High-carbohydrate diets promote endurance capacity. We have known since the 1960s that low-carbohydrate diets decrease endurance capacity by 25 percent or more. Carbohydrates are the principal fuels used at exercise intensities above 65 percent of maximum effort. A sophisticated study, led by Martin Gibala from McMaster University in Canada, showed that exercising with low muscle glycogen levels caused an increased use of protein as fuel, and reduced the capacity for protein synthesis after exercise.
Why is that a big deal if you’re looking to build and maintain muscle mass? Protein synthesis increases slightly – or stays the same – after a workout, while protein breakdown significantly increases. Muscle growth occurs when there is a positive protein balance during recovery, which isn’t the case if protein synthesis is reduced because of depleted glycogen stores.
The researchers caused muscle glycogen depletion through a combination of exercise and a low-carbohydrate diet. This study has important implications for athletes who often consume low-carbohydrate diets when trying to cut fat. Severe glycogen depletion induced through training and exercise make it impossible to maintain muscle mass.
Source: Journal of Applied Physiology, 109: 431-438, 2010