A person is considered underweight if his or her body mass index (BMI) falls below a certain threshold (body mass index is a measure determined by a person's age, height, and weight). For infants and children, a BMI below the 10th percentile for a specific age indicates an individual who is underweight. For adults, a BMI below 19.1 for females and 20.7 for males is considered underweight. A BMI of 17.5 indicates an individual is very underweight.
Individuals who are underweight are at high risk for malnutrition. Being underweight can affect growth and development, and it can cause infertility or delayed menstruation. It can also result in fatigue, irritability, and a lack of concentration, as well as impairing the body's ability to thermoregulate itself. Due to a decreased immune response, underweight individuals are less resistant to infections and disease.
It is recommended that underweight individuals gain one pound per week until an appropriate weight is reached. This can be accomplished by consistently (daily) increasing one's intake of calorically denser foods (i.e., nuts instead of pretzels), eating more frequently, and drinking fluids between meals rather than with meals.
Duyff, Roberta L. (1996). The American Dietetic Association's Complete Food and Nutrition Guide. Minneapolis, MN: Chronimed.
American Dietetic Association. Available from <http://www.eatright.org>