Osteoporosis is a major health concern worldwide. While there are drugs available for the treatment and prevention of osteoporosis, they are not practical for population-wide prevention efforts. Demonstrating the effectiveness of safe and widely available dietary interventions to prevent osteoporosis could have important public health ramifications. Different food sources of dietary protein may have different effects on bone metabolism. Animal foods provide a dietary acid load that may lead to negative calcium balance and increased bone resorption. In contrast, vegetable sources of protein, while providing some acid due to their protein content, provide proportionally more base that counters the dietary acid load. The effect of dairy products, which are rich in animal protein but also contain potential base precursors not found in vegetable foods, has not been established. Finally, soy protein sources may have a dual benefit: soy foods provide base precursors as well as plant estrogens that may have a beneficial effect on bone. We are resubmitting this proposal to randomize postmenopausal women to one of four diets equal in calories, protein, calcium, and sodium. The diets will differ by having 80 percent of the protein from one of four sources: non-dairy animal, vegetable, dairy, or soy foods, resulting in significant differences among the diets in acid, base, and isoflavone content. All food will be prepared and provided by the General Clinical Research Center. The subjects will consume the diets for 6 weeks with measurements of acid-base status, isoflavone excretion, and calcium metabolism. This will be the first intervention study to examine the effect of different sources of dietary protein in whole foods on calcium metabolism. Eventually our findings could have substantial public health implications and provide a widely available and low risk means to help prevent osteoporosis.