Carbohydrate during exercise: when a session needs fuel, and how much

Carbohydrate during exercise means sugar or starch taken in while training or racing, usually as a sports drink, a gel or an easily digested food. Whether it helps depends mainly on how long the session lasts. In sessions of up to about an hour that start a few hours after a normal meal, the body's own stores cover the fuel demand, and any benefit from carbohydrate is small and appears to work through the mouth and brain rather than the muscles. From one to two hours, about 30 g per hour becomes useful, and the recommended amount rises with duration from there.

Where the fuel comes from

The body stores carbohydrate as glycogen, chains of glucose held in the liver and in muscle. The International Society of Sports Nutrition (ISSN) position stand on nutrient timing gives typical stores of about 80–100 g in the liver and 300–400 g in skeletal muscle. Glycogen is a main fuel for up to a few hours of moderate to high-intensity aerobic exercise, and as it runs low, the ability to hold intensity declines.

That is why duration decides the question. A hard 45-minute class uses part of the stores but rarely empties them. A two-hour run or ride, or a session after a day of eating very little, can bring muscle glycogen low enough to limit pace.

Two ways carbohydrate improves performance

A 2014 systematic review by Stellingwerff and Cox included 61 placebo-controlled studies with 679 participants, all using endurance or all-out tests rather than team sports. Carbohydrate improved performance compared with water in 82% of them, and the gain grew with the length of the exercise. The authors describe two mechanisms. In efforts of about one hour, the benefit comes from oral carbohydrate sensing: receptors in the mouth detect carbohydrate and signal brain areas linked to reward and motor control, so the amount absorbed hardly matters. In exercise longer than two hours, carbohydrate supplies fuel to muscles as glycogen runs short, and the amount absorbed matters a great deal.

Asker Jeukendrup's 2014 review of carbohydrate intake during exercise describes the experiments behind this split. Glucose infused directly into the blood during about an hour of exercise was taken up by the muscles but did not improve performance, while rinsing the mouth with a carbohydrate solution, without swallowing it, did. The receptors involved have not been identified, so the brain-based mechanism is well supported but not settled.

What a mouth rinse does

A carbohydrate mouth rinse means swilling a carbohydrate drink around the mouth for several seconds and spitting it out. A 2019 meta-analysis of 13 studies on cycling time trials found that rinsing raised mean power output slightly compared with a placebo rinse, but did not significantly shorten finishing time. A 2026 meta-analysis of 90 controlled trials found small improvements in distance, time, speed and workload, no effect on maximal power or muscular endurance, and evidence drawn mainly from men.

Many of these studies tested people after an overnight fast, which limits how far the results reach. The ISSN position stand cites reviews concluding that carbohydrate during exercise shorter than about 70 minutes has mixed effects, with a consistent benefit from about 90 minutes, particularly when exercise starts after a meal. Someone who had lunch two hours before an evening class is in a different position from a study participant cycling before breakfast.

Guidelines by duration

Jeukendrup's review proposes these amounts:

The 60 g figure reflects absorption. Glucose crosses the gut wall through a transporter called SGLT1, which becomes saturated at about 60 g per hour. Fructose uses a different transporter, so multiple transportable carbohydrates, mixtures of glucose and fructose, produced oxidation rates up to 75% higher in the studies reviewed. The 90 g band therefore requires a mixture.

The review adds three qualifications. The amounts are absolute grams per hour, not scaled to body weight. They come mostly from trained athletes, and people with less experience or working at a lower absolute intensity burn less carbohydrate and should adjust downwards. And the intake should be practised in training, because the gut adapts: in one study, more of the ingested carbohydrate was used after 28 days on a higher-carbohydrate diet.

What heart rate can and cannot show

Heart rate does not measure glycogen or blood glucose. A dashboard tile that stays orange shows how hard the heart is working, not how much fuel is left. Late in a long session, falling pace at a steady or rising heart rate, with higher perceived effort, can reflect low glycogen. Heat and fluid loss produce a similar pattern through cardiovascular drift, so the trace has to be read alongside session length, the last meal, temperature and drinking.

Worked examples

Illustrations, not prescriptions. A 6% carbohydrate drink contains 6 g of carbohydrate per 100 ml.

1. A 45-minute class after a normal meal. Water covers the session; a carbohydrate drink is optional and likely to make little difference.
2. A 6:00 session after an overnight fast. This matches the conditions of many mouth-rinse studies, so a small carbohydrate drink, or a rinse for someone who cannot eat beforehand, has the clearest reason to help.
3. A race or simulation expected to take about 80 minutes. About 30 g per hour gives roughly 40 g in total; 650 ml of a 6% drink supplies about 39 g, taken in several portions. No studies have tested carbohydrate intake in HYROX races specifically, so this applies evidence from running and cycling to a mixed format.
4. A two-hour long run or ride. About 60 g per hour gives around 120 g in total. Going beyond 60 g per hour requires glucose-fructose products and a trained gut.

Key points

1. Match the intake to the duration. After a normal meal, sessions under about 75 minutes need no carbohydrate for fuel; plan about 30 g per hour from one to two hours and about 60 g from two to three, less at lower intensity.
2. Use a small drink or a rinse when training fasted. Early sessions without breakfast are where short-duration benefits are most likely.
3. Choose glucose-fructose mixtures only for high intakes. A single sugar is enough up to about 60 g per hour.
4. Rehearse race fuelling in training. The gut adapts, and race day is the wrong time to test a new gel.
5. Read heart rate as effort, not fuel. Interpret late-session changes alongside duration, food, heat and hydration.

Sources

• Jeukendrup A. A step towards personalized sports nutrition: carbohydrate intake during exercise. Sports Medicine, 2014.
• Stellingwerff T, Cox GR. Systematic review: carbohydrate supplementation on exercise performance or capacity of varying durations. Applied Physiology, Nutrition, and Metabolism, 2014.
• Brietzke C et al. Effects of carbohydrate mouth rinse on cycling time trial performance: a systematic review and meta-analysis. Sports Medicine, 2019.
• Zalagkitis C et al. Effects of carbohydrate mouth rinse on human capacity and performance: a systematic review and meta-analysis of 90 controlled trials. Nutrients, 2026.
• Kerksick CM et al. International Society of Sports Nutrition position stand: nutrient timing. Journal of the International Society of Sports Nutrition, 2017.

Heart rate is a training signal, not medical advice, and this article gives general nutrition information rather than a personal plan. People with diabetes or other conditions affecting blood sugar should agree on fuelling around exercise with their doctor or dietitian.