Underweight

Lore & Background

Underweight is assessed primarily through body mass index (BMI), a ratio of weight to height, with a cut-off point of 18.5 kg/m². Another measure compares an individual's weight to the average of a similar age and height cohort, with those 15% to 20% below considered underweight. Body fat percentage has also been suggested as an alternative assessment, with the American Council on Exercise defining essential fat thresholds below which a person is underweight as 10–13% for women and 2–5% for men, reflecting the greater essential fat needed for reproductive function in women.

Causes of being underweight include genetics, poor nutrient absorption, increased metabolic rate or energy expenditure, lack of food (often due to poverty), low appetite, drugs affecting appetite, illness (physical or mental), and the eating disorder anorexia nervosa. Associated medical conditions include type 1 diabetes, hyperthyroidism, cancer, and tuberculosis. Underweight individuals tend to eat about 12% fewer calories than those with normal BMI and are 23% less physically active.

Being underweight can be a symptom of an underlying condition (secondary) or can cause other conditions (primary). Severely underweight individuals may have poor physical stamina, a weak immune system, and increased infection risk. In women, severe underweight can result in amenorrhea, infertility, or pregnancy complications. It is also an established risk factor for osteoporosis, even in young people. Treatment approaches include increasing calorie intake through diet or liquid supplements, exercise to promote muscle hypertrophy and bone density, and appetite stimulants such as certain antidepressants, antipsychotics, or medicinal cannabis where approved.

Reader's Guide

Underweight is a significant health concern globally, particularly in developing countries where it is a primary cause of mortality. Its prevalence has declined worldwide from 1975 to 2014, with age-standardized rates dropping from 14.6% to 9.7% in women and from 13.8% to 8.8% in men, indicating progress in reducing undernutrition. However, the condition remains a major public health issue, carrying risks comparable to those of morbid obesity in terms of increased mortality, though this effect is less drastic when restricted to non-smokers without disease history, suggesting smoking and disease-related weight loss are leading contributors.

The assessment of underweight has evolved beyond simple BMI to include body fat percentage, which accounts for differences between adipose and muscle tissue. This nuance is important because underweight individuals may have inadequate intake of not only calories but also essential amino acids and micronutrients, leading to conditions like anemia, hair loss, and weakened immune defenses. The condition's association with osteoporosis, even in young people, highlights the long-term consequences of low body weight, particularly in cases of relative energy deficiency in sport.

Treatment strategies focus on weight gain through increased caloric intake, exercise for muscle and bone health, and appetite stimulants when necessary. The complexity of underweight—ranging from genetic predisposition to poverty and eating disorders—requires a multifaceted approach. Its recognition as both a symptom and a cause of disease underscores the need for careful medical evaluation, especially when weight loss is unexplained.

Did You Know?

• Underweight is defined as a BMI under 18.5 or a weight 15% to 20% below the average for a person's age and height group.
• Global prevalence of underweight in women declined from 14.6% in 1975 to 9.7% in 2014.
• Underweight individuals eat about 12% fewer calories and are 23% less physically active than those with normal BMI.
• Being underweight is an established risk factor for osteoporosis, even in young people.