For the past 10 years, evidence has been building that the risk of Alzheimer’s is declining in high-income countries. The newest report comes from the most diverse sample yet. Scientists led by David Weir and Kenneth Langa, University of Michigan, Ann Arbor, reported November 21 in JAMA Internal Medicine that between 2000 and 2012 the prevalence of dementia in the United States fell a stunning 24 percent among people older than 65. The data come from the Health and Retirement Study (HRS), which includes people from different racial and socioeconomic backgrounds. It may mean a million fewer dementia cases over the next two or three decades than were projected, Langa said.
“This could have huge public health and policy implications,” he told Alzforum. Langa said that while the number of people with dementia will undoubtedly increase significantly over the next 20 or 30 years as the population ages, the total burden might be smaller than expected.
Outside scientists praised the work, saying it was in line with similar findings from other longitudinal observational studies. “A 25 percent reduction is a pretty big effect,” said Lon Schneider, University of Southern California, Los Angeles, who was not involved in the work. “If you saw this kind of an effect with a drug, you’d be talking about a cure.”
The studies from the United States, United Kingdom, and Europe that have reported declines in incidence and prevalence of dementia in recent decades all draw their samples from mostly white populations, often from geographically distinct locations (for a review, see Langa et al., 2013; May 2013 news). In an effort to survey a more representative cohort, Langa and colleagues relied on the HRS, which samples from the entire U.S. population. Nearly a quarter of the volunteers were minorities—14 percent identified as black, 8 percent Latino, and 2 percent as other non-Caucasian groups. HRS scientists had previously reported a decline in dementia risk between 1993 and 2002 (Langa et al., 2008). In this study, the researchers wanted to see if that trend continued into the current decade.
The HRS collects data on cognitive health through its biennial survey. That survey contains items adapted from the Telephone Interview for Cognitive Status (TICS), such as immediate and delayed free recall, serial seven subtraction, and backward count tests. If participants are unable or unwilling to take those tests, researchers ask a spouse or adult child about the volunteer’s memory. People are then classified as having either dementia, cognitive impairment without dementia, or normal cognition. In the current study, the researchers compared 10,546 survey responses from the year 2000 with 10,516 from 2012. Each cohort averaged age 75. The 2012 group had a higher proportion of people who were older than 85 and averaged one extra year of education compared with the respondents from 2000.
Despite being older, the prevalence of dementia in the 2012 cohort was 8.8 percent, compared with 11.6 percent in 2000, a relative decline of 24 percent. Likewise, the percentage of those with cognitive impairment fell from 21.2 percent to 18.8. Linear regression modeling suggested that better education and higher net worth best explained the lower risk of dementia. Advanced age, minority status, and a history of stroke or diabetes increased the odds of getting dementia. Being overweight or obese appeared to be protective, a result that seems to agree with a recent controversial report that extra pounds in old age are good for memory (Apr 2015 news).
The falling incidence supports the cognitive reserve hypothesis, which posits that education and cognitive stimulation early in life can help guard against dementia down the road, the authors wrote (Jun 2014 news). However, they pointed out that genetic factors can also underlie education levels, and that better-educated people have healthier lifestyles, more stimulating jobs, and better health care, all of which could influence dementia risk. In addition, the authors noted that obesity, diabetes, hypertension, and heart disease were on the rise in this group over the same time period. A declining rate of dementia at the same time could mean that better control of these cardiovascular risk factors lowers dementia risk.
“Even if we don’t have breakthrough with a medication or other intervention, it appears there are things we as individuals and as a society can do to decrease dementia risk,” said Langa. Furthering education and improving control of cardiovascular risk are obvious starting points, but other factors such as better nutrition and reduced environmental pollution could also come into play, Langa said. The authors acknowledged that the study was based on self-reports that could have led to misdiagnosis, especially where participants suffered from memory loss. New funding from the National Institute on Aging will enable them do more detailed cognitive assessments in future studies, they said.
The growing consensus of a potential decrease in dementia prevalence is encouraging, wrote Ozioma Okonkwo and Sanjay Asthana, University of Wisconsin School of Medicine and Public Health, Madison, in an accompanying commentary. “The focus now should be on better understanding the factors that underlie this trend.”
Carol Brayne, University of Cambridge, U.K., agreed. “We need to understand the influences that have led to these changes and ensure that policy makers are aware that practices begun many decades ago seem to be playing out now,” she wrote to Alzforum. “Current policies should be assessed for their potential to maintain and continue such positive trends,” which are likely different in lower- and middle-income countries, she added. Langa said his group will next collaborate with scientists in England, Mexico, India, and China to try to understand risk factors and trends in dementia in diverse regions of the world.
While the message is optimistic, Schneider cautioned that scientists should not be lulled by the finding. “Just because we see this trend now doesn’t mean it is going to continue,” he said. “We might be maximizing what can be done environmentally and it might flatten out.”—Gwyneth Dickey Zakaib
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