Anorexia nervosa is a psychological condition that involves an eating disorder. In the disorder individual loses more weight than is healthy for their height and age. The individual will maintain a body weight of 85 percent or less of their expected weight. Symptoms include a very low body mass index (BMI), a refusal to eat, and attempts to lose weight, even when body mass index is very low. It is thought to be triggered by a combination of biological, environmental, and genetic factors.