Carbohydrate for training

Glycogen

Glycogen is carbohydrate stored in muscle and liver.⁶ Bergström and colleagues measured it in a 1967 study, with needle biopsies from the thighs of 9 men who then cycled to exhaustion at 75% of maximal oxygen uptake.¹

Each man’s time to exhaustion rose and fell with the glycogen in his thigh at the start.¹ A high-carbohydrate diet after hard exercise raised it above the normal range in every man.¹

How much

A 2011 review by Burke and colleagues advised matching an athlete’s carbohydrate to what muscle and brain use in each day’s training.³ In a 2016 joint position statement with the Academy of Nutrition and Dietetics and Dietitians of Canada, the American College of Sports Medicine (ACSM) sets daily carbohydrate by training load.^2,3

Eating too little

In 2014, an IOC panel named Relative Energy Deficiency in Sport, a syndrome of poorer health and performance in athletes who eat too little for their training.¹⁰ Its 2023 update drew on more than 170 studies published since 2018, with new data on the part played by low carbohydrate availability.¹⁰

Low-carbohydrate diets

A 2017 study by Burke and colleagues fed 29 elite race walkers one of three diets of equal energy through 3 weeks of intensive training.⁴ Both high-carbohydrate groups, whether they ate it evenly or in planned highs and lows, cut their 10 km times by 6.6% and 5.3%.⁴

The third group ate a ketogenic high-fat diet and did not get faster.⁴ Its walkers burned far more fat, and walking at race pace took them extra oxygen, which the authors named as part of the reason.⁴

Timing

Twelve cyclists in Ivy’s 1988 study rode 70 minutes twice, drinking 2 g/kg of carbohydrate straight after one ride and 2 hours after the other.⁵ After the late drink, their muscles stored glycogen 45% more slowly over the next 2 hours than after the early one.⁵

With less than 8 hours between two hard sessions, the ACSM advises 1.0–1.2 g/kg of carbohydrate an hour for the first 4 hours.^2,3

Glucose and fructose

Glucose and fructose are absorbed mainly through different transport proteins in the gut, so taking both raises the total that can be absorbed.⁶ A 2017 review found that liver glycogen refills about twice as fast with glucose and fructose together as with glucose alone, and that muscle glycogen refills no faster.⁶

Fuchs and colleagues, in 2016, gave 15 trained cyclists 1.5 g/kg an hour of sucrose (table sugar) or of glucose for 5 hours after exhausting exercise.⁷ Sucrose refilled the liver faster, by about 3.4 g of glycogen an hour, and muscle glycogen did not differ between the two.⁷

Protein

For building and keeping muscle, the ISSN finds 1.4–2.0 g/kg a day enough for most people who exercise, in 20–40 g doses every 3–4 hours.⁹ It advises endurance athletes to focus on eating enough carbohydrate.⁹

Morton and colleagues pooled 49 randomised trials in a 2018 meta-analysis, with 1,863 people who did resistance training for 6 weeks or more.⁸ Extra protein added to gains in muscle size, and lean-mass gains stopped rising above a total intake of about 1.6 g/kg a day.⁸