underweight

Underweight describes a body weight that is too low to be healthy. In adults, it is commonly defined as a body mass index (BMI) below 18.5. For children and adolescents, weight-for-age and height percentiles are used to assess underweight status. BMI has limitations and does not measure body composition; some individuals with a low BMI may be healthy, particularly if they have a lean body habitus or high muscle mass. Unintentional underweight—meaning weight loss without trying—can indicate an underlying health problem and should be evaluated.

Causes and risk factors include inadequate caloric intake relative to needs, high energy expenditure (for example due to intense physical activity or illness), malabsorption, chronic diseases (such as cancer or gastrointestinal disorders), metabolic or hormonal conditions, infections, medications, and eating disorders. Social and economic factors may contribute as well.

Health effects associated with underweight can include weakened immune function, poor wound healing, anemia, electrolyte disturbances, menstrual irregularities or fertility problems, osteoporosis, fatigue, and hair loss. In older adults, underweight raises the risk of frailty and falls.

Assessment typically involves a medical history, documentation of weight trends, physical examination, calculation of BMI, and growth charts for younger people. Laboratory tests may be used to identify nutritional deficiencies and underlying conditions.

Management aims to address the root cause and promote safe, gradual weight gain when appropriate. This may involve nutrition therapy with energy- and nutrient-dense foods, behavioral counseling for eating disorders, treating illnesses, and monitoring for complications such as refeeding syndrome in severely malnourished individuals. Regular follow-up with healthcare providers guides progress and safety.