Dietary Intake of Tryptophan and Metformin Response

Inclusion Criteria: 1. healthy adult male or non-pregnant female volunteers (age 20-40) 2. able and willing to give consent Exclusion Criteria: 1. Age less than 20 or greater than 40 2. Women who are pregnant, nursing, or at risk of becoming pregnant 3. Body mass index (BMI) less than 20 or greater than 28 4. Changes of more than 5 pounds in weight (increase or decrease) during the month prior to enrollment in the study 5. Participation in more than 300 minutes of exercise per week during the month prior to enrollment in the study 6. Known diabetes or pre-diabetes (based on prior diagnoses, use of medications to lower glucose; or fasting blood glucose \> 100mg/dL at screening) 7. Untreated hypertension (defined as systolic blood pressure \> 140mmHg and diastolic blood pressure \> 90mmHg) 8. Current or past mood disorder, including major depression, anxiety, or bipolar disorder 9. Conditions causing intestinal malabsorption, including celiac disease or a history of intestinal or gastric bypass surgery 10. History of liver disease and/or aspartate aminotransferase (AST) or alanine aminotransferase (ALT) more than 3 times upper limit of normal (ULN) 11. Estimated glomerular filtration rate (eGFR) \< 60 ml/min/1.73 m2 as calculated by the Modification of Diet in Renal Disease equation 12. Currently taking or intending to take during the study duration any medication known to affect glycemic parameters, such as glucocorticoids, growth hormone, or fluoroquinolones 13. Use of medications and herbal or vitamin supplements during the study 14. Contraindications to safe use of metformin, including planned radiologic or angiographic study requiring contrast within one week of the study completion 15. Objection to taking metformin 16. Participation in any other interventional study during the study duration 17. Use of nicotine-containing products, including inhaled, chewed, or patches during the study. 18. Use of drugs of abuse. 19. Restrictions that prevent adherence to standardized meals or unwillingness to adhere to a pre-specified meal plan, including abstinence from alcohol and limitation of caffeine intake to one drink daily.