Low-energy diets before metabolic bariatric surgery: A systematic review of the effect on total body weight, liver volume, glycemia and side effects.

There is no consensus regarding energy content or duration of hypocaloric diets used for preoperative optimization of patients before metabolic bariatric surgery. In this systematic review, we aimed to compare the effect of different hypocaloric diets on reductions in total body weight, liver volume, glucose and insulin concentrations, and side effects. Six databases were searched for articles including adults with BMI ≥35 kg/m² treated with hypocaloric diets before metabolic bariatric surgery. Hypocaloric diets were categorized as (1) low-energy diet containing 800-1200 kcal/day for 2-4 weeks, (2) very low-energy diet containing 450-<800 kcal/day for 2-4 weeks, and (3) low-energy diet containing 800-1200 kcal/day for >4 weeks. Thirty-three articles (1868 patients) were included, and if data were sufficient, synthesis without meta-analysis was conducted. A low-energy diet and very low-energy diet for 2-4 weeks resulted in similar reductions in total body weight, but longer treatment correlated to a more pronounced weight reduction. In addition, a low-energy diet for 2-4 weeks led to decreased liver volume, which might facilitate the surgical procedure. Insulin resistance was generally reduced after a low-energy diet for 2-4 weeks. However, most studies were within-group control, and since more than 60% of the studies lacked variance measures for our outcomes, we did not perform a meta-analysis. Accordingly, our results should be interpreted carefully. The protocol was registered in the International Prospective Register of Systematic Reviews (PROSPERO) with registration number: CRD42022295757; available at: https://www.crd.york.ac.uk/prospero/display_record.php?RecordID=295757.