These values were lower than what was reported for 1975 as 14.6% and 13,8%, respectively, indicating a worldwide reduction in the extent of undernutrition. Causes
A person may be underweight due to genetics, improper metabolism of nutrients, lack of food (frequently due to poverty), drugs that affect appetite, illness (physical or mental) or the eating disorder anorexia nervosa.Being underweight is associated with certain medical conditions, including type 1 diabetes, hyperthyroidism, cancer, and tuberculosis. People with gastrointestinal or liver problems may be unable to absorb nutrients adequately. People with certain eating disorders can also be underweight due to one or more nutrient deficiencies or excessive exercise, which exacerbates nutrient deficiencies.A common belief is that healthy underweight individuals can ‘eat what they want’ and then burn it off either by high levels of activity or elevated metabolism. It has been shown however that individuals with BMI < 18.5 eat about 12% less calories than individuals with normal BMI (21.5 to 25) and they are 23% less physically active (by accelerometry). Problems
Being underweight can be a symptom of an underlying condition, in which case it is secondary. Unexplained weight loss may require a professional medical diagnosis.Being underweight can also cause other conditions, in which case it is primary. Severely underweight individuals may have poor physical stamina and a weak immune system, leaving them open to infection.
This can be done by eating a sufficient volume of sufficiently calorie-dense foods. Body weight may also be increased through the consumption of liquid nutritional supplements. Exercise
Another way for underweight people to gain weight is by exercising, since muscle hypertrophy increases body mass. Weight lifting exercises are effective in helping to improve muscle tone as well as helping with weight gain. Weight lifting has also been shown to improve bone mineral density, which underweight people are more likely to lack.Exercise is catabolic, which results in a brief reduction in mass. However, during recovery, anabolic overcompensation causes the muscles to grow, which results in an overall increase in mass. This can happen through an increase in muscle proteins, or through enhanced storage of glycogen in muscles. Exercise can also help stimulate the appetite of a person who is not inclined to eat. Appetite stimulants
Certain drugs may increase appetite either as their primary effect or as a side effect. Antidepressants, such as mirtazapine or amitriptyline, and 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 cannabis may be prescribed for severe appetite loss, such as that caused by cancer, AIDS, or severe levels of persistent anxiety. Other drugs or supplements which may increase appetite include antihistamines (such as diphenhydramine, promethazine or cyproheptadine). See also
Essential nutrient
List of phytochemicals in food
Body image
Emaciation
Malnutrition
Stunted growth

References


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