| Outcome Measures: |
Primary: Dipeptidyl peptidase 4 (DPP4), DPP4 activity and antigen concentration, during study days 1 and 2 | Secondary: Glucose Area Under the Curve, Glucose will be measured before after the mixed meal during sitagliptin and placebo. Area under the curve will be calculated based on at least 10 time points after the meal., Before the meal and for 4 hours after (t=-15 through t=240 min) on study days 1 and 2|Intact DPP4 substrates with cardiovascular properties (other than GLP-1), Cardiovascular biomarkers include: CXCL12, substance P, neuropeptide Y, and brain natriuretic peptide. These are peptides that are also DPP4 substrates and are rapidly inactivated by this peptidase., Before the meal (t=-15 or -1 min) on study days 1 and 2|Disposition index, Disposition index will be calculated from insulin sensitivity and insulin secretion. These variables will be computed using mathematical modeling of insulin and c-peptide. We will collect insulin and c-peptide at least 10 time points after the meal., Calculated from samples collected before the meal and for 4 hours after (t=-15 through t=240 min) on study days 1 and 2|Mean blood pressure, Measured via an automated blood pressure cuff approximately every 15 minutes, Before the meal and for 4 hours after (t=-15 through t=240 min) on study days 1 and 2|Glucagon-like peptide-1 (GLP-1), This is released in response to a meal and rapidly degraded by DPP4. We will collect samples for at least six time points after the meal., Before the meal and for 4 hours after (t=-15 through t=240 min) on study days 1 and 2|CD26, CD26 is DPP4 on T cells and monocytes., Before the meal or at baseline (t=-15 or -1 min) on study days 1 and 2|Surrogate markers of lipolysis, Triglycerides, free fatty acids; We will collect samples at least six time points after the meal., Before the meal and for 4 hours after (t=-15 through t=240 min) on study days 1 and 2|Mean heart rate, Measured via an automated blood pressure cuff approximately every 15 minutes, Before the meal and for 4 hours after (t=-15 through t=240 min) on study days 1 and 2
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