the last 30 minutes. Late fuel does not reach the muscles before the finish, and it can cause trouble at the moment you least want it.
5. Fuel on time, not on feel. Hunger and flatness arrive after the deficit, not before it.

Write the schedule on your hand or your watch alongside your pace plan, as the race-day pacing guide describes.

Training your gut

The gut adapts to carbohydrate intake during exercise the way muscles adapt to load, and a stomach that has never handled 60 g an hour at race pace will not handle it on the day. Rehearsal is the whole of the method.

• Fuel every long run over 90 minutes in the specific block of a marathon build, using the exact products and timing you plan to race with.
• Start at 30 g per hour and add about 10 g per hour every two or three long runs, holding when your stomach complains.
• Practice at race pace, because gut blood flow and tolerance differ between an easy run and a hard one. The marathon-pace blocks in the marathon blocks guide are the right place.
• Practice in the conditions you expect. Heat lowers gut tolerance.
• Practice the mechanics: opening a gel at pace, drinking from a cup on the move, carrying what you need.

The half marathon guide applies the same rehearsal rule to long runs in that build, at lower amounts.

Fueling a half marathon

A half marathon that takes more than about 90 minutes benefits from some carbohydrate. Below an hour, the benefit is small. A gel at around 45 and 75 minutes, taken with water, covers most runners, and the amount matters less than having practiced it once.

Common mistakes

• Trying a new product on race day. Course gels are free and unpredictable. Carry what you practiced.
• Waiting until you feel bad. By then the deficit has already built.
• Taking a gel without water. Concentration is what upsets the stomach.
• Counting a drink twice. If a sports drink provides 30 g an hour, the gels only need to cover the rest.
• Chasing 90 g per hour without practice. The upper limit is for runners who have trained their gut to it. Start lower.
• Treating fueling as the hydration plan. They are different problems with different answers.

What to log

Write down what you took and when in each long run, alongside how your stomach felt and whether the marathon-pace block held up late. Over a build that log tells you your tolerance, which is the only number that matters on race morning. Vitadel Run's marathon plans place the long runs where the rehearsal happens, and the fueling calculator turns your tolerated grams per hour into a timed race-day plan you can download.