Background. Modern technologies of enteral and parenteral nutrition, along with antibiotic therapy, respiratory and inotropic support, and rational infusion therapy, are now an integral part of the complex of therapeutic measures performed in patients with peritonitis, sepsis, polytrauma, and multiple organ dysfunction. Despite this, among patients who underwent operative intervention, malnutrition syndrome is diagnosed in 14-15%. In our opinion, this is due, first of all, to an insufficient assessment of the trophic status of the patient. The aim of the study. To compare different methods for assessing a nutritional status and calculating real energy requirements and plastic material (protein) in patients with peritonitis. Materials and methods. The medical history of 85 patients operated on for peritonitis of various etiologies in Belgorod City Hospital 1. The data obtained from clinical, laboratory, anthropometric research methods were aimed at studying the nutritional status of patients by the known method proposed by Luft V.M. and Kostyuchenok A.L. (2001) in a comparative aspect with the method of questioning patients on the scale "Mini research of nutritional status" (MNA). The energy requirement was calculated by the degree of catabolism and using the original program for smartphones "Optimizing the Nutrition of Patients with Malnutrition Syndrome" based on the Harris-Benedict formula. Results. The first stage of the study was a comparative analysis of methods for determining the trophic status, which showed that it was possible to conclude that in urgent surgery for rapid diagnosis of trophic status, the most convenient method for interviewing was the use of the MNA scale. At the second stage of our study, when conducting a comparative analysis of the energy needs of a patient with peritonitis, it was proven that the MNA scale is a more cost effective and convenient scale for assessing the nutritional status, and in order to calculate real energy requirements and protein in patients in emergency surgery, the use of the original program "Optimizing the Nutrition of Patients with Malnutrition Syndrome" on the basis of the supplemented Harris-Benedict formula is justified. Conclusion. Thus, timely preoperative and early postoperative nutritional support in elderly and senior patients who underwent surgery, especially in patients with peritonitis, helps reduce the risk of malnutrition syndrome, as well as reduce the costs of treatment and hospital stay.