Is the incidence of Alzheimer disease decreasing?
Several recent studies support the notion that the incidence of dementia and Alzheimer disease has been decreasing over at least the past 25 years, and at a remarkable rate. In cohort studies from Stockholm just published in the journal Neurology (Qiu et al 2013) and from Rotterdam published last year in the same journal (Schrijvers et al 2012) key observations were decreased mortality by 35-37 %, increased longevity, increased education, and increased statin use are associated with lower dementia incidence.
In Rotterdam the decrease was about 25% over 10 years. An earlier US study suggested a 3% annual decline in dementia incidence over 10 years around the early 1990s in Rochester, Minnesota (Rocca et al 2011). These observations – if supported by other studies – somewhat temper the dire predictions for a boomer-fueled, Alzheimer’s “Silver Tsunami” as Martha Stewart and some charities call the projected increase in dementia.
It is probably not too speculative to consider that any decrease in age-specific incidence of dementia in cohorts separated by only about 10 years may result from environmental and public health interventions that were implemented just within the last 40 years. Both the Flynn effect (2009) and Fries’ prediction of optimal longevity and compression of morbidity (Fries 1980) may be in action. At least in Western countries people may be getting smarter and healthier, surviving better, and pushing morbidity back toward the end of life such that they get cardiovascular disease and dementia later and die sooner afterwards.
Baby-boomers and their slightly older sibs may have been weaned on bacon, eggs, toast, butter, frosted flakes, and Marlboros; but beginning in the 1970s they have had some profoundly improved public health options: better food information and choices; gyms as destinations for purposeful exercise; smoking awareness and restrictions; cardiovascular medications and statins, and arguably better air quality. Any one or several of these options combined with genetics, better education, and luck could have helped to lower the chances for cardiovascular disease or dementia. Looking forward there may be continuing benefits from mandatory restrictions in trans fats, sweetened drinks, and no cigarettes until age 21.
The decrease in dementia as well as in cardiovascular deaths, interestingly, may be occurring in the presence of greater obesity and diabetes. Some factors may outweigh others and the trends may not last. As these are observational studies over short periods, and not experiments, little can be proven, and both suspected and unknown biases may be at play. However, the idea that public health interventions may not only work but have marked effects, that we might grow our way out of a dementia “tsunami” and substantially reduce risk is intriguing and wholly consistent with the range of hypotheses about Alzheimer disease and the amyloid theory.
It appears that the boomers – even at their advanced age – can look forward to a longer life with less heart disease and dementia. Of course they will need to have their retirement plans in order and work into their 70s.
Qiu C, von Strauss E, Bäckman L, Winblad B, Fratiglioni L. Twenty-year changes in dementia occurrence suggest decreasing incidence in central Stockholm, Sweden. Neurology 2013.
Schrijvers EMC, Verhaaren BFJ, Koudstaal PJ, Hofman A, Ikram MA, Breteler MMB. Is dementia incidence declining? Neurology 2012; 78(19): 1456-63.
Rocca WA, Petersen RC, Knopman DS, et al. Trends in the incidence and prevalence of Alzheimer’s disease, dementia, and cognitive impairment in the United States. Alzheimer’s & Dementia 2011; 7(1): 80-93.
Fries JF. Aging, Natural Death, and the Compression of Morbidity. New England Journal of Medicine 1980; 303(3): 130-5.
Flynn, JR. What Is Intelligence: Beyond the Flynn Effect. Cambridge: Cambridge University Press2009 pp. 1–2. ISBN 978-0-521-74147-7.