title: Vitamin D, season, and risk of prostate cancer: a nested case-control study within Norwegian health studies.
abstract: BACKGROUND
  It is biologically plausible that vitamin D might prevent prostate cancer. However, recent meta-analyses concluded that there is no consistent relation between circulating 25-hydroxyvitamin D [25(OH)D] concentrations and prostate cancer, and several large studies have actually reported an increased risk of prostate cancer associated with high 25(OH)D.
  
  
  OBJECTIVE
  We aimed to assess the prospective relations between serum 25(OH)D, serum retinol, and risk of prostate cancer.
  
  
  METHODS
  In this nested case-control study, Norwegian men who participated in population-based health studies between 1981 and 1991 were followed with respect to prostate cancer throughout 2006. For each case (n = 2106), a matched control was selected. Stored serum was analyzed for 25(OH)D by using HPLC atmospheric pressure chemical ionization mass spectrometry.
  
  
  RESULTS
  We showed a positive relation between an increasing 25(OH)D concentration and prostate cancer risk [rate ratio (RR): 1.15 (95% CI: 1.04, 1.27) per 30-nmol/L increase in 25(OH)D concentration]. Predefined analyses stratified by season showed no relation for subjects with serum collected during winter and spring (RR: 1.00 per 30-nmol/L increase), whereas a strengthened positive association [RR: 1.27 (95% CI: 1.09, 1.47) per 30-nmol/L increase] was observed in men with serum collected during the summer and autumn. There was no relation between serum retinol and prostate cancer.
  
  
  CONCLUSIONS
  The cause for increased risk of prostate cancer related to a high 25(OH)D concentration only during the summer and autumn is not obvious. The effect may be related to vitamin D itself or to other factors associated with sun exposure.
authors.affiliation: Department of Community Medicine, Faculty of Medicine, University of Oslo, Oslo, Norway. h.e.meyer@medisin.uio.no
authors.forename: Haakon E
authors.initials: HE
authors.lastname: Meyer
authors.forename: Trude E
authors.initials: TE
authors.lastname: Robsahm
authors.forename: Tone
authors.initials: T
authors.lastname: Bjørge
authors.forename: Magritt
authors.initials: M
authors.lastname: Brustad
authors.forename: Rune
authors.initials: R
authors.lastname: Blomhoff
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D016022:Case-Control Studies; D002648:Child; D023381:Endpoint Determination; D005500:Follow-Up Studies; D006801:Humans; D016015:Logistic Models; D008297:Male; D008875:Middle Aged; D009664:Norway / Q000453:epidemiology; D011471:Prostatic Neoplasms / Q000097:blood / Q000453:epidemiology* / Q000517:prevention & control; D012307:Risk Factors; D012621:Seasons; D014801:Vitamin A / Q000097:blood; D014807:Vitamin D / Q000097:blood*; D014815:Vitamins / Q000097:blood; D055815:Young Adult
chemical_list: D014815:Vitamins; D014801:Vitamin A; D014807:Vitamin D