Preoperative Very-Low-Calorie Ketogenic Diet Versus Low-Calorie Diet in Bariatric Surgery: A Prospective Comparative Study

Background: The very-low-calorie ketogenic diet (VLCKD) is increasingly used before bariatric surgery (BS), but its effects on surgical and long-term outcomes remain unclear. Objective: The aim of this study was to compare the impact of a 4-week VLCKD with a 4-week low-calorie diet (LCD) on preoperative, perioperative and postoperative outcomes for up 12 months in patients undergoing BS. Methods: In this prospective study, 72 (n = 36: VLCKD; n = 36: LCD) patients (mean age 43.2 ± 10.6 years; BMI 45.6 ± 6.4 kg/m2; 87.5% female) submitted to sleeve gastrectomy were enrolled at a tertiary care center from 2022 to 2024. Results: No adverse events were detected with both diets. The VLCKD was associated with a greater preoperative median weight loss percentage (−5.5 vs. −2.6 kg, p < 0.001), BMI reduction (−2.6 vs. −1.2 kg/m2, p < 0.001), shorter hospital stay (3.0 ± 0.2 vs. 3.4 ± 0.9 days, p = 0.017), higher day-1 postoperative hemoglobin (12.7 ± 1.3 vs. 12.0 ± 1.2 g/dL, p = 0.024), and lower day-1 postoperative median C-reactive protein levels (9.7 vs. 13.4 mg/L, p = 0.042). These associations were confirmed in a multiple regression model, after adjustments for BMI at enrolment, age and sex. After 6 months, the VLCKD was associated with greater reductions in mean weight loss percentage (−24.9 ± 7.8 vs. −19.6 ± 9.4 kg, p = 0.012), BMI reduction (−11.7 ± 4.4 vs. −8.7 ± 3.9 kg/m2, p = 0.003), neck circumference (−4.9 vs. −3.6 cm, p = 0.027) and lower aminotransferase levels. At 12 months, VLCKD patients maintained significant advantages over the same variables, except for neck circumference. Conclusions: A short preoperative VLCKD was safe and was associated with greater short-term weight reduction compared with the LCD, with potential benefits extending to early postoperative recovery and 1-year outcomes.