Date Published: November 13, 2009
Publisher: Public Library of Science
Author(s): Greta Snellman, Håkan Melhus, Rolf Gedeborg, Sylvia Olofsson, Alicja Wolk, Nancy L. Pedersen, Karl Michaëlsson, Zoltán Bochdanovits. http://doi.org/10.1371/journal.pone.0007747
Abstract: Although environmental factors, mainly nutrition and UV-B radiation, have been considered major determinants of vitamin D status, they have only explained a modest proportion of the variation in serum 25-hydroxyvitamin D. We aimed to study the seasonal impact of genetic factors on serum 25-hydroxyvitamin D concentrations.
Partial Text: Vitamin D is unique among vitamins because it can be synthesised endogenously by skin exposure to UV-B radiation. Globally, sun exposure is the most important source of vitamin D . At northern latitudes, however, dietary intake by natural food sources, food fortification or dietary supplements is also of importance because exposure to UV-B radiation is low during the winter season . Relatively little is known about genetic influences on vitamin D levels and virtually nothing about seasonal differences in these effects.
Descriptive characteristics as a function of season are summarised in Table 1. The average total 25-hydroxyvitamin D concentration was 84.8 nmol/L (95% CI 81.0–88.6); however, 17 twins (8%) had a concentration below 50 nmol/L. There were no differences in plasma PTH between twins with 25-hydroxyvitamin D concentrations above this cut-off (mean PTH 1.93 pmol/L) compared to below (mean PTH 1.95 pmol/L). Only one individual had a 25-hydroxyvitamin D concentration below 25 nmol/L. This twin had a higher serum PTH of 2.8 pmol/L. Only 40 twins (20%) had detectable 25-hydroxyvitamin D2. The mean serum 25-hydroxyvitamin D2 concentration in these twins was 7.8 nmol/L and the mean 25-hydroxyvitamin D3 concentration 79.2 nmol/L, giving a mean of the total 25-hydroxyvitamin D of 86.9 nmol/L.
The results of our twin study indicate a moderate genetic impact on vitamin D status in the summer season and a predominantly shared environmental influence during the winter season, presumably due to solar altitude. We are not aware of any previous study that has addressed these issues. During the winter season when UV-B radiation is minimal at latitude 60° north and the cutaneous synthesis of pre-vitamin D is non-existent , the heritability was undetectable. Finding decreased vitamin D concentrations in winter compared to summer was expected and is most likely due to the low vitamin D3 synthesis in the skin during the winter. Theoretically, genetic factors could be important at several levels, such as the capacity to synthesise vitamin D in the skin, to assimilate vitamin D from the diet and to store the vitamin in body fat (which is used as a reservoir during the winter). Assuming that dietary intake and body fat mass are relatively constant throughout the year , it seems reasonable to conclude that the key genetic influence is on the cutaneous synthesis of vitamin D.