Anorexia Nervosa with dangerously low body weight meets all definitional requirements for Anorexia Nervosa, with BMI under 14.0 kg/m² in adults or under the 0.3rd percentile for BMI-for-age in children and adolescents. In the context of Anorexia Nervosa, severe underweight status is an important prognostic factor that is associated with high risk of physical complications and substantially increased mortality.