title: Seasonal reduction in vitamin D level persists into spring in NSW Australia: implications for monitoring and replacement therapy.
abstract: BACKGROUND
  Seasonal variation in 25-hydroxyvitamin D [25OHD] status and its relationship to gender, age, socioeconomic and geographic determinants in Australians has not been described in large biomedical sampling cohorts.
  
  
  OBJECTIVE
  To analyse 25OHD levels in all primary tests undertaken consecutively in a 2-year period to determine the prevalence of 25OHD deficiency and its relation to patient setting, gender, age, season, urban or rural residency, socioeconomic status, latitude and longitude.
  
  
  METHODS
  We assessed 24 819 ambulatory and inpatient samples taken from the largest reference laboratory in NSW, Australia between 01 July 2008 and 30 July 2010.
  
  
  METHODS
  Serum 25OHD was measured using chemiluminescent immunoassay. Vitamin D deficiency was defined as 25OHD <50 nm.
  
  
  RESULTS
  Median 25OHD was 54 nm ranging from 63 nm in summer to 44 nm in spring and was lowest in inpatient women (49 nm) and highest in ambulatory men (64 nm). Mean 25OHD peaked in January (67 nm) and reached a nadir in August/September (39 nm). During summer, 36% subjects overall had a level below 50 nm, increasing to 58% in spring. The highest prevalence of deficiency occurred in female inpatients (42% in summer and 62% in spring). Factors associated with lower 25OHD included being tested in spring, an inpatient, female, aged 20-39 or >79 years, socioeconomically disadvantaged and from a major city.
  
  
  CONCLUSIONS
  This cross-sectional study demonstrates the extent and duration of 25OHD deficiency is greater than expected, and particular individuals are at higher risk. Our findings imply that supplementation guidelines need to be modified and strengthened.
authors.affiliation: Westmead Hospital and eHealth NSW Initiative, NSW, Australia.
authors.forename: Steven
authors.initials: S
authors.lastname: Boyages
authors.forename: Kellie
authors.initials: K
authors.lastname: Bilinski
mesh_terms: D000328:Adult; D000367:Age Factors; D000368:Aged; D000369:Aged, 80 and over; D001315:Australia; D003430:Cross-Sectional Studies; D005260:Female; D006801:Humans; D007118:Immunoassay; D008297:Male; D008875:Middle Aged; D012621:Seasons; D012737:Sex Factors; D014807:Vitamin D / Q000031:analogs & derivatives* / Q000097:blood; D014808:Vitamin D Deficiency / Q000097:blood; D055815:Young Adult
chemical_list: D014807:Vitamin D; C104450:25-hydroxyvitamin D