Learn exercises and information to help underweight people gain weight. Jack Lalanne cites poor nutrition and lack of exercise as the reason for being underweight. He recommends the correct balance of vitamins and minerals to help combat the poor nutrition that leads to a low body weight. He also recommends whole grains and many other natural foods to correct a personal weight problem. Lalanne believes that exercise is a normalizer that helps correct weight for both the underweight and overweight.