KEY POINTS  A low-carbohydrate diet in athletes impairs their exercise tolerance and their ability to beneficially adapt to long-term physical training.  Physical performance and mood state seem better maintained with a high vs. moderate carbohydrate diet, thus reducing the symptoms of overreaching and possibly overtraining. KEY POINTS  A low-carbohydrate diet in athletes impairs their exercise tolerance and their ability to beneficially adapt to long-term physical training.  Physical performance and mood state seem better maintained with a high vs. moderate carbohydrate diet, thus reducing the symptoms of overreaching and possibly overtraining.