Multiple sclerosis (MS) is more common than previously thought in African-Americans. In fact the risk is higher in black women compared to white women, a new population-based study shows.

Researchers published their findings in the May 7 issue of Neurology. They examined more than 3.5 million patients in the Kaiser Permanente Southern California health plan database from January 2008 to December 2011. Out of the 496 patients recently diagnosed with MS, 21 percent were African-American.

MS is a debilitating disease in which the immune system damages myelin, the protective coating that envelopes nerves. Although a person living with MS lives as long as the general population, the disease costs the U.S. billions of dollars. In this study, 70 percent of the cases were in women.

"Our findings do not support the widely held belief that blacks have a lower risk of MS than whites, but that MS risk is determined by complex interactions between race, ethnicity, sex, environmental factors and genotypes," said lead author Dr. Annette Langer-Gould of the Kaiser Permanente Southern California Department of Research & Evaluation. "Although additional research is needed, possible explanations for the higher incidence of MS in black women include a greater prevalence of hormonal, genetic, or environmental risk factors such as smoking, compared to patients from other racial or ethnic groups."

Although African-Americans represented 10 percent of the total study, they had a 47 percent higher risk of developing MS compared to whites. Hispanic and Asians have less risk than Caucasians by 58 and 80 percent, respectively.

"One explanation for our findings is that people with darker skin tones have lower vitamin D levels and ultimately, an increased risk, but this would not explain why Hispanics and Asians have a lower risk than Caucasians," Dr. Langer-Gould said.

Specifically, the greatest risk was found in African-American women, while the risk was lower for both men and women of Hispanic and Asian ethnicities.

Nearly 19,000 people per year, or 250 people every week, are diagnosed with MS in the United States, researchers say. Experts estimate that 2.1 million people have the disease worldwide.

"These numbers highlight the need for more minorities to be included in MS studies, so that we can fully understand how race may play a role in developing the disease," Dr. Langer-Gould added.