Eating is the first and most important factor that is needed to gain weight. It's simple - you need to eat more calories than you burn. Muscle tissue, unlike fat, requires calories to sustain muscles. Therefore if you are training to build muscle you need to increase your calorie consumption, due to the fact you will have more muscles which demand more calories.
Muscle is often consumed by the body when fat reserves are depleted, so a portion of the weight gained needs to be muscle. Muscles contain a great deal of protein and this protein is required for development of new muscle and mass, so high-protein foods are key for weight gain. Read my review on optimum anabolics for a shockingly weird but very effective way to stimulate rapid muscle growth by manipulating diet and exercise.
