title: Seasonality of vitamin D status in older people in Southern Germany: implications for assessment.
abstract: BACKGROUND
  from a clinical and public health perspective, it is important to understand the influence of seasonality on the serum vitamin D level to adequately assess and interpret an individual measurement. Therefore, the aim of this study was to analyse the effects of seasonal conditions on 25-hydroxyvitamin D (25(OH)D) serum levels in a population-based cohort of older people.
  
  
  METHODS
  between March 2009 and April 2010 the 25(OH)D serum level was assessed in 1,418 community-dwelling individuals living in Germany aged ≥65 years (56.7% men) with no subscribed vitamin D supplementation. Least-square means of monthly 25(OH)D serum levels with 95% confidence intervals (CI) were estimated, adjusted for gender, age and body mass index. Additionally, the proportion of vitamin deficiency (<20 ng/ml), insufficiency (20-<30 ng/ml) and sufficiency (30 ng/ml or higher) were estimated for each month. Finally, mean values of daily total global solar radiation and daylight were calculated for each month.
  
  
  RESULTS
  the minimum 25(OH)D serum level was observed in March with 15.4 ng/ml (SD = 6.56 ng/ml) and the maximum in August with 25.6 ng/ml (SD = 6.59 ng/ml). Compared with daylight and global solar radiation the progression over the year was similar but delayed by ∼2 months. The proportion of vitamin D deficiency, insufficiency and sufficiency were 78.8, 19.2 and 1.9% in March and 16.1, 63.4 and 20.5% in August, respectively.
  
  
  CONCLUSIONS
  vitamin D insufficiency was very common in this cohort and showed a strong seasonal effect with lowest values in March.
authors.affiliation: Institute of Epidemiology and Medical Biometry, Ulm University, Ulm, Germany. jochen.klenk@uni-ulm.de
authors.forename: Jochen
authors.initials: J
authors.lastname: Klenk
authors.forename: Kilian
authors.initials: K
authors.lastname: Rapp
authors.forename: Michael Dieter
authors.initials: MD
authors.lastname: Denkinger
authors.forename: Gabriele
authors.initials: G
authors.lastname: Nagel
authors.forename: Thorsten
authors.initials: T
authors.lastname: Nikolaus
authors.forename: Richard
authors.initials: R
authors.lastname: Peter
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Koenig
authors.forename: Bernhard Otto
authors.initials: BO
authors.lastname: Böhm
authors.forename: Dietrich
authors.initials: D
authors.lastname: Rothenbacher
mesh_terms: D000367:Age Factors; D000368:Aged; D000369:Aged, 80 and over; D015415:Biomarkers / Q000097:blood; D003430:Cross-Sectional Studies; D005260:Female; D005858:Germany / Q000453:epidemiology; D006801:Humans; D016014:Linear Models; D008297:Male; D017440:Photoperiod; D012621:Seasons; D013472:Sunlight; D013997:Time Factors; D014807:Vitamin D / Q000031:analogs & derivatives* / Q000097:blood; D014808:Vitamin D Deficiency / Q000097:blood* / Q000453:epidemiology
chemical_list: D015415:Biomarkers; D014807:Vitamin D; C104450:25-hydroxyvitamin D