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

Abstract

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). Related biochemical indices such as iPTH, alkaline, phosphatase, creatinine, albumin, Ca, Mg, and P were also measured. Factors such as demographic characteristics, intake of foods containing vitamin D, and proxy measure of sunlight 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 vitamin D status and to explore the relationship between vitamin D and the variables 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 regarding the role of vitamin D in Korean women. The mean serum 25-hydroxyvitamin D (25-OHD) level was 25.8 ng/ml. Of the total subjects, 16.5% showed vitamin 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 positive correlation between vitamin D intake and serum at the 25-OHD level. Significant 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 outdoors. The relative importance of the two sources of vitamin D such as dietary intake (33.6% explained) and endogenous production of the time spent outdoors (19.7% explained) in serum 25-OHD were also evaluated. Multiple regression analysis revealed that the effects of aging on serum 25-OHD could largely be accounted for. Both the decline in dietary vitamin D intake and the time 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 intake, serum calcium level and dietary calcium intake, and serum alkaline phosphatase.