Seasonal vitamin D changes and the impact on health risk assessment

ObjectiveTo investigate seasonal variation of vitamin D levels in 148,821 serum samples during a 2 year time period in a northern-latitude city in the United States.MethodsTotal vitamin D assay testing by chemiluminescence was performed on the DiaSorin Liaison. Vitamin D results were extracted from the laboratory information system without patient identification during 2011 and 2012 and separated by season and vitamin D results: less than 10 ng/mL (deficient), 10–20.0 ng/mL (insufficient), 20.1–30 ng/mL (borderline), 30.1–40 ng/mL (sufficient), 40.1–100 ng/mL, and greater than 100 ng/mL.ResultsThe seasonal winter period constituted the months of January through March; spring, April through June; summer, July through September; and fall, October through December. The data set analyzed included 36,643 samples during the winter, 38,299 in spring, 36,141 in summer, and 37,738 in fall and demonstrated an expected rise and fall in vitamin D levels.ConclusionThis retrospective epidemiological study demonstrates seasonal variation of vitamin D levels at clinical decision points. Although not unexpected, this variation has an impact on studies relating low vitamin D levels to higher rates of cancer, cardiovascular disease, multiple sclerosis, diabetes, autoimmune disease, and a host of other health risk assessments.