Conversion Bariatric Surgery, Ketogenic Diet, and Intermittent Fasting in Bariatric Surgery Patients with Recurrent Weight Gain: a Prospective Randomized Controlled Trial

Recurrent weight gain (RWG) after metabolic and bariatric surgery (MBS) increases the need for alternative treatment strategies. This study evaluated the effects of conversion bariatric surgery (CBS), very low-calorie ketogenic diet (VLCKD), and time-restricted intermittent fasting (TRIF) on anthropometric measurements, biochemical parameters, and dietary habits in patients who experienced suboptimal clinical response (SCR) or RWG after bariatric surgery. This study included 56 patients, allocated to four groups (CBS, VLCKD, TRIF, and control; n = 14 each). Weight, waist-hip measurements, body composition, glycemic/lipid profile, and serum levels of specific vitamins and minerals were assessed at baseline and at week 6. Energy and nutrient intakes were calculated using BeBiS-9. Data were analyzed with SPSS 22.0. The percentages of total and excess weight loss differed significantly among the groups (p < 0.001), with CBS (9.07–28.5%), VLCKD (9.12–31.85%), TRIF (5.09–14.97%), and control (0.97–3.40%). Additionally, the pre- and post-intervention differences in fasting insulin, HOMA-IR, HbA1c, cholesterol, LDL-C, triglyceride, and uric acid levels varied significantly among the groups. VLCKD showed a more prominent effect on glycemic parameters, whereas CBS had a more beneficial impact on the lipid profile. In intervention groups, daily energy, carbohydrate and fat intake (g/day) decreased; protein percentages increased; the frequency of consumption of energy-dense foods decreased; and healthy food preferences increased. Consequently, clinically significant improvements in weight management and metabolic parameters were observed in CBS, VLCKD, and TRIF groups under multidisciplinary team follow-up. These findings suggest that dietitian-led VLCKD and TRIF interventions may be considered as alternative treatment options before deciding on CBS.