By Ed Edelson
THURSDAY, Sept. 24 (HealthDay News) -- Low blood levels of vitamin D in younger women tripled their risk of high blood pressure 15 years later, new research has found.
Vitamin D deficiency, defined as less than 80 nanomoles per liter of blood, was measured in 1993 at the start of the Michigan Bone Health and Metabolism Study, explained study author Flojaune C. Griffin, a doctoral candidate at the University of Michigan School of Public Health.
By that measure, more than 80 percent of the 559 women first tested in the study had vitamin D deficiency, while 2 percent were being treated for high blood pressure and another 4 percent had undiagnosed high blood pressure.
No association between vitamin D levels and high blood pressure was seen at that time. But in 2008, when 19 percent of the women had been diagnosed with high blood pressure and 6 percent had the condition but didn't know it, the incidence of high blood pressure was three times higher for women who had vitamin D deficiency at the study's start, after adjusting for the effects of age, obesity and smoking, Griffin said.
Griffin was to report on the findings Thursday at the American Heart Association's High Blood Pressure Research Conference in Chicago.
What happened to the women in the intervening years in terms of vitamin D intake is unknown, Griffin said. "We don't have any information about how the women were eating beyond that baseline measurement," she noted.
The recommended intake of vitamin D has risen since the study began. Current guidelines call for an intake of 400 International Units (IU) for people under 60 and 600 IUs for those aged 60 and older, Griffin said.
"Exposure of skin to the sun is the most potent way to increase vitamin D levels," she added. "The main food sources include fatty fish, such as wild salmon. Also, milk and milk products are fortified with vitamin D."
There is no way of knowing whether increased vitamin D intake over the years might have affected the incidence of high blood pressure, a major risk factor for such cardiovascular problems as heart attack and stroke, Griffin said.
"This study underscores a growing amount of accumulated data that low vitamin D levels are associated with high blood pressure," said Dr. John P. Forman, an associate physician in the renal division of Brigham and Women's Hospital.
But it's still not certain that raising vitamin D intake can help prevent high blood pressure, Forman added. "We need large randomized trials on that," he said.
Still, he noted, "there are a growing number of studies associating lower vitamin D levels and high blood pressure. This one probably has the longest follow-up."
Known risk factors for high blood pressure are listed by the U.S. National Heart, Lung, and Blood Institute.
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