title: The effects of seasonal variation of 25-hydroxyvitamin D and fat mass on a diagnosis of vitamin D sufficiency.
abstract: BACKGROUND
  The effect of season on vitamin D status is often overlooked in studies of optimal vitamin D concentrations and in clinical practice.
  
  
  OBJECTIVE
  We aimed to determine the effects of seasonal variation of 25-hydroxyvitamin D [25(OH)D] on a previously selected minimum concentration for vitamin D sufficiency (50 nmol/L) and to determine whether fat mass modifies these effects.
  
  
  METHODS
  A cross-sectional study evaluated 1606 healthy postmenopausal women and 378 older men from Auckland, New Zealand, who were undergoing single measurements of 25(OH)D.
  
  
  RESULTS
  Concentrations of <50 nmol 25(OH)D/L were seen in 49% (range: 23%-74%) of women and 9% (range: 0%-26%) of men when measured, but 73% of women and 39% of men were predicted to have trough 25(OH)D concentrations < 50 nmol/L, according to the demonstrated seasonal variation. The predicted duration of 25(OH)D concentrations < 50 nmol/L was 250 d/y in women and 165 d/y in men.
  
  
  CONCLUSIONS
  Seasonal variation significantly affects the diagnosis of vitamin D sufficiency, which requires seasonally adjusted thresholds individualized for different locations. Clinicians should consider the month of sampling and the amount of body fat when interpreting 25(OH)D measurements.
authors.affiliation: Department of Medicine, University of Auckland, Auckland, New Zealand. m.bolland@auckland.ac.nz
authors.forename: Mark J
authors.initials: MJ
authors.lastname: Bolland
authors.forename: Andrew B
authors.initials: AB
authors.lastname: Grey
authors.forename: Ruth W
authors.initials: RW
authors.lastname: Ames
authors.forename: Barbara H
authors.initials: BH
authors.lastname: Mason
authors.forename: Anne M
authors.initials: AM
authors.lastname: Horne
authors.forename: Greg D
authors.initials: GD
authors.lastname: Gamble
authors.forename: Ian R
authors.initials: IR
authors.lastname: Reid
mesh_terms: D000273:Adipose Tissue / Q000378:metabolism*; D000368:Aged; D019540:Area Under Curve; D015331:Cohort Studies; D003430:Cross-Sectional Studies; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009520:New Zealand; D009752:Nutritional Status; D017698:Postmenopause / Q000097:blood; D012016:Reference Values; D012621:Seasons; D012737:Sex Factors; D014807:Vitamin D / Q000031:analogs & derivatives* / Q000097:blood; D014808:Vitamin D Deficiency / Q000150:complications / Q000175:diagnosis*
chemical_list: D014807:Vitamin D; C104450:25-hydroxyvitamin D