BACKGROUND: Despite a lack of demonstrated patient benefit, many insurance providers mandate a physician-supervised diet before financial coverage for bariatric surgery. OBJECTIVES: To compare weight loss between patients with versus without insurance mandating a preoperative diet. SETTING: University hospital, United States. METHODS: Retrospective study of all patients who underwent laparoscopic Roux-en-Y gastric bypass or sleeve gastrectomy over a 5-year period, stratified based on whether an insurance-mandated physician-supervised diet was required. Weight loss outcomes at 6, 12, and 24 months postoperation were compared. Linear mixed-models and backward-stepwise selection were used. P<0.05 was considered significant. RESULTS: Of 284 patients, 225 (79%) were required and 59 (21%) were not required to complete a preoperative diet by their insurance provider. Patients without the requirement had a shorter time to operation from initial consultation (P = .04), were older (P<.01), and were more likely to have government-sponsored insurance (P<.01). There was no difference in preoperative weight or body mass index or co-morbidities. In unadjusted models, percent excess weight loss was superior in the group without an insurance-mandated diet at 12 (P = .050) and 24 (P = .045) months. In adjusted analyses, this group also had greater percent excess weight loss at 6 (P<.001), 12 (P<.001), and 24 (P<.001) months; percent total weight loss at 24 months (P = .004); and change in body mass index at 6 (P = .032) and 24 (P = .007) months. There was no difference in length of stay or complication rates. CONCLUSIONS: Insurance-mandated preoperative diets delay treatment and may lead to inferior weight loss.