Each eligible participant will be administered an oral glucose tolerance test so he/she can be assigned to the group with "normal glucose tolerance", "impaired glucose tolerance" (between normal and diabetic), or type 2 diabetes mellitus. Each study subject will then be administered a meal tolerance test (MTT) on 4 separate occasions. For the MTT, a liquid meal (Boost Plus)will be ingested following an overnight fast. A primed-continuous infusion of vehicle alone, GIP alone, xenin-25 alone, or the combination of GIP plus xenin-25 (each peptide at a dose of 4 pmoles x kg-1 x min-1) will be initiated at the same time the meal is ingested. Blood samples will be collected before and during the MTT for the measurement of glucose, insulin, C-peptide, glucagon, GIP and xenin-25 levels.