Season-specific vitamin D thresholds offer best risk prediction
MedWire News: Researchers report that blood 25-hydroxyvitamin D (25-[OH]D) levels below 50 nmol/L predict an increased risk for adverse outcomes in elderly individuals, including hip fracture, myocardial infarction, cancer, and death.
The 50 nmol/L threshold identified in this study is lower than the value recommended by most professional societies and expert panels for the identification of at-risk individuals.
Furthermore, the association between vitamin D levels and risk varied by season, leading the team to suggest "that season-specific targets for 25-(OH)D concentration are more appropriate than static targets when considering potential implications for long-term health."
The study, which appears in the Annals of Internal Medicine, used data from the Cardiovascular Health Study, a longitudinal cohort study conducted in four US communities between 1992 and 2006.
The present study included information on 1621 individuals; their average age at baseline was 74.0 years, 30% were men, and all were White. During a median follow-up duration of 11 years, the composite clinical outcome (incident hip fracture, myocardial infarction, cancer, or death) occurred in 1018 participants (62.8%).
Using a 25-(OH)D threshold of 50 nmol/L, 495 (30.5%) participants were classified as having low vitamin D levels and the remainder as having normal levels. Levels varied markedly by season, however, with the lowest levels occurring in winter (January‑March) and the highest levels in summer (July‑September).
When participants were classified into deciles of 25-(OH)D, those in the lowest 2‑3 deciles (ie, the lowest 20‑30%) had an increased risk for the composite clinical outcome compared with those in the highest 7‑8 deciles.
This association varied by season, say the authors, who calculated that a 25-(OH)D level below a season-specific Z-score of ‑0.54 offers the optimal discrimination of risk. This score corresponded to 25-(OH)D levels of 43, 50, 61, and 55 nmol/L in winter, spring, summer, and autumn, respectively.
Using this cutpoint, participants with low 25-(OH)D levels had a 24% higher risk for the composite outcome than those with normal levels; the risk increase was similar for each of the four components of the composite.
"In conclusion, we found that 'optimal' concentrations of 25-(OH)D, gauged by associations with major clinical disease events, centered near 50 nmol/L, the level recently recommended by the Institute of Medicine for bone health," write Ian de Boer (University of Washington, Seattle, USA) and team.
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By Joanna Lyford