Men and postmenopausal women, age 50 and older, will be enrolled in this 42-day study in order to have 20 completers. Subjects may continue in the study for up to an additional two weeks should this be necessary for reasons related to intercurrent illness (unrelated to the research) or scheduling circumstances. Over the first 15 days, subjects will be placed on gradually increasing doses, up to a maximum of 90 mmol/d of potassium bicarbonate or placebo. They will remain on this dose for the rest of the study. Subjects will then have two 10-day metabolic diet cycles each, in random order, as follows: