[4] The American Council on Exercise defines the amount of essential fat, below which a person is underweight, as 10–13% for women and 2–5% for men.

[citation needed] Using the body mass index as a measure of weight-related health, with data from 2014, age-standardised global prevalence of underweight in women and men were 9.7% and 8.8%, respectively.

[6] A person may be underweight due to genetics,[7][8] poor absorption of nutrients, increased metabolic rate or energy expenditure, lack of food (frequently due to poverty), low appetite, drugs that affect appetite, illness (physical or mental) or the eating disorder anorexia nervosa.

[15][16] 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.

Severely underweight individuals may have poor physical stamina and a weak immune system, leaving them open to infection.

This is seen in individuals suffering from relative energy deficiency in sport, formerly known as female athlete triad: when disordered eating or excessive exercise cause amenorrhea, hormone changes during ovulation leads to loss of bone mineral density.

[29] Another way for underweight people to gain weight is by exercising, since muscle hypertrophy increases body mass.

[30] Weight lifting has also been shown to improve bone mineral density,[31] which underweight people are more likely to lack.

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.