Quickly gaining weight through weight training is not as easy as it may sound. Because you’re trying to build up muscle mass and not fat, a carefully monitoring your diet is a must. Two methods are the best to gain weight quickly. Option one is to eat an almost exclusively protein diet, and experience the results immediately. Option two is to eat a high protein diet with some fat, and burn the body fat during the later winter months. This second method is known as cutting, though you really don’t want to keep gaining and losing a lot of weight all of the time.