Anorexia nervosa is achronic disease classified as an eating disorder. It has amultifactorial aetiology and should be treated by a multidisciplinary team of specialists. Anorexia has many signs and symptoms, and usually affects young women. The complex clinical picture of anorexia is formed by numerous somatic and psychological symptoms. A high risk of mortality is associated with complications in all organs and systems. Body mass index (BMI) is an indicator in the diagnosis of anorexia, and BMI <15 kg/m² defines an extreme state in a patient. Data from a literature review and clinical practice show that a BMI <10 kg/m² is not uncommon. Aspecific BMI value associated with a particular medical complication has not been established, but many relationships have been reported. This article presents gastrointestinal and endocrine complications related to anorexia. It explains metabolic adaptations activated by the body to enable the survival of patients with very low body mass index (the lowest reported BMI was only 6.7 kg/m²).