The effects of seasonal variation of 25-hydroxyvitamin D on diagnosis of vitamin D insufficiency.
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.
Bolland
Mark J
MJ
Osteoporosis Research Group, Department of Medicine, University of Auckland, Private Bag 92 019, Auckland, New Zealand. m.bolland@auckland.ac.nz
Chiu
Weldon W
WW
Davidson
James S
JS
Grey
Andrew
A
Bacon
Catherine
C
Gamble
Greg D
GD
Reid
Ian R
IR
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
D014807:Vitamin D; C104450:25-hydroxyvitamin D