Grape juice contains a high level of fructose. Fructose is one of the worst carbohydrate sources for bodybuilders, especially in things like fruit juices. Specifically spiking insulin post-workout can actually be detrimental. Post-workout, starving muscles are wide open to receiving nutrients for repair and growth, and when we flood our bodies with simple carbohydrates the muscles quickly slam the door shut. Consuming BCAAs, protein and some fat is preferential immediately following a workout.