You need vitamin D to absorb calcium and build bones. But more isn’t better. Researchers randomly assigned 311 adults aged 55 to 70 to take one of three daily doses of vitamin D: 400 IU, 4,000 IU, or 10,000 IU. Their blood levels ranged from 12 to 50 nanograms per milliliter when the study started, so some were in the “inadequate” range (12 to 20 ng/mL). None were “deficient” (less than 12 ng/mL). After three years, arm bone density was lower in those who took 4,000 IU or 10,000 IU a day than in those who took 400 IU, and leg bone density was lower in those who took 10,000 IU than in those who took 400 IU. Bone strength was no different.