Interrelationship of Vitamin D Supplementation, Adiposity and CVD Risk Factors in a Randomized Clinical Trial

Observational studies suggest that low 25-hydroxyvitamin D (25(OH)D) levels are associated with high BMI and fat mass (FM), but it is unknown whether vitamin D supplementation can alter body composition or adiposity. Despite enthusiasm for the use of vitamin D supplements to reduce FM, the hypothesis that vitamin D can modify adiposity remains unproven. Finding strategies to prevent obesity is of critical public health importance due to the high prevalence of overweight/obesity and its role in causing diabetes, hypertension, dyslipidemia, cardiovascular disease (CVD), among others. Observational studies also link low 25(OH)D levels to CVD risk factors related to obesity, but sequestration of vitamin D in fat tissue may be a confounding factor. The effects of vitamin D supplementation on body composition, adiposity and CVD risk factors are best tested in a randomized clinical trial (RCT), and according to the Institute of Medicine (IOM) 2011 report, more data from randomized clinical trials on these outcomes are needed. Previous trials of vitamin D have been limited by the inability to separate effects of supplemental calcium from vitamin D, small sample size, insufficient vitamin D dose, failure to monitor 25(OH)D levels or inadequate ascertainment of body composition. The NIH-funded VITamin D and OmegA-3 TriaL (VITAL) (1 U01 CA138962) affords a unique and cost-effective opportunity to investigate the effect of vitamin D on changes in body composition and to assess whether changes in CVD risk factors are mediated, at least in part, by these parameters. VITAL is a large-scale, randomized, primary prevention trial testing 2000 IU/d vitamin D3 (cholecalciferol) and 1 g/day omega-3 fatty acids (840 mg EPA+DHA in 1.3:1 ratio) in a 2x2 factorial design among 20,000 men and women (≥50 and ≥55 years, respectively), with mean participant follow-up of 5 years for CVD and cancer. This ancillary study will address understudied areas and two overarching hypotheses that vitamin D supplementation (1) lowers total and regional (trunkal and abdominal/androidal) body fat as measured by dual x-ray absorptiometry (DXA) scans, improving biomarkers of adiposity (leptin, adiponectin) and (2) impacts CVD risk factors, at least in part through adiposity. The investigators also seek to define how circulating achieved 25(OH)D levels, due to supplementation, may be affected by body composition and BMI, thus elucidating how adiposity, BMI, and body composition (total and regional) may influence vitamin D intake needs in the population. To critically evaluate these hypotheses, the investigators will examine a representative, randomized subcohort of 1000 racially diverse VITAL participants (25% African American) over two years.