Underweight can be caused by heredity, a serious illness, not eating enough, or a combination of intense ongoing exercise and insufficient calories in the diet. Commonly it is caused by an eating disorder such as anorexia or bulimia.
Severe depression can also affect the appetite, resulting in weight loss.
To gain body weight the number of calories consumed in the diet must exceed the number of calories the body needs to maintain its current weight. Medical problems such as depression, bulimia, or anorexia must be addressed and treatment underway before weight gain will occur. The nutrition considerations of a healthful diet still apply when gaining weight is desired. The goal is to increase the number of calories in the diet while making healthful choices from a variety of foods. Choose dairy products, meats, fish, poultry, eggs, legumes, nuts, fruits, vegetables, whole grain cereals, breads, rice and pasta.