Vitamin D deficiency and seasonal and inter-day variation in circulating 25-hydroxyvitamin D and parathyroid hormone levels in indoor daytime workers: a longitudinal study

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 involv- ing 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 two- week periods in February and October, 2008. Plasma 25OHD, serum intact parathyroid hor- mone (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 indi- vidual 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.