In the presence of adequate food resources, being underweight can sometimes be the result of mental or physical disease. There are hundreds of possible medical causes for excessive weight loss or a person being underweight. Some of the more prevalent include:
Underweight can also be a primary causative condition. Severely underweight individuals may have poor physical stamina and a weak immune system, leaving them open to infection. According to Robert E. Black of the Johns Hopkins School of Public Health, "Underweight status ... and micronutrient deficiencies also cause decreases in immune and non-immune host defenses, and should be classified as underlying causes of death if followed by infectious diseases that are the terminal associated causes. People who are malnutrative underweight raise special concerns, as not only gross caloric intake may be inadequate, but also intake and absorption of other vital nutrients, especially essential amino acids and micronutrients such as vitamins and minerals.
Underweight is an established risk factor for osteoporosis even for young people. This is a particular insidious consequence, because the affected persons do not notice the danger, they can feel fit and may be brilliant for example in endurance sports. After the occurrence of first spontaneous fractures the damage is often already irreversible.
If an individual is severely underweight to the point where problems with his or her health develop, it may be necessary for the person to make a concentrated effort to gain weight. The treatment for an underweight individual is to increase the food energy intake so that more food energy is consumed than is being used as work. It is usually suggested that weight training is also to be undertaken to increase muscle mass.
If weight loss results from a disease, resolving the illness and consuming adequate calories can bring many underweight individuals to a healthy body weight.