@article {1353, title = {Seasonal variation in 25-hydroxyvitamin D concentrations in the cardiovascular health study.}, journal = {Am J Epidemiol}, volume = {174}, year = {2011}, month = {2011 Dec 15}, pages = {1363-72}, abstract = {

Low circulating concentrations of 25-hydroxyvitamin D (25(OH)D) are associated with adverse health outcomes in diverse populations. However, 25(OH)D concentrations vary seasonally with varying exposure to sunlight, so single measurements may poorly reflect long-term 25(OH)D exposure. The authors investigated cyclical trends in average serum 25(OH)D concentrations among 2,298 individuals enrolled in the Cardiovascular Health Study of community-based older adults (1992-1993). A sinusoidal model closely approximated observed 25(OH)D concentrations and fit the data significantly better than did a mean model (P < 0.0001). The mean annual 25(OH)D concentration was 25.1 ng/mL (95\% confidence interval: 24.7, 25.5), and the mean peak-trough difference was 9.6 ng/mL (95\% confidence interval: 8.5, 10.7). Male sex, higher latitude of study site, and greater physical activity levels were associated with larger peak-trough difference in 25(OH)D concentration (each P < 0.05). Serum concentrations of intact parathyroid hormone and bone-specific alkaline phosphatase also varied in a sinusoidal fashion (P < 0.0001), inversely to 25(OH)D. In conclusion, serum 25(OH)D varies in a sinusoidal manner, with large seasonal differences relative to mean concentration and laboratory evidence of biologic sequelae. Single 25(OH)D measurements might not capture overall vitamin D status, and the extent of misclassification could vary by demographic and behavioral factors. Accounting for collection time may reduce bias in research studies and improve decision-making in clinical care.

}, keywords = {Aged, Biomarkers, Continental Population Groups, Exercise, Female, Humans, Male, Residence Characteristics, Seasons, Sex Factors, Vitamin D}, issn = {1476-6256}, doi = {10.1093/aje/kwr258}, author = {Shoben, Abigail B and Kestenbaum, Bryan and Levin, Gregory and Hoofnagle, Andrew N and Psaty, Bruce M and Siscovick, David S and de Boer, Ian H} } @article {1384, title = {Serum 25-hydroxyvitamin D concentration and risk for major clinical disease events in a community-based population of older adults: a cohort study.}, journal = {Ann Intern Med}, volume = {156}, year = {2012}, month = {2012 May 01}, pages = {627-34}, abstract = {

BACKGROUND: Circulating concentrations of 25-hydroxyvitamin D [25-(OH)D] are used to define vitamin D deficiency. Current clinical 25-(OH)D targets based on associations with intermediate markers of bone metabolism may not reflect optimal levels for other chronic diseases and do not account for known seasonal variation in 25-(OH)D concentration.

OBJECTIVE: To evaluate the relationship of 25-(OH)D concentration with the incidence of major clinical disease events that are pathophysiologically relevant to vitamin D.

DESIGN: Cohort study.

SETTING: The Cardiovascular Health Study conducted in 4 U.S. communities. Data from 1992 to 2006 were included in this analysis.

PARTICIPANTS: 1621 white older adults.

MEASUREMENTS: Serum 25-(OH)D concentration (using a high-performance liquid chromatography-tandem mass spectrometry assay that conforms to National Institute of Standards and Technology reference standards) and associations with time to a composite outcome of incident hip fracture, myocardial infarction, cancer, or death.

RESULTS: Over a median 11-year follow-up, the composite outcome occurred in 1018 participants (63\%). Defining events included 137 hip fractures, 186 myocardial infarctions, 335 incidences of cancer, and 360 deaths. The association of low 25-(OH)D concentration with risk for the composite outcome varied by season (P = 0.057). A concentration lower than a season-specific Z score of -0.54 best discriminated risk for the composite outcome and was associated with a 24\% higher risk in adjusted analyses (95\% CI, 9\% to 42\%). Corresponding season-specific 25-(OH)D concentrations were 43, 50, 61, and 55 nmol/L (17, 20, 24, and 22 ng/mL) in winter, spring, summer, and autumn, respectively.

LIMITATION: The observational study was restricted to white participants.

CONCLUSION: Threshold concentrations of 25-(OH)D associated with increased risk for relevant clinical disease events center near 50 nmol/L (20 ng/mL). Season-specific targets for 25-(OH)D concentration may be more appropriate than static targets when evaluating health risk.

PRIMARY FUNDING SOURCE: National Institutes of Health.

}, keywords = {Aged, Cause of Death, Female, Follow-Up Studies, Hip Fractures, Humans, Male, Myocardial Infarction, Neoplasms, Proportional Hazards Models, Risk Assessment, Risk Factors, Seasons, United States, Vitamin D, Vitamin D Deficiency}, issn = {1539-3704}, doi = {10.7326/0003-4819-156-9-201205010-00004}, author = {de Boer, Ian H and Levin, Gregory and Robinson-Cohen, Cassianne and Biggs, Mary L and Hoofnagle, Andy N and Siscovick, David S and Kestenbaum, Bryan} }