This is a randomized, controlled trial of calcium carbonate supplementation (1500 milligrams per day) in third-decade women with low calcium-to-protein intakes. We accept women aged 19-27 on the basis of good health and the 7-day food diary demonstrating a dietary calcium-to-protein ratio (in milligrams:grams) that does not exceed 13. The outcome variable is the change in BMD at hip, total body, forearm, and spine. Treatment lasts for 3 years. We expect that bone mass will increase in both groups but will increase to a greater extent in the calcium-supplemented group than in the nonsupplemented group.