Determining the optimal level of vitamin D

In testing older patients' blood vitamin D levels, there's uncertainty about where the dividing line falls between enough and not enough. To help resolve this debate, University of Washington 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. Vitamin D is measured in the blood as 25-hydroxy-vitamin D (25(OH)D). Researchers tested 25(OH)D concentrations from a biorepository of blood samples of 1,621 adults who had enrolled in a study originally designed to look at risk factors and progression of heart disease in people age 65 and over in the early 1990s. Over an 11-year follow-up, researchers looked at the association between each individual's 25(OH)D test results and the time that a first defining medical event occurred. Through statistical analysis, researchers concluded that the risk of these disease events rose when the concentration of 25(OH)D fell below 20 ng/milliL or 50 nmol/L.