['Roux-en-Y gastric bypass (RYGB) surgery is an effective and frequently used surgical treatment for severe obesity. Postoperative weight loss varies markedly, but biomarkers to predict weight loss outcomes remain elusive. Levels of the satiety gut hormones glucagon like peptide-1 (GLP-1) and peptide YY (PYY) are attenuated in obese patients but elevated after RYGB surgery. We aimed to evaluate the preoperative responses of GLP-1 and PYY to a standard meal as a predictor of weight loss after RYGB surgery. We hypothesized that weak satiety gut hormone responses preoperatively, would predict poor weight loss after RYGB surgery.', 'Preoperatively 43 patients (F = 25/M = 18) had GLP-1 and PYY measured in the fasting state and at 30-minute intervals over 180 minutes after a standard 400 kcal mixed meal. Weight loss was assessed at weight stability after surgery (mean 16.2 mo [CI 15.516.9]).', 'Body mass index decreased from 44.0 kg/m(2) (CI 42.2-45.7) before surgery to 30.3 kg/m(2) (CI 28.4-32.2) after surgery (P<.001). Preoperative GLP-1 and PYY responses to food intake; as delta value between fasting and maximum as well as total responses during 180 minutes did not correlate to total weight loss (GLP-1; rho = .060 and rho = -.089, PYY; rho = -.03 and rho = -.022, respectively) or to excess weight loss % (GLP-1; rho = .051 and rho = -.064, PYY; rho = -.1 and rho = -.088, respectively).'] Does preoperative assessment of gut hormones correlate to weight loss after Roux-en-Y gastric bypass surgery?