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Carb loading: what is new?

Carb loading has been a popular practice for athletes in the days leading up to an event. There is a a lot of discussion about the best ways to do this. Starting with a full tank is generally believed to be a good idea. But early protocols to achieve high glycogen concentrations in the muscle were extreme and had a lot of side effects. So the methods of carb loading have evolved! Here we will discuss the current thinking and I will give my personal interpretation and practical recommendation.

Muscle glycogenolysis is dependent on glycogen stores

How it all began

In the 1960s the biopsy needle was redeveloped and this allowed researchers to collect a small amount of muscle tissue and measure muscle glycogen (1), the storage form of carbohydrate in the muscle. A number of discoveries were made:

  • Glycogen concentration in the muscle is dependent on diet. The more carbohydrate in the diet the higher the glycogen stores

  • Glycogen concentration declines during exercise, especially higher intensity exercise

  • Higher glycogen concentrations in the muscle resulted in less fatigue and better performance

These findings have only been confirmed since then.

Early observations

It was also observed that if you deplete muscle glycogen first, then reduce carbohydrate intake for 3 days followed by a very high carbohydrate intake for 3 days, that muscle glycogen bounced back much more than just eating carbohydrate every day. Supercompensation referred to the observation that glycogen did not just return to normal after depletion, it actually “supercompensated” and achieved “supra-normal” concentrations. This observation resulted in the development of the classical supercompensation diet which was then successfully used by runners like European Marathon Champion Ron Hill in the 1970s.

Supercompensation referred to the observation that glycogen did not just return to normal after depletion, it actually “supercompensated” and achieved “supra-normal” concentrations.

Side effects

This protocol involved an extremely hard workout 7 days before the race, followed by carbohydrate restriction for 3 days. It may not be ideal to have such a hard workout 7 days before. Without carbohydrate recovery in the days after 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 a lot of gastro-intestinal problems in many runners. So, overall, although this protocol was highly effective, the side effects may have outweighed the potential benefits (2).


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.

2 days may be enough

Then 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.

Studies also demonstrated that the rate of glycogen breakdown during exercise was directly proportional to the amount of glycogen present in the muscle. In other words, if you have 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.

Practical guidelines

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

  • Make sure you start exercise with sufficient muscle glycogen

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

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

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

  • 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

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

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


Bergstrom, J., Hermansen, L., Hultman, E., and Saltin, B. Diet, muscle glycogen and physical performance. Acta Physiol Scand. 71: 140-150, 1967.

Jeukendrup and Gleeson Sports Nutrition Human Kinetoics Champaign IL, 2010

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