Underweight

An underweight person is a person whose body weight is considered too low to be healthy. A person who is underweight is malnourished. The body mass index, a ratio of a person's weight to their height, has traditionally been used to assess the health of a person as it pertains to weight: under the cut-off point at a BMI of 18.5, a person is considered underweight. The calculation is either weight in kilograms divided by height in meters, squared, or weight in pounds times 703, divided by height in inches, squared. Another measure of underweight is through comparison to the average weight of a cohort of people of a similar age and height: people who are at least 15% to 20% below the average weight for the group are considered underweight. Body fat percentage has been suggested as another way to assess whether a person is underweight. Unlike the body mass index, which is a proxy measurement, the body fat percentage takes into account the difference in composition between adipose tissue (fat cells) and muscle tissue and their different roles in the body. 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. The greater amount of essential body fat in women supports reproductive function. 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. 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. A person may be underweight due to genetics, poor absorption of nutrients, increased metabolic rate or energy expenditure, 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.