Vitamin D supplements are widely recommended to promote bone health in the general population, but data are inconsistent regarding whether they prevent fractures, which are a major public health problem. A randomized, controlled trial investigated whether supplemental vitamin D, as compared with placebo, would result in a lower risk of incident fractures among generally healthy U.S. adults who weren't recruited for the trial based on vitamin D deficiency, low bone mineral density, or fracture history.

A total of 25,871 men (ages 50 and older) and women (ages 55 and older) were enrolled from all 50 states and followed for a median of 5.3 years. Participants were randomly assigned to one of four groups: vitamin D (2000 IU/day) plus n-3 fatty acids (1 g/day), vitamin D plus placebo, n-3 fatty acids plus placebo, or double placebo. Baseline characteristics were balanced between the trial groups. Only 2.4% of participants had baseline 25-hydroxyvitamin D levels of less than 12 ng/mL.

During follow-up, there were 1,991 confirmed fractures in 1,551 participants. Compared with placebo, supplemental vitamin D didn't have a significant effect on first incident total fractures, which occurred in 769 of 12,927 participants in the vitamin D group and 782 of 12,944 participants in the placebo group. The risk of nonvertebral and hip fractures was also similar in the vitamin D and placebo groups. The effects of vitamin D supplementation weren't modified by baseline age, sex, race or ethnic group, body mass index, personal use of supplemental calcium or vitamin D, or serum 25-hydroxyvitamin D levels. The incidence of hypercalcemia and kidney stones wasn't substantially different in the vitamin D and placebo groups.

The authors note that only one dose of vitamin D was evaluated, and the trial wasn't designed to evaluate the effects of supplementation in adults who are deficient in vitamin D. Also, the results may not be generalizable to adults who have osteoporosis or osteomalacia or older adults who are institutionalized.