CHAPEL HILL - After eliminating the influence of smoking, high blood pressure, diabetes, education levels and existing heart disease, black Americans still face a 38 percent higher risk of stroke than U.S. whites do, a large new study indicates. No one knows why.
Before accounting for those factors, black men face a two-and-a-half times greater chance of stroke than white men do. Black women's stroke rate falls about half way between black men and white men, and the rate for white women is lowest of the four groups studied.
"We already have good data on the fact that blacks and whites differ in stroke mortality, with blacks being more likely to die from stroke," said Dr. Wayne D. Rosamond, assistant professor of epidemiology at the University of North Carolina at Chapel Hill School of Public Health. "What this study gives us is new data on the incidence of the disease, which is a direct measure of their risk of getting it.
"Differences between whites and blacks may result from obesity or diet or factors we haven't identified yet."
A report on the research appears in the just-published April issue of the journal Stroke. Besides Rosamond, UNC-CH authors are Drs. Lloyd E. Chambless, research professor of biostatistics, and Chin-Hua Wang, research investigator in biostatistics.
The study, known as Atherosclerosis Risk in Communities, or ARIC, involved following a group of 15,792 men and women ages 45 to 64 an average of 7.2 years from 1987 to 1995.
Researchers recruited subjects from four geographically diverse U.S. communities - Washington County, Md.; Jackson, Miss.; Minneapolis, Minn.; and Forsyth County, N.C. They determined stroke rates from yearly telephone contacts with participants and careful searches of area hospital records.
During the period analyzed, they found 267 strokes, 83 percent of which were classified as ischemic stroke and 17 percent internal bleeding in or near the brain.
Hemorrhagic -- or internal bleeding - stokes were 4.5 times as likely to be fatal as the ischemic strokes, which are caused by insufficient blood flow through the brain.
"Although further understanding of the reasons for the observed excess in death from stroke among blacks is needed, this study provides a partial explanation for the observed excess," Rosamond said. "It's likely due to the fact that blacks get this disease more often, rather than a reduced ability to survive a stroke once it happens."
Knowing that much of the difference in stroke mortality between the races is due to incidence and not case fatalities suggests health-care specialists need to focus more on prevention.
"We found that differences in smoking history and prevalent heart disease explained only part of the race-ethnicity difference in stroke occurrence," he said. "Much of the excess stroke risk in blacks was attributable to more hypertension and diabetes, with a further proportion attributable to differences in educational achievement."
An estimated 600,000 to 731,100 stokes occurred in 1996 in the United States, making stroke the third-leading cause of death and the leading cause of severe neurological disability, Rosamond said. Although the rate of death from stroke declined 17.3 percent from 1985 to 1995, aging of the U.S. population contributed to a 3.2 percent increase in the actual number of stroke deaths.
North Carolina, South Carolina and Georgia experience the highest rates of stroke mortality in the nation, he said.
Researchers did not study obesity directly because they believed eliminating the effects of high blood pressure covers obesity, Rosamond said. They also did not study the influence of diet directly because people's reports of what they eat often are inaccurate.
Other authors of the paper include Drs. Aaron R. Folsom, Paul G. McGovern and Eyal Shahar of the University of Minnesota, Dr. George Howard of Wake Forest University and Dr. Lawton S. Copper of the National Heart, Lung and Blood Institute.
Note: Rosamond can be reached at 919-962-3230.