For years doctors have assumed that black people are less likely to develop multiple sclerosis (MS) than whites, but a new study suggests that the opposite may be true

Researchers found that black women were more likely than white women to be diagnosed with MS, in which the protective coating around nerve fibers breaks down, slowing signals traveling between the brain and body. Among men, there was no difference. "The thing I was taught in medical school is that this is a disease primarily of white people," said Dr. Annette Langer-Gould, who led the new study at Kaiser Permanente Southern California in Los Angeles. But most of that older data came from potentially unrepresentative research, she said, such as a study of veterans from the 1950s that found white men were twice as likely to get MS as black men. What's more, she pointed out, most MS clinics are in predominantly white areas - skewing how people perceive the disease. The new report, Langer-Gould said, is the first to look at a group of patients representative of a large population. She and her colleagues analyzed three years' worth of medical records for the 3.5 million patients in the Kaiser Permanente health system. During that time, 496 were diagnosed with MS. The researchers found that over an average year, 10 out of every 100,000 blacks developed the disease, compared to 7 white patients, 3 Hispanics and just over 1 Asian per 100,000. More than two-thirds of all MS diagnoses were in women, and that gender gap was particularly strong among blacks, Langer-Gould and her colleagues found. She said that it's still not clear why the frequency of the disease varies by race, although her team is doing a follow-up study to try to answer that question. Up to 25% of an individual's risk for MS is thought to be due to genes, Langer-Gould said - but the rest likely has to do with everything from environmental influences, such as smoking, to vitamin D and hormone levels. Langer-Gould's group cannot say whether past research truly underestimated how often blacks develop MS, or whether the rates in the new study represent a more recent rise in cases. The new study "very strongly implies that the rate has really gone up in blacks," said Dr. George Ebers, a neurologist who studies MS at John Radcliffe Hospital at the University of Oxford in Britain but wasn't involved in the new research. "This may be the conjunction of the Western lifestyle, whatever that is … plus the fact that they're living in the relatively northern section of the world," where there's less vitamin D from sunlight, for example, Ebers said. If there has been an increase in blacks' risk, he said it would have to be due to the environment - since genes wouldn't change that much over a couple of generations. MS symptoms typically start with numbness and tingling from the waist down or weakness on one side of the body - such as after a stroke. Because of the notion that they're at lower risk, many black patients are initially misdiagnosed, Langer-Gould said.