The aim of our study was to evaluate the safety and efficacy of a 2-week ultra-low-calorie, high-protein, low-carbohydrate diet to reduce body weight, left liver volume, and correct biochemical parameters in patients with planned bariatric surgery. A retrospective analysis of the treatment of 32 patients (23 men and 9 women) with morbid obesity who were in the surgical department from 2018 to 2021. They underwent preoperative preparation followed by laparoscopic hose resection of the stomach. The mean age of patients was 43.8 ± 11.7 years, body weight - 175.5 kg (167.5-193.8 kg), body mass index (BMI) - 58.8 ± 10.3 kg/m2, area left lobe of the liver - 80.9 ± 18 cm2, the average thickness of subcutaneous tissue - 36.5 (25-40) mm, the average level of ALT - 45.8 (41.4 - 61.8) μmol/l, AST - 42, 4 (34.8 - 52.5) μmol/l, total cholesterol - 6.09 ± 0.79 mmol/l, HDL-cholesterol - 0.93 ± 0.22 mmol/l, LDL-cholesterol - 3.75 ± 0.99 mmol/l and triglycerides - 2.09 ± 0.66 mmol/l. 2 weeks after the start of the diet, the percentage of weight loss (% BMD) reached 9.9 ± 2.7% (5.6-16.5%), body weight - 164.5 kg (153.8-184.3 kg) and the average value of BMI - 55.4 ± 9.7 kg/m2, the area of the left lobe of the liver - 69.6 ± 17.5 cm2, the average thickness of subcutaneous tissue - 31.5 (22-37) mm, the average level of ALT - 42.8 (28.8 - 49.2) μmol/l, AST - 29.3 (22.9 - 37.7) μmol/l, total cholesterol - 5.24 ± 0.83 mmol/l, HDL -HS - 1.19 ± 0.2 mmol/l, LDL-cholesterol - 4.57 ± 0.87 mmol/l and triglycerides - 1.65 ± 0.78 mmol/l. Thus, prescribing an ultra-low-calorie, high-protein, low-carbohydrate diet is a safe and effective way to reduce excess weight, reduce anterior abdominal wall thickness, reduce left liver size, and correct biochemical parameters in high-risk patients before bariatric surgery.