Summer/winter differences in the serum 25-hydroxyvitamin D3 and parathyroid hormone levels of Japanese women

Serum 25-hydroxyvitamin Dg [25(OH)D-3] is produced in the skin in response to exposure to ultraviolet radiation, and is a good indicator of vitamin D nutritional status. The aim of this study was to determine summer/winter differences in serum 25(OH)D-3 and parathyroid hormone (PTH) in Japanese women and how the summer and winter values are related. The subjects were 122 healthy Japanese women aged 45-81 years (average age: 65.7 years), They were medically examined twice, in September 1997 and February 1949. Serum 25(OH)D-3 and intact PTH were determined by high-performance liquid chromatography and a two-site immunoradiometric assay respectively. Lifestyle information was obtained through an interview. The seasonal differences (winter minus summer) in 25(OH)D-3 [Delta 25(OH)D-3] and intact PTH concentrations were -18.8 nmol/l (SD 19.2, P<0.0001) and 0.98 pmol/l (SD 1.02, P<0.0001) respectively. The correlation coefficient between summer (x) and winter (y) 25(OH)D-3 levels was 0.462 (P<0.0001), with a linearly fitted line of y=0.42x+26.4. This relationship was interpreted as subjects with higher summer 25(OH)D-3 values having greater reductions in winter 25(OH)D-3 concentrations. There were inter-individual differences in 25(OH)D-3, although the summer and winter 25(OH)D-3 concentrations were well-correlated. Since Delta 25(OH)D-3 was not associated with any of the lifestyle factors, seasonal differences in the 25(OH)D-3 concentrations of an individual appeared to reflect her ability to produce 25(OH)D-3 photochemically in the skin. Sun bathing would be a less effective means of attaining adequate vitamin D nutritional status in a person with a small seasonal difference in 25(OH)D-3, i.e., one with a low 25(OH)D-3 level.