High carbohydrate diets promote endurance capacity.  We have known since the 1960’s 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.  The researchers caused muscle glycogen depletion through a combination of exercise and a low carbohydrate diet.  This study has important implications for bodybuilders who often consume low carbohydrate diets when trying to cut fat for a contest.  Severe glycogen depletion induced through training and exercise make it impossible to maintain muscle mass.  (Journal of Applied Physiology, 109:  431-438, 2010)