Serum 25-Hydroxyvitamin D Concentrations Fluctuate Seasonally in Young Adults of Diverse Ancestry Living in Toronto

Previous research indicates that circulating vitamin D levels are low in many otherwise healthy adults and that there is considerable seasonal variation in 25-hydroxyvitamin D[25(OH)D] concentrations at high latitudes We examined seasonal variation in 25(OH)D levels in a sample of young adults of diverse ancestry living in the Greater Toronto Area Three hundred and fifty-one (351) healthy young adults completed both a fall and winter visit during this study The study was conducted over 2 y (y 1 fall 2007 to winter 2008 and y 2 fall 2008 to winter 2009) At both visits each participant s serum 25(OH)D concentration was measured Information was also obtained on skin pigmentation (measured via reflectometer) vitamin D intake and extent of sun exposure Overall the serum 25(OH)D concentration was 54 4 +/- 1 3 nmoVL in the fall and 38 4 +/- 1 1 nmoVL in the winter Concentrations differed among ancestral groups at both visits (P < 0 001) with South Asians and East Asians having substantially lower concentrations than Europeans Skin pigmentation (r(2) = 0 14 P < 0 001) supplemental vitamin D intake (r(2) = 0 09 P < 0 001) sun exposure (r(2) = 0 04 P< 0 001) and study year (r(2) = 0 02 P = 0 017) were predictors of fall 25(OH)D concentrations During the wintertime serum 25(OH)D concentrations were associated with concentrations taken in the fall (r(2) = 0 45 P < 0 001) supplemental = 0 15 P < 0 001) and dietary vitamin D intake (r(2) = 0 06 P < 0 001) and with study year (r(2) = 0 02 P = 0 009) Our study confirms that serum 25(OH)D concentrations undergo strong seasonal variation at high latitudes and are influenced by vitamin D intake skin pigmentation and sun exposure J Nutr 140 2213-2220 2010