You need to take a fast digesting protein/carbohydrate shake immediately after a workout because you just finished damaging your muscles, precisely what a workout does, and now your muscles are breaking down because you've use most of your stored fuel(glycogen) your body is breaking down the muscles to use as fuel and also to save energy, the protein/carb shake stops the breakdown and stimulates muscle repair and growth, right after a workout your blood vessel are dilated and the blood is pumping in your muscles like crazy, perfect conditions for protein and carbs to get into your muscles. I actually do 10 minutes of light cardio on my Exercycle after my workout to keep my heartrate up for a little longer to get those nutrients to my muscles. The protein, stops the muscle breakdown, the carbs replenish your fuel stores and TOGETHER they stimulate muscle growth.
