THE VITAMIN D DOSE RESPONSE IN OBESITY

Objective: The prevalence of vitamin D inadequacy is high in obese individuals. Determining the response of serum 25-hydroxyvitamin D (25[OH]D) to vitamin D-3 supplementation in obese and nonobese individuals may lead to concurrent recommendations for optimal vitamin D intake in these populations. The objective of this study was to determine the dose response of vitamin D-3 in subjects with a body mass index >= 35 kg/m(2). Methods: Randomized, double-blind, placebo-controlled study. This study is an extension of our previous study of vitamin D dosing in healthy adults. After an assessment of baseline 25(OH)D levels, participants were randomized to a vitamin D supplementation arm (100 mu g daily if baseline 25[OH]D was <50 nmol/L, or 50 mu g daily if baseline 25[OH]D was >= 50 nmol/L) or placebo arm. Subjects with baseline 25(OH) D level >= 80 nmol/L were excluded from the study. Two months following randomization, a repeat 25(OH)D measurement was done. Results: Final analysis included 25 subjects (14 placebo, 11 active). At 2 months, serum 25(OH)D concentration increased to a mean of 75 nmol/L in the active group. Mean slope (i.e., vitamin D-3 response), defined as 25(OH) D change/baseline dose, was 0.398 nmol/L/mu g/day. Conclusion: The dose response of vitamin D-3 (slope) in obese subjects was significantly lower (P<.03) at 0.398 nmol/L/mu g/day compared to the slope in the previous study of healthy subjects (0.66 nmol/L/mu g/day). These results suggest that obese individuals may require 40% higher vitamin D intake than nonobese individuals to attain the same serum 25(OH)D concentration.