Underweight

The term underweight refers to a human who is considered to be under a healthy weight. The definition is usually made with reference to the body mass index (BMI). Most consider individuals under 18.5 to be underweight, though some authorities use a higher value of 20. It is important to note that the BMI is a statistical estimate and some individuals classified as underweight may be perfectly healthy.

Causes
The most common cause of underweight is primary malnutrition caused by the unavailability of adequate food, which can run as high as 50% in parts of sub-Saharan Africa and south Asia. The effects of primary malnutrition may be amplified by disease; even easily treatable diseases such as diarrhea may lead to death.

In the presence of adequate food resources, underweight is generally the result of mental or physical disease. There are hundreds of possible medical causes for excessive weight loss or underweight. Some of the more prevalent include:
 * Anorexia nervosa and bulimia
 * Cancer
 * Tuberculosis
 * Hyperthyroidism
 * Diabetes (especially type 1)
 * Anxiety and depressive disorders
 * Drug abuse
 * Diseases of digestive organs
 * Feebleness, especially in the untended elderly
 * Dental pain
 * Obsessive overtraining
 * HIV/AIDS

Problems
The most immediate problem with underweight is that it might be secondary to, and/or symptomatic of, an underlying disease. Unexplained weight loss requires professional medical diagnosis.

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 defences, and should be classified as underlying causes of death if followed by infectious diseases that are the terminal associated causes." Malnutrative underweight raises 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.

In women, being grossly underweight can result in amenorrhoea (absence of periods) and possible complications during pregnancy. It can also cause anemia and hair loss.

Weight gain
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, with only 3 or 4 sessions of under 45 minute period of weight training, using heavy weights and lesser number of reps.

It is easier to increase Caloric intake by drinking food energy rather than eating food energy. There are several high–food energy drinks available in supermarkets that dramatically supplement daily intake, even adding 350 Calories (1.5 kJ) per six fluid ounce (180 mL) bottle.

Another simple method of increasing one's daily food energy is to add high-calorie/high-fat condiments to foods or follow a meal with a simple snack.

Papers

 * Google Scholar