Vitamin D nutrition in relation to season and occupation.

Seasonal variations in vitamin D nutrition were assessed by measurements of serum 25-hydroxycholecalciferol levels in outdoor workers, indoor workers and long-term hospital inpatients. All three groups showed seasonal changes and the outdoor workers had, as might be expected, the highest levels at all seasons. However, the highest levels of 25-hydroxycholecalciferol were found in October in the indoor workers and in November for the outdoor workers whereas the peak in ultraviolet exposure was in July. The possible reasons for this long lag are discussed; the most likely explanation is that vitamin D continues to be formed and stored during the autumn especially in outdoor workers.