Among smokers, those with the least education are at the greatest risk for stroke, Danish researchers report.
"The combined effect of low educational level and smoking on the risk of stroke is the most surprising finding of our paper," said study author Helene Nordahl, an epidemiologist in the department of public health at the University of Copenhagen.
Wide number of stroke risk factors
She added that reducing smoking among those with the least education could potentially reduce strokes more than targeting smoking in those with the most schooling.
Although the study included Danish participants, Nordahl believes her findings are applicable to other countries.
"Since the most disadvantaged groups are often exposed to a wide number of stroke risk factors, it seems plausible that these people are at increased risk of stroke, not only in Denmark but also in the U.S.," Nordahl said.
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Other factors increase the risk for stroke, she said. "However, in this study we only included two of the main risk factors, smoking and high blood pressure, which were both more frequent in the lowest educated than in the highest educated," Nordahl said.
Previous studies by the same researchers have shown that the most disadvantaged groups are often exposed to a wide number of behavioural risk factors, such as obesity and physical inactivity, she said. "So it seems plausible that these risk factors could influence the risk of stroke in these groups," Nordahl added.
The report was published online in the journal Stroke.
Combined effect of risk factors
Dr. Ralph Sacco, chairman of neurology at the University of Miami Miller School of Medicine, said, "This very large study from Denmark shows an increased risk of stroke among smokers with low education and among smokers with high blood pressure."
In other words, he said, "the combined effects of these two risk factors on stroke risk is greater than one would expect if we just added each of them up independently."
Although this kind of study can only show an association between different factors, such as education and stroke, and not prove a cause-and-effect link, Sacco said the size of the study helps validate its conclusions.
"Statistical interactions are difficult to detect without very large numbers of people, so these findings are novel," he said.
Maximum of 10 years' education
For the study, Nordahl's team collected data on more than 68,000 adults between the ages of 30 and 70. They classified these participants as having either low-, medium- or high-education levels. Low and medium levels were defined as grade school or lower secondary school – a maximum of 10 years' education.
The researchers also assessed for smoking and high blood pressure.
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The investigators found that 16 percent of men and 11 percent of women had the highest risk for stroke based on their level of education, smoking and high blood pressure.
Over 14 years of follow-up, 10 percent of the men and 9 percent of the women in the lowest education groups suffered strokes. The greatest risk for stroke was among the least educated, regardless of blood pressure, Nordahl said.
The risk of stroke was higher among men than women and increased with age, she noted.
"Smokers with low educational level had a greater risk of stroke than smokers with high education, suggesting that people, particularly men, with lower educational level were more vulnerable to the effect of smoking than those with higher educational level," Nordahl said.
But, Sacco added, "Regardless of education or high blood pressure, smoking is to be avoided at all costs. We need even more intensive efforts among those with lower educational backgrounds to eliminate smoking and reduce stroke risk."