There may be some benefit to eating by bodytype since hormone levels, metabolic rate, sympathetic nervous system, and carbohydrate tolerance are factors in determining the optimization of one's physique. The latter factor (carb tolerance) is what's emphasized in the bodytype diet. The timing of carb ingestion (esp. simple carbs) can affect the appearance of your physique. Since ectomorphs have a high tolerance for carbs, they should eat more simple carbs during and immediately after their workouts in order to gain muscle mass and strength. Mesomorphs have a moderate tolerance for carbs and therefore should eat some simple carbs during and after their workouts in order to continue building muscle mass while maintaining a low bodyfat. Endomorphs have a low tolerance for carbs and therefore should eat simple carbs only during their workouts in order to lose bodyfat.
Brian Danley, CFT
"Motivation is what gets you started. Habit is what keeps you going."