Inclusion Criteria:

• Age: 18-65 years old.
• Willing to provide consent.
• Gender: Men or women.
• Women of childbearing potential will have negative pregnancy tests within 48 hours of enrolment.
• BMI: ≥ 35.
• Individuals who have elected to have RYGB at the Mayo Clinic Bariatric Center of Excellence, and individuals with a BMI ≥ 40 who are otherwise healthy with controlled cardiac, pulmonary, gastrointestinal, hepatic, renal, hematological, neurological, or endocrine disorders. The RYGB participants will be invited to participate when their surgery is scheduled at the Mayo Clinic Bariatric Center of Excellence.
• The non-RYGB participants (BMI ≥ 40) will be otherwise healthy individuals with no unstable psychiatric disease and no unstable cardiac, pulmonary, gastrointestinal, hepatic, renal, hematological, neurological, or endocrine disorders.

Exclusion Criteria:

• Positive history of chronic gastrointestinal diseases, or systemic disease that could affect gastrointestinal motility, or use of medications that may alter gastrointestinal motility, appetite or absorption; e.g., orlistat, phentermine.  
• Significant untreated psychiatric dysfunction based upon screening with the Hospital Anxiety and Depression Inventory (HAD), a self-administered alcoholism screening test (AUDIT-C) and the Questionnaire on Eating and Weight Patterns (binge eating disorders and bulimia). If such a dysfunction is identified by a depression or anxiety score > 11 or difficulties with substance or eating disorders, the participant will be excluded and given a referral letter to his/her primary care doctor for further appraisal and follow-up. The AUDIT-C is a 3-item alcohol screening questionnaire that reliably identifies participants who are hazardous alcohol drinkers or have active alcohol use disorders. In men, a score of 4 or more is above the recommended limits will be reviewed by study personnel.In women, a score of 3 or more is above the recommended limits will be reviewed by study personnel.However when the points are above recommended limits, the provider will review the patient’s alcohol intake over the past few months to confirm accuracy and determine study eligibility.
• Any acute or chronic condition or other disease that, in the opinion of the Investigator, would limit the subject’s ability to complete and/or participate in this clinical study.
• Participant activities: if recruited for the investigation participants will be asked to refrain from donating blood; refrain from participating in other research studies; avoid taking any additional over the counter or prescription medications or herbal supplements that have not been reviewed and approved by the physician or the study coordinator until the study has been completed.