THURSDAY, Jan. 21, 2010 (HealthDay News) -- The more overweight you are, the more likely you are to have a stroke, a new study reports.
The study, which followed 13,549 middle-aged Americans for 19 years, looked at stroke risk associated with several measures of obesity, emphasizing body mass index (BMI), a ratio of weight and height, but also such measures as waist circumference.
"We found that the risk of stroke was increased with each measure of obesity," said Dr. Hiroshi Yatsuya, a visiting associate professor of public health at the University of Minnesota and lead author of a report published online Jan. 21 in Stroke.
The degree of risk varied by sex and ethnic group. For example, people in the highest BMI category had a 1.43 to 2.12 times higher risk for stroke, with variations by race and sex. The risk ratios ranged from 1.65 to 3.19 when obesity was measured by waist circumference and from 1.69 to 2.55 when the ratio of waist to hip was used.
The risk was especially high for blacks, the study found. For example, the incidence of stroke was 1.2 per 1,000 person-years for white women and 4.3 per 1,000 person-years for black women. In the highest BMI category, rates ranged from 2.2 for white women to 8.0 for black men.
That higher incidence of stroke for blacks has been found in many previous studies, and it also is seen in Asians, Yatsuya said. The reason is unknown, but "there may be a genetic difference," he said.
But the increased risk linked to weight was evident in every ethnic group, the study found. Throughout, men and women in the highest obesity category had about double the risk for stroke as did those in the lowest category.
Obesity appears to act by increasing the incidence of high blood pressure and diabetes, two major risk factors for stroke and other cardiovascular problems, the study indicated. When blood pressure readings and diabetes were factored into the calculations, the association between obesity was weakened, "suggesting these major risk factors explain much of the obesity-stroke association," Yatsuya said.
And despite the mountain of evidence linking obesity and stroke, "we don't have clear evidence that obesity reduction reduces stroke rate," he said. Controlled trials are needed to prove that obesity prevention or reduction reduces stroke risk, he said.
Daniel Lackland, a professor of epidemiology at the Medical University of South Carolina and a spokesman for the American Heart Association, said that the exact relationship between obesity and stroke risk has not been established, but the new study "contributes to a better understanding of it."
"Obesity contributes to both diabetes and hypertension [high blood pressure], which are associated with stroke and at an earlier age," Lackland said.
And though definitive proof might be lacking that obesity prevention or reduction reduces the risk for stroke, the American Heart Association and other major organizations stress the need for weight control by proper dieting and physical activity, he said.
A BMI of 20 is regarded as normal, with overweight beginning at 25 and obesity at a reading of 30.
The U.S. National Heart, Lung and Blood Institute offers a simple way to calculate your BMI.