Leslie McClure, PhD, of the University of Alabama at Birmingham, says past research has shown a link between sunlight and cognitive impairment. That's a condition marked by greater memory loss than would be expected by normal aging.
"Previous research has shown a strong relationship between cognitive impairment and stroke. So it made sense to look at the relationship between sunlight and stroke," she tells WebMD.
The study involved more than 16,000 men and women involved in a long-term study looking at racial and geographic differences in stroke risks.
They filled out detailed questionnaires asking where they had lived throughout their lives. Then, the researchers used a program developed by NASA that takes into account clouds, smog, and other factors to calculate sunlight exposure based on latitude and longitude.
Every six months, people were contacted and asked about their health. Over the five years they were followed, 351 people had a stroke.
An analysis of the data that took into account a host of factors that can affect stroke risk showed the greater the sun exposure, the lower the stroke risk.
Conversely, people who lived in areas with less than average exposure were at 60% increased stroke risk.
The Vitamin D-Sunshine Link
McClure isn't sure of the biologic explanation for the observation. But radiation from the sun is a main source of the body's vitamin D, she notes.
Vitamin D has been previously linked in some studies to heart and blood vessel health, but there has been no conclusive data showing that taking vitamin D supplements helps prevent heart attacks or strokes.
A link between vitamin D and the brain was supported by the second study, also conducted at the University of Alabama.
More than 30,000 people were divided into three groups, depending on how much vitamin D they reported they took in through diet and supplements.
People in the group that took in the most vitamin D were 13% less likely to have a stroke and 25% less likely to have cognitive impairment than those who took in the least.
Again, the researchers took into account other risk factors when performing the analysis. But you can't account for everything, says American Stroke Association spokesperson Mark Alberts, MD, a neurologist at Northwestern University in Chicago.
"People who live in gloomy, rainy areas may be less likely to exercise, and that may account for a higher stroke risk. And getting a lot of vitamin D may be a marker for overall good health," he tells WebMD.
Both studies reveal interesting associations deserving of further study, Alberts says.
Both studies were funded by the National Institutes of Health.
These findings were presented at a medical conference. They should be considered preliminary as they have not yet undergone the "peer review" process, in which outside experts scrutinize the data prior to publication in a medical journal.