Your muscles are made of fibres, surrounding those fibres are "loose" cells... once your muscle fibres start to break down and cause "holes" in the fibres, certain chemicals and hormones are released which "call" these "loose" cells to the fibre where they attach. This is how muscles begin to grow. In order for the chemicals to be released, the muscle fibre should be fed... I heard that this time period is up to 1 hour after heavy lifting. Remember, when you target one area (ie. when working a single body part), blood (which carries nutrients) will go to that area... once you do cardio, blood will start to distribute away from your target.... ergo... eat after lifting!