To address this question we will recruit patients with T2DM whose diabetes is controlled with either diet+exercise or with metformin (another medicine commonly used to treat T2DM). Subjects will undergo measurement of body fat and bone mineral density by DEXA scanning and a 3-hour mixed meal test. During the mixed meal test blood samples will be taken to measure how much GLP-1 and GIP are produced. Markers of bone formation will also be measured in blood samples obtained during the mixed meal test. Subjects will then be randomly assigned to 8 weeks of treatment with either sitagliptin (100 mg/day) or matching placebo (an inactive tablet that does not contain medication). Subjects will be seen 4 weeks after commencing treatment to assess safety and tolerability. After 8 weeks of treatment the meal test will be repeated. Subjects will then be washed off of their initial treatment (sitagliptin or placebo) for 1 week (that is, they will receive no study medication during this period). After the washout period, they will commence a second 8-week period of treatment with the other study medication (that is, if they received sitagliptin initially, they will receive placebo during period 2 and vice-versa). At the end of period 2, subjects will undergo a third mixed meal test with measurement of GLP-1, GIP and markers of bone turnover.