The VITAL trial randomly assigned roughly 770 adults to take vitamin D (2,000 IU a day) or a placebo. (About 80 percent of them started with adequate vitamin D levels.)

After two years, vitamin D takers had no higher bone density—and no less bone loss—in the spine, hip, or whole body than placebo takers.

Findings for "free" vitamin D

Among people who entered the study with lower blood levels of "free" (unbound) vitamin D, vitamin D takers had a slight increase in spine bone, and less loss in hip bone, than placebo takers.

(Most blood tests measure total, not free, vitamin D.)

But those findings could be due to chance, so they need further study.

What to do now

Aim for recommended intakes of vitamin D (600 IU a day up to age 70 and 800 IU a day over 70), and no more.

Fortified foods typically have only 40 to 100 IU per serving, so most people need a daily multivitamin or vitamin D supplement to reach 600 or 800 IU.