Superiority of Free 25-OH D Levels in Detecting Relationships
Abstract
Context: Inverse associations between 25-OH vitamin D levels and cardiovascular morbidity and mortality have been reported.
Objectives: Our goals were to 1) investigate effects of correcting inadequate D status on lipids, 2) determine whether free 25-OH D is better correlated with lipids than total 25-OH D.
Design: A randomized, double-blind placebo-controlled trial was performed.
Setting: Participants resided in the general community.
Participants: Adults with inadequate D status were randomized to D3: 14 men, 12 women, age 60 ± 8 years (mean ± SD) or placebo: 12 men, 11 women: 59 ±12 years.
Intervention: Responses to 12-week oral vitamin D3 titrated (1000–3000 IU/d) to achieve 25-OH D levels ≥25 ng/mL were compared to placebo.
Main Outcome Measures: Measurements were 25-OH D (tandem mass spectometry), free 25-OH D (direct immunoassay), lipids (directly measured triglyceride, cholesterol, and subfractions; plant sterols and cholesterol synthesis precursors), and safety labs before and after 6 and 12 weeks D3 or placebo. Data were analyzed by repeated measures ANOVA and linear regression.
Results: Vitamin D3 was titrated to 1000 IU/d in 15/26 (58%), to 2000 IU/d in 10, and 3000 IU/d in one patient. D3 had no effect on cholesterol or cholesterol subfractions except for trends for decreases in atorvastatin-treated patients (cholesterol, P = .08; low-density lipoprotein [LDL] cholesterol, P = .05). Decreased campesterol concentrations (P = .05) were seen with D3 but not placebo in statin-treated patients. Relationships between total 25-OH D and lipids were not detected, but inverse linear relationships were detected between free 25-OH D and triglycerides (P= .03 for all participants [n = 49], P = .03 in all statin-treated [n = 19], and P = .0009 in atorvastatin-treated [n = 11]), and between free 25-OH D and LDL cholesterol (P = .08 overall, P = .02 in all statin-treated, and P = .03 for atorvastatin-treated), and total cholesterol (P = .09 overall;P = .04 for all statin-treated, and P = .05 for atorvastatin-treated).
Conclusions: Vitamin D lipid-lowering effects appear limited to statin-treated patients and are likely due to decreased cholesterol absorption. Relationships between lipids and D metabolites were only detected when free 25-OH D was measured, suggesting the superiority of determining free 25-OH D levels compared to total 25-OH vitamin D levels when analyzing biologic responses.
Graham
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