Although the number of people suffering a stroke has declined 40 percent over the last 16 years, the decrease hasn’t been seen in young people or black people, according to a study by researchers at King’s College London.

The team analyzed stroke incidence from the South London Stroke Register, which covered more than 350,000 people. Between January 1995 and December 2010, 4,245 patients had a first-ever stroke, a 39.5 percent decline, and the average first stroke decreased from 71.7 years to 69.6.

But the number of 15-44 year-olds who had a stroke increased significantly, from 5.1 percent to 8.4 percent, and from 11.1 percent to 19.9 percent for those 15-54. The increased risk for younger people could be due to a rise in cardiovascular risk factors like diabetes, obesity and high cholesterol.

More black people — up nine percent to 25.6 percent — also had a stroke in the same time period. The ethnic disparities may be because of cardiovascular risk factors such as high blood pressure and diabetes. There may also be cultural differences in perceptions of health and the healthcare system, environmental exposures, genetic factors, socioeconomic status and education level, according to the study authors.

“This study shows a 40 percent overall reduction of stroke over 16 years, which is good news. However, we noticed strikingly higher stroke rates in black groups and in younger people,” said Dr. Yanzhong Wang, lecturer in medical statistics at King’s College London. “The reasons for this are not entirely clear but it could be because of a rise in diabetes and obesity in these groups. If this trend is not reversed we could face a major public health concern because long-term disability, as a result of stroke, will put a strain on health and social care services.”

The researchers say more work needs to be done to understand the reasons and help with widespread stroke prevention.