Impact of a very low-calorie ketogenic diet on metabolic and microbiota outcomes in post-bariatric patients and bariatric-Naïve individuals: A comparative pilot study

Aims: To date, bariatric surgery (BS) is the most effective long-term treatment for obesity, but weight regain (WR) is common. The very low-calorie ketogenic diet (VLCKD) is effective for weight loss and may influence gut microbiota (GM) composition, but it has been scarcely evaluated in post-bariatric patients. This study compared the efficacy and safety of a VLCKD in patients with WR post-bariatric surgery (BS+) and in bariatric surgery-naïve patients (BS-). Methods: In this prospective, case–control study, 33 patients (15 BS+, 18 BS-) underwent an 8-week-long VLCKD. Outcomes included weight loss, metabolic profile, safety and GM composition. Results: Both groups achieved significant weight loss (BS+: −6.9%, BS-: −8.3%), but the BS+ group showed slightly less metabolic improvement, particularly in insulin resistance and triglycerides. GM composition differed at baseline, reflecting the lasting effects of BS, and VLCKD led to significant changes in both groups. Microbial diversity and specific taxonomic shifts were more pronounced in BS- patients. Mild renal function changes were noted in BS+ patients, though these remained within clinically acceptable ranges. Conclusion: VLCKD is effective in both BS+ and BS- patients, though metabolic and microbial responses may be less robust post-surgery, possibly due to anatomical and physiological changes. Tailored approaches may be therefore needed to optimize outcomes in post-bariatric patients.