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JACKI LYDEN, HOST:

Back in May, we reported a story on the projected rise in Alzheimer's cases. The National Institute on Aging estimated that the number of people living with that disease would triple by 2050. Now a pair of new studies has shown that rates of dementia, a broader category of brain disease of which Alzheimer's is the most common form, may actually be on the decline.

Dr. Carol Brayne of Cambridge University was the lead researcher on one of the studies. Back in the early '90s, she helped compare dementia rates across different locations in England.

DR. CAROL BRAYNE: Because it was standardized, because we had to be very clear about how we were doing everything, we were able to repeat that study near 20 years later.

LYDEN: The new study, published last week in the British medical journal The Lancet, used the same methodology on today's population. Here's the good news.

BRAYNE: What we found was that there was lower prevalence. That is, the proportion of the population who met the diagnostic criteria had dropped fairly consistently across the age groups and across the genders as well.

LYDEN: In the original study from the early '90s, 8.3 percent of the people met all the criteria for dementia. But in the new findings, the number fell to just 6.2 percent. The prevalence had fallen by a quarter. Brayne says it's tough to pin down why exactly this is happening.

BRAYNE: If you think about what's going on in the population more generally, in that time, life expectancy has increased.

LYDEN: Dr. Brayne points to better medical care for people at risk for dementia. Better diets also play and as do other social factors.

BRAYNE: And also very importantly, our populations are better educated than they were.

LYDEN: Also published recently was a study conducted in Denmark. Though it had different methods from Dr. Brayne's report, these researchers also came to the conclusion that dementia rates are decreasing.

BRAYNE: It adds to the confidence in thinking that there really is a change. And we can perhaps revise our estimates for this cohort downwards.

LYDEN: At least in Western Europe, that is. As for whether those of us in the States can share in the good news, Dr. Brayne says it depends.

BRAYNE: Particularly in those populations who have been able to make the most of all the health messages and health opportunities, treatment opportunities and education and so on.

LYDEN: But she also notes that the United States is much bigger and more diverse than Britain or Denmark, and therefore harder to pin down. The National Institute on Aging and the Alzheimer's Association have been cautious about responding to the studies. They point out that a smaller percentage of people may be getting dementia, but the sheer size of the aging population in the U.S. means that the total number of people with dementia and Alzheimer's will continue to rise.

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