Carb loading is one of the best-known nutrition strategies in endurance sport. Its main aim is to reach high stores of muscle glycogen before an important race, and so delay exhaustion. But how do you do it correctly?
Why does endurance performance depend on glycogen?
Glycogen is a polysaccharide made up of glucose molecules and is the primary and most efficient source of energy for muscle work at moderate to high intensity.
During long-lasting exercise these stores are gradually used up. Once the level of muscle glycogen falls below a critical threshold, the muscles lose the ability to hold the required pace or power, marked fatigue sets in and you “hit the wall".
Source: Hawley et al., 1997
When does carb loading make sense (and when is it pointless)?
For short efforts it is usually not necessary. Even then, though, it is a good idea not to start with depleted stores. For longer efforts it is an advantage to have glycogen stores high enough — but it does not automatically follow that the more glycogen you store, the better your performance will be.
A very high concentration also increases the rate at which the body uses it up at the start of the activity. After the first 30 to 60 minutes the glycogen stores may therefore be similar regardless of whether they were high or extremely high before the start (Juekendrup & Gleeson, 2025).
Before you start deliberately maximising glycogen, consider whether it can really be a limiting factor in the activity in question. Carb loading is usually not needed for efforts shorter than 30 minutes. In longer endurance activities, however, it can help delay fatigue and support performance (Juekendrup & Gleeson, 2025).
The evolution of carbohydrate timing: from myths to modern science
The scientific view of carb loading has changed fundamentally over time:
1. The classic protocol
1960s – Saltin, Bergström, Hultman
- Depleting session (7 days before): a demanding session to completely empty glycogen stores
- Depletion phase (3 days): minimal carbohydrates, high fat and protein content, training continues
- Loading phase (3 days): a high-carbohydrate diet, minimal training
Why it was abandoned: it caused hypoglycaemia, digestive problems, extreme fatigue, mood swings and an increased risk of injury just before the race.
2. The modern moderate protocol
Sherman et al., 1981
Research showed that the preceding exhausting depletion phase is not necessary to reach maximum glycogen stores.
How it works: gradually reduce your training volume (tapering) 3–6 days before the race and in the last 1–3 days (36–48 h) increase your carbohydrate intake.
The result: practically the same high level of muscle glycogen (~200–210 mmol/kg wet tissue) without the negative effects.
Adapted from Juekendrup & Gleeson (2025)
The amount of carbohydrates: how much is a lot and how much is too little?
According to Burke (2001), in normal training the intake of most male athletes is between 5–7 g of carbohydrates/kg of body weight a day; during periods of demanding training or racing it may rise to 7–10 g/kg/day.
| Situation | Recommended carbohydrate intake |
|---|---|
| Carb loading – most athletes | 6–8 g/kg/day |
| Carb loading – competitive / elite athletes | 8–10 g/kg/day |
| Extreme load (e.g. a stage race) | 10–12 g/kg/day |
You need to consider your own everyday diet — a sharp jump from, say, 4 g/kg/day to 8–10 g/kg/day could cause more complications than benefit. For most athletes 6–8 g/kg/day is enough (Jeukendrup, 2025).
Practical rules: how much and what exactly to eat?
1. Tailored dosing — 6–8 g/kg/day is enough for most recreational athletes, 8–10 g/kg/day for competitive and elite athletes. Under extreme load (e.g. the Tour de France) up to 10–12 g/kg/day. There is no need to force carb loading if you simply cannot eat any more.
2. Choosing high-GI foods — the most suitable are easily digestible foods with a high glycaemic index: white rice, pasta, white bread, porridge, potatoes, juices, jam or sports carbohydrate drinks and gels.
3. Limit fibre, fat and protein — in the 24–48 hours before the start, significantly reduce your intake of them. They slow down digestion and increase the risk of gastrointestinal problems (bloating, cramps, diarrhoea) during the race.
Watch out for hydration and weight gain
A common surprise for endurance athletes is a slight increase in weight on race day. This phenomenon is completely physiological:
- Every 1 gram of stored glycogen binds roughly 3 grams of water with it.
- If you store an extra 400–500 g of glycogen, the body retains roughly 1.2–1.5 kg of water.
- A total weight increase of 1.5 to 2 kg is normal.
Common mistakes in carb loading
- Overeating — before a race training volume is usually reduced, and with it energy expenditure. Carb loading should not consist of simply increasing the amount of food, but of adjusting its composition — more carbohydrates, less fat. Carb loading is not the same as overeating.
- The type of carbohydrate — the particular kind or form does not matter much: solid food, carbohydrate drinks and energy gels all have a similar effect.
- The goal is “high enough", not “maximum" — excessively high glycogen stores are broken down faster during exercise.
Conclusion for athletes
Carb loading is not about cramming in as many carbohydrates as possible and hoping for the best. It is a controlled process that gives the body the time and space to store reserves that it will then really use during the race. Just as we train our muscles and cardiovascular system, here too it is about systematic preparation — a gradual reduction in load combined with a targeted increase in carbohydrates, not a last desperate attempt the day before the start. When you get this process right, the result is not just a time that is a few per cent better. It is the certainty that you will enjoy the whole race considerably more.
References
- Bergstrom, J., and E. Hultman. 1967b. Synthesis of muscle glycogen in man after glucose and fructose infusion. Acta Med Scand 182(1):93-107.
- Burke, L.M. 2001. Nutritional practices of male and female endurance cyclists. Sports Med 31(7):521-532.
- Sherman, W.M., D.L. Costill, W.J. Fink, and J.M. Miller. 1981. The effect of exercise and diet manipulation on muscle glycogen and its subsequent utilization.
- Hawley, J.A., E.J. Schabort, T.D. Noakes, and S.C. Dennis. 1997. Carbohydrate loading and exercise performance. Sports Med 24(1):1-10.
- Jeukendrup, A., Gleeson, M. (2025). Sport Nutrition (4th ed.). Human Kinetics.
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