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.