This program is basically the opposite of Weight Loss with similar emphasis on avoiding junk foods, excessive fats, and other poor nutritional choices. People who want to gain weight need to eat more calories and more food. To most of the over-weight population, this would be a dream come true, but for underweight people who have trouble gaining weight, it can be a real problem.

Like obesity, being underweight involves many factors. Undernourishment during infancy and adolescence or nutrient deficiencies of the mother during pregnancy can lead to a lower number of fat cells and thus the potential for lower amounts of fatty tissue. Genetics and conditioning are also factors. Thinness, of course, runs in families, as does obesity. Poor eating habits and low food intake are other causes of underweight. Illness can cause weight loss. Flus or debilitating diseases such as cancer can lead to loss of weight, which can be hard to regain, especially when those with these problems are not used to higher-calorie diets. Stress and anxiety are often found with low body weight. High-strung people and worriers can have trouble putting on weight. During times of extreme emotional upset, it may be difficult to eat, and that can be a problem. People who use stimulants such as caffeine and cigarettes are more commonly underweight than those who do not abuse these stimulants. The caffeine-cigarette combination generates a lot of nervous energy and frenetic mental activity. It may be productive for office work, but not for health.

Several medical problems can be associated with weight loss or the inability to gain weight. Thyroid problems, mainly hyperthyroidism, are the most common of these, and are associated with many other symptoms, such as a rapid heart rate, sweaty palms, and insomnia. Some psychologically related medical problems are also associated with underweight. Bulimia and anorexia nervosa are the two serious food-oriented maladies; they are, in fact, often symptoms of much deeper emotional, attitudinal problems. Treatment of bulimia, or voluntary vomiting, may require a combination of medical and psychological care. The cycle of regular overeating and vomiting is hard on the body, as it can irritate the upper gastrointestinal tract with hydrochloric acid and cause a loss of potassium and other nutrients. Bulimia may or may not be a part of anorexia nervosa, which is becoming a more common eating disorder.

Anorexia nervosa, loss of appetite due to nervous or psychological factors, mainly afflicts young women from ages 13–25. It commonly entails a delusion about weight and body image and a fear of eating food because of possible weight gain. Anorexics are often malnourished, deficient in both calories and nutrients. They may also be involved in regular vigorous exercise, such as aerobics or ballet, and may use laxatives, both of which may lead to further loss of body nutrients. And they may become socially separated because of a fear of group pressure toward eating.

Anorexia nervosa is a serious problem! People with this syndrome should receive immediate treatment with education, emotional support, understanding, and food. This problem is more common in the teen years, which is even more a concern because nutrient needs are very high and poor nutrition is more common in this age group. Fortunately, though, this eating avoidance (not really loss of appetite) usually passes in time.