[Seasonal rhythm in 25-hydroxyvitamin D blood levels].

A retrospective study of the serum concentrations of 25-hydroxyvitamin D was performed over a one-year period, using data from 158 healthy subjects aged 40-80 years. Serum 25-hydroxyvitamin D was determined by competitive protein binding assay; the method included a chromatographic step. A noticeable seasonal variation was observed, with a maximum in summer.