You can drink some of your juice during exercise, and it should be high protein because protein helps to prevent muscle breakdown, which leads to a better recovery post-workout. It also ensures that you adapt better to training as you progress, which means that you are better able to power your way through your workout. You don’t need a massive amount of protein in your juice; just a small amount is enough to prevent protein breakdown. If you like exercising on an empty stomach, then you don’t need any more than around 15 grams of protein during training.  Athletes who do punishing training bouts need more protein, as does anyone who is looking to gain a significant amount of mass.