The basic criterion for the diagnosis of anorexia (AN - anorexia nervosa) by ICD-10 (International Classification of Diseases, version 10) is the body weight less than 15% of the expected normal body weight. According to DSM-IV (Diagnostic and Statistical Manual for Mental Disorders, version IV) the basic feature of AN is a refusal to maintain body weight equal or greater than the minimal normal weight. The prevalence of anorexia nervosa is 0.3-0.5% or even 1.3-3.7% if include pre-anorexic states (eg. the phenomenon of pro-ana). The main feature of anorexia is a reduction of caloric intake. According to the recommendations of the American Psychiatric Association (APA) for nutritional treatment of patients with AN the main goals in therapy of AN are: restoration of body weight, normalization of eating patterns, achievement a normal feeling of hunger and satiety and correction of the consequences of improper nutrition. APA suggests that achievable weight gain is about 0.9-1.4 kg per week in the case of hospitalized patients and approximately 0.23-0.45 kg per week in the case of outpatients. During the nutritional treatment of AN numerous side effects including anxiety, phobia, occurrence of obsessive thoughts and compulsive behavior, suicidal thoughts and intentions may occur. According to National Institute for Clinical Excellence (NICE) the most important goal of AN therapy is weight gain in the range of 0.5-1 kg per week in hospitalized patients and 0.5 kg per week for outpatients. A person suffering from anorexia in the initial period of nutritional treatment spends twice more energy to maintain elevated body temperature, which significantly increases during the night rest. This phenomenon is called nocturnal hyperthermia and has a negative effect on the healing process. "Refeeding syndrome" is an adverse effect of nutritional treatment in anorexia. It is caused by too rapid nutrition in a patient suffering from chronic starvation. It can endanger the patient's life.