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Seasonal reduction in vitamin D level persists into spring in NSW Australia: implications for monitoring and replacement therapy


Correspondence: Kellie Bilinski, PO Box 143 Westmead 2145, NSW, Australia. Tel.: 02 9843 3175, 0430 395 129; Fax: 9843 3133; E-mail:



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.


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.


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.

Main outcome measures

Serum 25OHD was measured using chemiluminescent immunoassay. Vitamin D deficiency was defined as 25OHD <50 nm.


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.


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.

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