This article may need to be rewritten entirely to comply with Wikipedia's quality standards. You can help. The discussion page may contain suggestions. (November 2011)

Underweight

The underweight range according to the Body Mass Index (BMI) is the white area on the chart.
ICD-10	R62.8
ICD-9	783.22
MeSH	D013851

Underweight refers to a human who is considered to be under a healthy "weight". "Underweight" means weighing less than what is expected to be a healthy person (underweight = insufficiency of weight). The definition is usually made with reference to the body mass index (BMI). A BMI of under 18.5 is usually referred to as underweight. Calculate your Body Mass Index, National Institutes of Health, http://www.nhlbisupport.com/bmi/, retrieved 2009-04-27 

Causes[link]

The most common cause of a person being underweight is primarily malnutrition caused by the unavailability of adequate food.

Some people are underweight due to genetics;^[1][2] others due to poverty.

Being underweight can sometimes be the result of mental or physical disease, such as hyperthyroidism, cancer, or tuberculosis. People with gastrointestinal or liver problems may be unable to absorb nutrients adequately. People with eating disorders, such as anorexia nervosa, are likely to become underweight.

Problems[link]

Underweight might be secondary to or symptomatic of an underlying disease. Unexplained weight loss may require 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 defenses, and should be classified as underlying causes of death if followed by infectious diseases that are the terminal associated causes."^[3] 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 micro-nutrients such as vitamins and minerals.

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

Underweight is an established^[4] risk factor for osteoporosis, even for young people. This is a particular insidious consequence, because the affected persons do not notice the danger.^[5][6] After the occurrence of first spontaneous fractures the damage is often already irreversible.

Being underweight causes increased mortality at rates comparable to that seen in morbidly obese people.^[7]

Treatment[link]

This section may require copy-editing.

Diet[link]

Underweight individuals may be advised to gain weight by increasing calorie intake by, for example, eating calorie-dense foods, such as dried fruits, cheese, raisins, and nuts.^[8] Body weight may also be increased through the consumption of liquid nutritional supplements, such as Ensure and Boost.^[9] Other nutritional supplements may be recommended for individuals with insufficient vitamin or mineral intake.^[10][11]

Exercise[link]

Another way for underweight people to gain weight is by exercising. The addition of muscles will increase your body mass. Weight-lifting exercises are effective in helping to increase muscle tone as well as helping with gaining weight.^[12]

Appetite stimulants[link]

Finally, certain drugs may increase appetite either as their primary effect, or as a side-effect as in the majority of cases. Antidepressants such as mirtazapine or amitryptiline, antipsychotics (particularly chlorpromazine and haloperidol, as well as tetrahydrocannabinol (found in cannabis), all present an increase in appetite as a side-effect. In states where it is approved, medicinal marijuana may be prescribed for severe appetite loss, such as that caused by cancer, AIDS or even severe levels of persistent anxiety. Other drugs which may increase appetite include certain benzodiazepines (such as diazepam), sedating antihistamines (such as diphenhydramine or promethazine, or B vitamin supplements.

Exercise itself is catabolic, which results in a brief reduction in mass. The gain in weight that can result of it comes from the anabolic over-compensation when the body recovers (via rest and eating) and overcompensates via muscle hypertrophy. This can happen via an increase in the muscle proteins, or through enhanced storage of glycogen in muscles. Exercise can help stimulate people's appetite if they are not inclined to eat.

See also[link]

• Body image
• Emaciation
• Malnutrition
• Stunted growth

References[link]