2 rules to gain weight
From the moment you wake up, eat every 3 hours including hour 0.
After your workout, you MUST eat 40 grams of protein within 45 minutes of completing your last set. This is perhaps the most important rule. Ideally whey protein powder, or egg white, lean chicken. Bring it to the gym with you !!! Eat this with some white bread, or a sugary drink (Gatorade, not coca cola!)
