A seasonal difference in serum 25-hydroxyvitamin D3 in a Finnish population

A chromatographic assay procedure for the analysis of 25-hydroxyvitamin D3 [25(OH)D3] in human serum is described. The procedure involves methanol/chloroform extraction of serum lipids followed by separation of the vitamin D metabolites and purification from interfering contaminents by chromatography on Sephadex LH-20 and by high-pressure liquid chromatography. 25(OH)D3 is quantitated by uv detection, and its peak height compared with those of standards. Values in normal human serum samples taken in July-August and January-February are: 33.0 +/- 13.6 ng/ml (82.5 +/- 34.1 nmol/1) and 14.5 +/- 12.2 ng/ml (36.3 +/- 30.5 nmol/1) (means +/- SD), respectively (p less than 0.001). The summer values are somewhat lower in the elderly subjects as compared to younger ones, but a seasonal difference was observed in both groups. This difference may be due to a relatively low vitamin D intake and a limited sun exposure.