My mother has always been a big proponent of calcium supplements. She had me swallowing those green pills in elementary school and was equally diligent about taking them herself. As a physician, she knows the importance of calcium for healthy bones; as the daughter of an osteoporosis patient who has suffered multiple fractures, she knows of the pain caused by brittle ones.

After her bone density scan last October, however, my mother received disturbing news: She was diagnosed with osteopenia, a...