Carbo-loading is a popular practice for athletes in days leading up to an event. The idea of starting with a full tank is thought to be a good idea. However, carbo-loading has evolved, and the old practice of having high glycogen is now extreme and can have negative side effects.

In the 1960’s discoveries showed that:

  • Glycogen concentration depended on diet
  • A high carb diet increased glycogen stores
  • Glycogen concentration decreases during exercise
  • Increasing glycogen concentration resulted in less fatigue.

Early observations showed that if you deplete glycogen, then reduce carbohydrates for 3 days, followed by a high carbohydrate diet for 3 days, then the muscle glycogen bounced back much more than just eating carbohydrates every day. This was known as “super compensation”. This resulted in the classical carbo-loading approach used by marathoners in the 1970’s. Side effects of a very high carb diet caused gastro-intestinal tract discomfort and training hard 7 days before an event to deplete carb intake can also take its toll on the body.

In the 1980’s a more moderate approach was proposed. Exercise was gradually decreased as carbohydrates slowly increased. Glycogen concentrations were then shown to be just as high as the traditional approach.

In the 1990’s studies showed that well-trained athletes could achieve similar glycogen concentrations in only 1-2 days of carbo-loading and reduced training on those days. Studies also showed that the role of glycogen breakdown during exercise was directly proportional to the amount of glycogen present in the muscle. I.e. 1-2 hours into an event, glycogen concentration was similar, regardless of high or very high carbohydrate concentrations.

In 2017, Louise Burke did research showing:

  • LCHF (Low Carbohydrate High Fat) diets impairs exercise economy and negates performance benefits from intensified training in elite race walkers
  • Adaptation to LCHF increases whole body fat oxidation during exercise
  • Increased rates of fat oxidation results in reduced economy e.g. need more oxygen for a given speed
  • Adaptation to LCHF diet impairs performance in elite endurance athletes compared to diets providing high or periodised carbohydrate availability.


  • Start with sufficient glycogen – it does not need to be high. In trained individuals this can be achieved by eating carbohydrate rich foods for 2 days before a race while decreasing training
  • It is important to note that you do not only have to eat more carbohydrates, but rather eat more carbohydrates whilst decreasing fat. The point here is to carbo load and not to overeat, which can result in unwanted weight gain.
  • Solid or liquid carbs have the same effect
  • Carbohydrate amounts of 5g per kg per day can be sufficient

In conclusion carbohydrate-loading has evolved over time, but is not outdated in its benefit in terms of improving glycogen stores. Pay attention to the practical guidelines. The quantity and timing of carbohydrates should be prescribed according to anticipated fuel needs of exercise, as well as to different cycles in the training calendar, and personalised to the athlete.