Primary objective: To evaluate the consistency of results between a novel oral glucose solution and glucose powder for oral glucose tolerance tests (OGTT). Secondary objectives: 1) To compare subjects' preferences for the two glucose formulations; 2) To evaluate the safety and tolerability of the novel oral glucose solution and the marketed oral glucose powder for glucose tolerance tests; 3) To evaluate the effects of different glucose formulations on subjects' plasma glucose, insulin, and C-peptide concentrations.