Time spent outdoors and seasonal variation in serum concentrations of 25-hydroxyvitamin D in Korean women

The vitamin D status of 179 Korean women between the ages of 20 and 75 were measured by the use of high performance liquid chromatography (HPLC). Rela ted biochemical indices such as iPTH, alkaline, phosphatase, creatinine, al bumin, Ca, Mg, and P were also measured. Factors such as demographic charac teristics, intake of foods containing vitamin D, and proxy measure of sunli ght exposure (time spent outdoors) were assessed to determine their effect on vitamin D status and used in the analysis. The purpose of this study was twofold. The first was to define reference data for the distribution of vi tamin D status and to explore the relationship between vitamin D and the va riables that affect the vitamin D status in Korean women. The second was to analyze the risk factors of the vitamin D status and the relation between the factors. The results of this study will provide valuable information re garding the role of vitamin D in Korean women. The mean serum 25-hydroxyvit amin D (25-OHD) level was 25.8 ng/ml. Of the total subjects, 16.5% showed v itamin D deficiency (i.e. s-25-OHD < 10 ng/ml). Serum 25-OHD was inversely related to iPTH and alkaline phophatase. There were significant changes in serum 25-OHD level from the pre- to the post-menopausal women with a positi ve correlation between vitamin D intake and serum at the 25-OHD level. Sign ificant seasonal variation of serum 25-OHD and PTH were noted in 26 of the subjects and the serum 25-OHD level also correlated with sunlight exposure especially at 12.00 p.m. to 2.00 p.m. as assessed by the time spent outdoor s. The relative importance of the two sources of vitamin D such as dietary intake (33.6% explained) and endogenous production of the time spent outdoo rs (19.7% explained) in serum 25-OHD were also evaluated. Multiple regressi on analysis revealed that the effects of aging on serum 25-OHD could largel y be accounted for. Both the decline in dietary vitamin D intake and the ti me spent outdoors were closely related to the decreasing serum 25-OHD level . Among the determinants of low serum 25-OHD were age, dietary vitamin D in take, serum calcium level and dietary calcium intake, and serum alkaline ph osphatase.