Objective: The aim of this study was to test whether daily doses of vitamin D3 at 400 or 1000 IU/d for 1 yr affected conventional markers of cardiovascular disease (CVD) risk.
Design: We conducted a parallel-group, double-blind, placebo-controlled randomized controlled trial. Randomization was computer generated. Participants and study investigators were blinded to intervention groupings throughout the trial.
Setting: The study was conducted at the Clinical Research Facility, University of Aberdeen, United Kingdom.
Participants: A total of 305 healthy postmenopausal women aged 60–70 yr were recruited for the study.
Intervention: Each woman received a daily capsule of 400 or 1000 IU vitamin D3 or placebo randomly allocated.
Main Outcome Measures: Primary outcomes were serum lipid profile [total, high-density lipoprotein, and low-density lipoprotein cholesterol; triglycerides; and apolipoproteins A-1 and B100], insulin resistance (homeostatic model assessment), inflammatory biomarkers (high-sensitivity C-reactive protein, IL-6, soluble intracellular adhesion molecule-1), and blood pressure.
Results: A total of 265 (87%) participants completed all study visits. Small differences between groups for serum apolipoprotein B100 change [repeated measures ANOVA, P = 0.04; mean (sd), -1.0 (10.0) mg/dl (400 IU); -1.0 (10.0) mg/dl (1000 IU); and +0.02 (10.0) mg/dl (placebo)] were not considered clinically significant. Other systemic markers for CVD risk remained unchanged. There was significant seasonal variation in systolic and diastolic blood pressure independent of vitamin D dose (P < 0.001, linear mixed model). Mean (sd) reduction in systolic blood pressure from winter to summer was -6.6 (10.8) mm Hg.
Conclusions: Improving vitamin D status through dietary supplementation is unlikely to reduce CVD risk factors. Confounding of seasonality should be recognized and addressed in future studies of vitamin D.