Modulating Glucose Tolerance With Dietary Tyrosine

Metabolic or Bariatric surgery is an effective treatment for type 2 diabetes mellitus (T2DM) diabetes associated with obesity. There remain some questions about the biochemical mechanism that drive how these surgeries work to reverse hyperglycemia. In the proposed human studies, the investigators will test the hypothesis that the amino acid tyrosine is a key metabolite in regulating blood sugar levels and that manipulation of the amount tyrosine supplied by nutrition is able to achieve some of the metabolic benefits seen in the early post-surgical period following bariatric surgery. The central hypothesis is that that the tyrosine content of the meal challenge affects post-prandial intestinal and plasma dopamine and levodopa and L-3,4-dihydroxyphenylalanine (L-DOPA) levels, which, in turn, impact β-cell insulin secretion and glucose excursions. The investigators now propose to characterize the possible effects of manipulating dopamine and L-DOPA levels in the gut and plasma on glucose tolerance, insulin secretion, and insulin sensitivity in healthy volunteers with a range of body mass indexes (BMIs).

Several biochemical mechanisms explaining how Roux-en-Y Gastric Bypass (RYGB) provides an effective treatment for obesity associated type 2 diabetes mellitus (T2DM) and improves hyperglycemia independently of weight loss have been proposed. Two are of particular interest; a) the hindgut hypothesis suggesting that nutrient delivery to the distal intestine drives the production of "incretins" which enhance insulin secretion (e.g. glucagon-like peptide-1 (GLP-1)), and b) the foregut hypothesis, positing that foregut bypass reduces the secretion of factors (i.e. anti-incretins) that normally defend against hypoglycemia. The investigators have been actively investigating this topic and have developed a hypothesis based on past studies that they wish to test in a limited human clinical study. In addition, preclinical data suggest that there exists a gut-to-beta cell pathway, responsive to nutritional tyrosine, regulating insulin secretion, and this pathway provides a mechanism for the early postoperative improvements in hyperglycemia observed in RYGB.

Eligibility Criteria:
i. Inclusion Criteria

1. Capable of giving written as well as oral informed consent.
2. A fasting plasma glucose level (FPG) \< 126 mg/dL (\< 7.0 mmol/L) and an Hb1ac in the 5.7-6.4 % range.
3. BMI in the range of 18-45 kg/m2.
4. Normal Complete blood count (CBC), renal and liver function tests.

ii. Exclusion Criteria:

1. Any diabetes medication within previous three (3) months.
2. Fasting plasma Glucose (FPG) \>126 mg/dl or HbA1c \> 6.4%
3. Current use (or within 6 months) of antipsychotic, anti-anxiety, or antidepressant medications (e.g. monoamine oxidase (MAO) inhibitors, 5-Hydroxytryptophan (5HT) inhibitors, tricyclic antidepressants, L-DOPA), reserpine, β-2-receptor agonists (e.g., terbutaline), steroids, weight loss medication, anticoagulant medication, over-the-counter nutritional supplements other than standard vitamin and mineral supplements
4. History of Phenylketonuria or other inherited disorders of amino acid metabolism.
5. History of movement disorder such as Parkinson's disease or Huntington's disease
6. Cardiovascular, renal, pulmonary, gastrointestinal, migraines or other medical conditions deemed significant by investigators
7. History of/ or psychiatric illness such as major depression, bipolar disease, anxiety or schizophrenia.
8. History of bariatric surgery with the exception of gastric band if the band has been removed
9. Female of child-bearing age, currently pregnant, breastfeeding or not using a form of birth control.
10. Previous or current use of cocaine, methamphetamine, ecstasy (3-4 methylenedioxymethamphetamine (MDMA))
11. Current daily intake of caffeine \>500 mg/day (\>4-5 cups of coffee; \>10 12-oz cans of soda)
12. Consumption of more than 1 alcoholic drink per day or smoking more than 5 cigarettes/day.
13. Systolic Blood Pressure (SBP) \> 150 mmHg; Diastolic Blood Pressure (DBP) \> 100 mmHg.
14. Recent history (in the past three months) of more than a 3% gain or loss in body wt.
15. Difficulty in swallowing capsules.
16. Concurrent use of antacids or proton pump inhibitors (e.g.,Prilosec Prevacid, dexilant, Aciphex, Protonix, Nexium, Vimovo, Zegerid)