title: The effects of seasonal variation of 25-hydroxyvitamin D on diagnosis of vitamin D insufficiency.
abstract: OBJECTIVE
  To explore the effects of seasonal variation on the diagnosis of vitamin D sufficiency and to determine whether age, gender, and ethnicity modify these effects.
  
  
  METHODS
  21,987 adults had a measurement of serum 25-hydroxyvitamin D (25OHD) at Labplus, Auckland City Hospital, between January 2002 and September 2003, and sine curves were fitted for 25OHD versus day of year to predict the 25OHD nadir for each individual.
  
  
  RESULTS
  48% (range: 30-63%) of individuals had 25OHD <50 nmol/L in the month of measurement, but 63% were predicted to have 25OHD <50 nmol/L in late winter or early spring based on expected seasonal variation. The 25OHD levels required to ensure 25OHD levels >50 nmol/L throughout the year varied substantially by season (in summer at least 60-75 nmol/L), and tended to be higher in men than women, decrease with age, and vary with ethnicity. Mean 25OHD levels were very low (<40 nmol/L) in people of Indian, Middle Eastern, and African descent.
  
  
  CONCLUSIONS
  Seasonal variation in 25OHD affects the diagnosis of vitamin D sufficiency. Clinicians should consider the month of sampling when interpreting the results of 25OHD measurements. In New Zealand, a summertime 25OHD <60-75 nmol/L is generally required to ensure year-round 25OHD levels <50 nmol/L.
authors.affiliation: Osteoporosis Research Group, Department of Medicine, University of Auckland, Private Bag 92 019, Auckland, New Zealand. m.bolland@auckland.ac.nz
authors.forename: Mark J
authors.initials: MJ
authors.lastname: Bolland
authors.forename: Weldon W
authors.initials: WW
authors.lastname: Chiu
authors.forename: James S
authors.initials: JS
authors.lastname: Davidson
authors.forename: Andrew
authors.initials: A
authors.lastname: Grey
authors.forename: Catherine
authors.initials: C
authors.lastname: Bacon
authors.forename: Greg D
authors.initials: GD
authors.lastname: Gamble
authors.forename: Ian R
authors.initials: IR
authors.lastname: Reid
mesh_terms: D000293:Adolescent; D000328:Adult; D000367:Age Factors; D000368:Aged; D000369:Aged, 80 and over; D001210:Asia, Southeastern / Q000208:ethnology; D003430:Cross-Sectional Studies; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009520:New Zealand / Q000453:epidemiology; D011114:Polynesia / Q000208:ethnology; D012621:Seasons; D012737:Sex Factors; D014807:Vitamin D / Q000031:analogs & derivatives* / Q000097:blood; D014808:Vitamin D Deficiency / Q000453:epidemiology* / Q000208:ethnology; D055815:Young Adult
chemical_list: D014807:Vitamin D; C104450:25-hydroxyvitamin D