Endogenous GLP-1 Secretion on Islet Function in People With and Without Type 2 Diabetes

Inclusion criteria - non-diabetic subjects: * Weight-stable, non-diabetic subjects Exclusion Criteria - non-diabetic subjects: * Age \< 25 or \> 65 years (to avoid studying subjects who could have latent type 1 diabetes, or the effects of age extremes in subjects with normal or impaired fasting glucose). * HbA1c ≥ 6.5% * Use of glucose-lowering agents. * For female subjects: positive pregnancy test at the time of enrollment or study * History of prior upper abdominal surgery such as adjustable gastric banding, pyloroplasty and vagotomy. * Active systemic illness or malignancy. * Symptomatic macrovascular or microvascular disease. Inclusion criteria - diabetic subjects: * Weight-stable, diabetic subjects treated with diet and lifestyle alone or with metformin monotherapy Exclusion Criteria - diabetic subjects: * Age \< 25 or \> 65 years (to avoid studying subjects who could have latent type 1 diabetes, or the effects of age extremes in subjects with normal or impaired fasting glucose). * Use of any glucose-lowering agent other than metformin. * 2 or more fasting glucose values \> 250mg/dl on medication or after medication withdrawal. * Unwillingness or inability to withdraw medication for three weeks prior to, and for the duration of the study. * For female subjects: positive pregnancy test at the time of enrollment or study * History of prior upper abdominal surgery such as adjustable gastric banding, pyloroplasty and vagotomy. * Active systemic illness or malignancy. * Symptomatic macrovascular or microvascular disease.