title: Seasonal genetic influence on serum 25-hydroxyvitamin D levels: a twin study.
abstract: BACKGROUND
  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.
  
  
  RESULTS
  204 same-sex twins, aged 39-85 years and living at northern latitude 60 degrees, were recruited from the Swedish Twin Registry. Serum 25-hydroxyvitamin D was analysed by high-pressure liquid chromatography and mass spectrometry. Genetic modelling techniques estimated the relative contributions of genetic, shared and individual-specific environmental factors to the variation in serum vitamin D. The average serum 25-hydroxyvitamin D concentration was 84.8 nmol/l (95% CI 81.0-88.6) but the seasonal variation was substantial, with 24.2 nmol/l (95% CI 16.3-32.2) lower values during the winter as compared to the summer season. Half of the variability in 25-hydroxyvitamin D during the summer season was attributed to genetic factors. In contrast, the winter season variation was largely attributable to shared environmental influences (72%; 95% CI 48-86%), i.e., solar altitude. Individual-specific environmental influences were found to explain one fourth of the variation in serum 25-hydroxyvitamin D independent of season.
  
  
  CONCLUSIONS
  There exists a moderate genetic impact on serum vitamin D status during the summer season, probably through the skin synthesis of vitamin D. Further studies are warranted to identify the genes impacting on vitamin D status.
authors.affiliation: Department of Surgical Sciences, Section of Orthopaedics, Uppsala University, Uppsala, Sweden. greta.snellman@surgsci.uu.se
authors.forename: Greta
authors.initials: G
authors.lastname: Snellman
authors.forename: Håkan
authors.initials: H
authors.lastname: Melhus
authors.forename: Rolf
authors.initials: R
authors.lastname: Gedeborg
authors.forename: Sylvia
authors.initials: S
authors.lastname: Olofsson
authors.forename: Alicja
authors.initials: A
authors.lastname: Wolk
authors.forename: Nancy L
authors.initials: NL
authors.lastname: Pedersen
authors.forename: Karl
authors.initials: K
authors.lastname: Michaëlsson
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000465:Algorithms; D004777:Environment; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D008957:Models, Genetic; D015233:Models, Statistical; D012621:Seasons; D013997:Time Factors; D014807:Vitamin D / Q000031:analogs & derivatives* / Q000097:blood / Q000378:metabolism; D014808:Vitamin D Deficiency / Q000235:genetics
chemical_list: D014807:Vitamin D; C104450:25-hydroxyvitamin D