Time of year matters for vitamin D risk

The threshold amount for older patients’ vitamin D levels has become controversial as several scientific societies set different targets. But a new study at the University of Washington supports recent recommendations for a lower threshold level, considerably lower than the recommendations of other expert panels. The researchers conducted an observational study to learn how much vitamin D must be circulating in the blood to lower the risk of a major medical event. This category included heart attack, hip fracture, diagnosis of cancer, or death. Their findings are reported in the Annals of Internal Medicine. Ian de Boer, assistant professor of medicine in the Division of Nephrology and a member of the Kidney Research Institute, led the project. The researchers tested Vitamin D concentrations from a biorepository of blood samples of 1,621 Caucasian adults. These adults had enrolled in the early 1990s in the Cardiovascular Health Study, originally designed to look at risk factors and progression of heart disease in people age 65 and over. The participants lived in one of four areas in the United States: Forsyth County, North Carolina; Sacramento County, California; Washington County, Maryland; and Philadelphia, Pennsylvania. Over about an 11-year follow-up, researchers looked at the association between each individual’s test results and the time that a first defining medical event occurred. Among the participants, 1,018 had such an event. There were 137 hip fractures, 186 heart attacks (myocardial infarctions), 335 incidences of cancer, and 360 deaths. Through their statistical analysis, the researchers concluded that the risk of these disease events rose when the concentration of Vitamin D fell below 20 ng/milliliter or 50 nmol/liter. “Our results,” de Boer says, “lend support to the threshold level recently recommended by the Institute of Medicine, a national, independent government advisory group.” He adds, “This target level for adults is considerably lower than that set by other expert panels.” His research team also found that the association of low Vitamin D blood concentrations with risk of major disease events varied with the season. Because people soak up most of their Vitamin D from the sun, their levels run highest in the summer and lowest in the winter. Levels in autumn are generally above those in spring. “In evaluating health risk,” the researchers conclude, “season-specific targets for Vitamin D blood concentration may be more appropriate than a static target when evaluating patients health risk.” Future research in this area, de Boer says, will require clinical trials. Researchers will look, for example, at the health effects of attempts to raise Vitamin D in patients whose levels fall below the target. Interventions might include supplements, dietary changes, and increasing outdoor activity. “Sun exposure is tricky,” de Boer says, “because people have to protect themselves from skin cancer and other sun damage.” Sunscreen blocks the UVB waves responsible for producing Vitamin D. Food sources for Vitamin D are milk, fortified juice and cereal, and oily fish like salmon, mackerel, cod, and herring.