An Inter-Method Reliability Study Comparing Interview Information on Sunlight, Tanning Beds, Food, and Supplement Exposures with Serum 25-Hydroxy Vitamin D Levels

Abstract Background: We developed a questionnaire designed to capture the vitamin D related exposures of sunlight, tanning bed use, dietary intake, and supplement use, and used an inter-method reliability approach to compare the study method (self-reported information on vitamin D related exposures nearest the blood draw) with serum 25-hydroxy vitamin D (25(OH)D) levels. Methods: This inter-method reliability study included 512 control women from a population-based, case-control study in Alberta, Canada. All women self-reported data on food/supplement intake (average µg/day), sun exposure (cumulative hours/year), and tanning bed exposure (cumulative hours/year) and provided fasting serum samples, measured in duplicate with a DiaSorin immunoassay for 25(OH)D levels. The correlation between participant characteristics and 25(OH)D are described. We used multivariable robust regression to estimate the percent of variation in 25(OH)D explained by our variables of interest. Results: Food intake, sun exposure, and tanning bed exposure had positive and significant correlations with 25(OH)D levels of a similar magnitude (Spearman r= 0.17 to 0.19). Supplement intake (average µg/day, Spearman r= 0.44) had the strongest positive correlation. In both crude and adjusted models (adjusted for age, body mass index, race, smoking, oral contraceptive use, and menopausal status/hormone therapy), we consistently found that food explained 3.1%, supplements 18.9%, sunlight 2.2%, and tanning bed use 3.0% of the variation in 25(OH)D levels, and all variables combined explained 27.5% - 36.0% of the total variation. Conclusions: These results suggest that our comprehensive dietary and light exposure questionnaire may be a reasonable proxy measure of vitamin D status in the recent past when either 25(OH)D measurements or serum samples are not available for study participants.