The VITamin D and OmegA-3 TriaL (VITAL; NCT 01169259) is a randomized clinical trial in 25,871 U.S. men and women investigating whether taking daily dietary supplements of vitamin D3 (2000 IU) or fish oil (1 gram of omega-3 fatty acids) reduces the risk of developing cancer, heart disease, and stroke in people who do not have a prior history of these illnesses. This ancillary study is being conducted among a sub-cohort of 771 participants in VITAL and will test the efficacy and safety of high-dose vitamin D supplementation vs. placebo on skeletal health and body composition.

The VITAL: Effects on Bone Structure and Architecture is an ancillary study of the parent VITAL. This study has enrolled a sub-cohort of 771 VITAL participants at the NIH-sponsored Harvard Catalyst Clinical and Translational Science Center (CTSC). The following measurements will be performed at baseline and 2 years post-randomization to determine whether high-dose vitamin D supplementation vs. placebo: 1) produces small increases or reduces bone loss in spine, hip, and total body areal bone density as assessed by dual x-ray absorptiometry (DXA); 2) reduces bone turnover as assessed by biomarkers of bone resorption and formation; 3) improves (a) volumetric bone mineral density (vBMD) and measures of bone structure as assessed by peripheral quantitative computed tomography (pQCT) and (b) bone microarchitecture as assessed by high resolution pQCT (HR-pQCT) at the distal radius and tibia as well as trabecular bone score (TBS) at the spine; and 4) results in changes in body composition as assessed by DXA. Parallel assessments of the effects of omega-3 fatty acids will be performed. This study may elucidate the mechanisms through which high-dose vitamin D may prevent age-related fractures and provide new insights into the role of vitamin D on skeletal health and body composition.

We are currently analyzing the effects of daily dietary supplements of fish oil (1 gram of omega-3 fatty acids) vs. placebo on skeletal health and body composition.

Participants in VITAL (NCT 01169259) who meet the following criteria are eligible to participate in this ancillary study:

• Those who are using bisphosphonates currently or within the past 2 years, or other bone-active medications currently or within the past year are not eligible for this study.

Baseline - 2 years

Timeline limited to 2 years of follow-up; results not adjusted for multiple hypothesis testing, so findings from the secondary and subgroup analyses are exploratory; results are not generalizable to younger individuals or adults with osteoporosis.

Double Blind