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.

Inclusion Criteria:

• Female
• Age 18-65
• 3-5 years post Roux-en-Y gastric bypass and experienced successful weight loss following surgery (up to n=10) or
• 3-5 years post Roux-en-Y gastric bypass and did not experience optimal weight loss following surgery (up to n=10) or
• Nonsurgical control participants who are approximately matched in sex, age, and BMI to the optimal weight loss group (up to n=10).

Exclusion Criteria:

• Tobacco use in past three months.
• Taking a medication which is known to significantly influence gastrointestinal transit time or affect the microbiome or other variables significantly (as determined by study pharmacist/MD).
• History of diabetes mellitus.
• Has taken an oral or IV/IM antibiotic in the past 3 months.
• Has taken probiotic and/or prebiotic agent in the past 3 months
• History of significant intestinal disease or disorder (e.g., Crohn's disease, ulcerative colitis, irritable bowel syndrome)
• History of gastrointestinal surgery that may impact measures of biological variables, as determined by the investigator.
• Medical condition expected to impact the biological variables of interest or interfere with providing a sample, as determined by the investigator.
• Unable to speak/read English.
• Breastfeeding, pregnant, or planning to become pregnant within the duration of the study.
• Unwilling to use a medically acceptable form of contraception.
• History of bipolar or psychotic spectrum disorder or alcohol or substance treatment in the past year

3-5 years post gastric bypass patients and control subjects