Maratona dles Dolomites: carb loading guidelines

Asker Jeukendrup’s point of view

Many endurance races, including the Maratona dles Dolomites, scheduled for 7 July next, have a carb loading dinner a day or 2 before the event.

This practice developed from research performed in the 1960s. These studies clearly showed a relationship between muscle carbohydrate stores (muscle glycogen) and endurance capacity.


The more muscle glycogen the subjects of the studies had, the longer they were able to exercise at a given intensity. Muscle glycogen stores were increased or decreased by changing the diet and the researchers also showed a linear relationship between carbohydrate intake and muscle glycogen.



Carb loading: full tank

British runner Ron Hill was one of the first to experiment with glycogen loading and when he became European Champion, carbohydrate loading became popular amongst runners.

There is a lot of evidence now that glycogen is important for performance and studies generally show that if exercise is 90 min or longer high glycogen stores can increase performance by 1-3%.

Although there is a lot of talk about low carbohydrate diets for athletes, these would lower glycogen and have a negative effect on performance (unless the intensity is very low, then the fuel doesn’t matter as much). So essentially: starting with a full tank is a good idea.


In the 1970s the protocols to achieve high glycogen concentrations in the muscle were quite extreme and had a lot of side effects. The original carb loading protocol that resulted in extremely high glycogen stores started 7 days before the important event. On that day glycogen in the muscle was depleted. In the next three days carbohydrate intake was kept to a minimum for 3 days, followed by 3 days on a very high carbohydrate intake. It was observed that muscle glycogen bounced back much more than just eating carbohydrate every day.

This observation resulted in the development of the classical supercompensation diet practiced by Ron Hill and others. Supercompensation referred to the observation that glycogen did not just return to normal after depletion, it actually “supercompensated” and achieved “supra-normal” concentrations.

Disadvantages of the classical supercompensation protocol

There are numerous disadvantages and unwanted side effects of this otherwise effective protocol: It may not be ideal to have such a hard workout 7 days before and without carbohydrate for 3 days afterwards recovery is likely to be very poor.


Athletes were also recommended not to exercise the week before the race. For many athletes this is a greater punishment than the extreme diet itself. The high fat, no carb diet in the 3 days after the glycogen depleting exercise also caused various gastro-intestinal problems in many runners. So, overall, although this protocol was highly effective, the side effects could outweigh the potential benefits.

Carb loading: the new approach

Therefore, a more moderate approach was proposed in the 1980s. The glycogen depleting exercise was removed and as training was reduced towards the race, the carbohydrate intake was gradually increased. Glycogen concentrations appeared to be very high as well after 6-7 days, even though they were not quite as with the traditional protocol.


Studies in the 1990s demonstrated that very well-trained athletes could achieve similar muscle glycogen concentrations with just 1 or 2 days of carbohydrate loading and reduced training on those days. In less trained individuals this appeared to take a little longer. After activities where muscle damage is substantial, glycogen restoration takes longer as well.


The more you have, the faster you use it

One more important finding is that the rate of glycogen breakdown during exercise is directly proportional to the amount of glycogen present in the muscle. In other words: if you start with extremely high muscle glycogen stores you will break them down faster than when you have normal or high glycogen stores. An hour or 2 hours into the exercise, glycogen concentrations may therefore be comparable whether you started with extremely high or just high glycogen stores.

Carb loading: practical guidelines

So, what we can take away from these studies are the following practical guidelines:

1. Make sure you start exercise with sufficient muscle glycogen.

2. Sufficient means high stores but these stores don’t need to be extremely high. Extremely high glycogen stores are also broken down faster.

3. For trained individuals this can be achieved by eating carbohydrate rich for 2 days prior to a race, whilst reducing glycogen use (reducing training).

4. Because training is reduced and therefore energy expenditure is reduced increased carbohydrate intake should not be the result of just eating more. It should be the result of selecting more carbohydrate sources and reducing fat intake. Very often carb loading and overeating seem to be confused by athletes.

5. There are many different ways to achieve high glycogen stores. The type of carbohydrate seems to have little or no effect, both solid and liquid carbohydrate sources seem to have the same effects.

6. A carbohydrate intake of 5-7 g/kg per day seems to be sufficient in the majority of cases (with low energy expenditure).

7. Athletes who often experience gastro-intestinal issues should select their carbohydrate sources more carefully and could benefit from a lower fiber intake.

8. The most common mistake by athletes is that carb loading is confused with overeating. Carb loading does NOT mean eating as much carbs as possible or eating as much as possible in general. Daily energy expenditure still needs to be considered and this often means that the percentage carbohydrate of total energy intake becomes greater in the days leading up to a race. This means that fat and perhaps protein intake have to be lowered somewhat.


  • Bergström J, Hermansen L, Hultman E, Saltin B. Diet, muscle glycogen and physical performance. Acta Physiol Scand 1967; 71, 140-150.
  • Hawley JA, Schabort EJ, Noakes TD, Dennis SC. Carbohydrate loading and exercise performance. Sports Med 1997 Aug;24, 73-81.
  • Jeukendrup AE, Gleeson M. Sport Nutrition: an introduction to energy production and performance. 3rd edition edn, Human Kinetics, 2018.



Professor Asker Jeukendrup is one of the world’s leading sports nutritionists/ exercise physiologists who spent most of his career as a professor at the University of Birmingham (UK). Dr Jeukendrup authored 8 books and over 200 research papers and book chapters. His expertise stretches from exercise metabolism and sports nutrition to training and overtraining. He is currently a (visiting) professor at Loughborough University and director of his own performance consulting business “Mysportscience” and communicates science through the popular website Asker works as Performance Manager for the Dutch Olympic team and is Head performance Nutrition for the Lotto Jumbo Pro cycling team. He also works with FC Barcelona, and other elite football clubs. Asker practices what he preaches and completed 21 Ironman races including 6 time the Ironman world Championship in Hawaii.