It’s been known that middle-aged blacks are more likely to die of stroke than whites.

Now researchers know why: African-Americans are having more strokes at younger ages.

The REasons for Geographic And Racial Differences in Stroke (REGARDS) Study — published recently in the American Heart Association journal Stroke — showed that at age 45, blacks were four times more likely to die of stroke than whites.

By age 85, there was no difference in stroke death rates among blacks and whites.

“The magnitude of public health burden of the racial disparity in stroke is staggering, with an estimated 22,384 ‘extra’ stroke events, i.e., above what would be expected relative to rates in whites occurring in blacks in 2014,” said George Howard, Dr.P.H., the study’s lead author and a professor of biostatistics at the University of Alabama at Birmingham School of Public Health.

“With a lifelong cost of stroke of $104,000 for each of the events, this black-white difference in stroke costs America more than $2.3 billion annually. Obviously, efforts that would even marginally reduce this burden would pay remarkable dividends for the United States,” he said.

REGARDS researchers analyzed data on 29,681 people selected at random from across the United States. Conducted from 2003 to 2007, they interviewed participants by mail and telephone to assess risk factors, followed by regular in-home visits to collect other biological samples and measure blood pressure, height and waist circumference.

“We need to do more to focus on prevention and control of risk factors (such as high blood pressure and diabetes that are more common in African-Americans) before they result in a stroke,” Howard said. “Unfortunately, most of the research resources are focused on treating stroke patients and preventing recurrent stroke.”

To reduce the racial disparity in stroke, “we need to go further upstream to make inroads” on racial differences in the traditional and non-traditional risk factors that lead to stroke, researchers said.