 The purpose of this study is to investigate mechanisms responsible for weight change in patients who have undergone weight loss surgery. Specifically, we will compare the gut microbiota, plasma bile acids, plasma gut peptides GLP-1, GLP-2, and PYY, and plasma LPS in three groups of subjects: 3-5 years post gastric bypass patients who experienced sub-optimal weight loss, 3-5 years post gastric bypass patients who had successful weight loss, and a control group who has not had a weight loss surgery and are of similar age, gender, body mass index as the gastric bypass groups. 