Fluctuation of Vitamin D Deficiency in Indoor Daytime Workers

Seasonal variation in circulating 25-hydroxyvitamin D (25OHD) levels related to seasonal and inter-day fluctuation in sunlight ultraviolet irradiation, may lead to misjudgments concerning 25OHD status in individual workers around threshold levels. Here, to examine seasonal and inter-day variations in plasma 25OHD, we conducted a longitudinal study involving indoor daytime workers. Subjects were four male indoor daytime workers aged 32–57 yr working in Kawasaki City, Japan. Blood samples were obtained on six days within two twoweek periods in February and October, 2008. Plasma 25OHD, serum intact parathyroid hormone (PTH) and 1α ,25-dihydroxyvitamin D [1α ,25(OH)2D] were measured. Individual monthly mean 25OHD levels were 16–56% higher in October than in February (p=0.03), while individual monthly mean intact PTH levels were 15–41% lower in October (p=0.09). No seasonal change was observed in 1α ,25(OH)2D (p=0.62). Notably, nearly all measured 25OHD levels in February were lower than the reference value of 20 ng/ml. Our study identified the occurrence of seasonal variation in circulating 25OHD and intact PTH levels, even after accounting for inter-day variability, and hypovitaminosis D in wintertime in indoor daytime male workers in Japan. Due to this variability, single spot measurements of 25OHD may lead to misjudgment of workers’ vitamin D status.