When the population as a whole is aging because of historical changes in birth and mortality rates, meaning that an increasing percentage of people are older rather than younger with each passing year, it is perfectly possible to observe both a growth in the total number of cases of age-related disease and at the same time a reduction in the rate at which individuals develop age-related disease. Both of these trends are underway at the present time. In this context, the short article noted here reviews some of the epidemiological research that indicates the risk of suffering dementia is falling.
Numerous studies have reported a dip in dementia incidence in the developed world. When did this trend begin? Researchers analyzed birth cohort data from the Einstein Aging Study, which enrolls cognitively healthy older adults living in the Bronx. Surprisingly, people born after 1928 were 85 percent less likely to develop dementia than those born before that year. The reason for such a stark drop in incidence is unclear. Neither better education nor improved cardiovascular health accounted for the effect. "The birth cohort effect is intriguing but will need replication in other populations. This important insight compels us to search for novel social and environmental factors that may have impacted this birth cohort. Changes in nutrition, education, pollutants, and infections all occurred and would be worth examining."
A growing number of studies have reported a drop in dementia incidence in the U.S. and Europe over the last two or three decades. Researchers have speculated that this may be due to better public health, particularly cardiovascular health. The finding is not uniform, however, with a handful of studies reporting higher dementia incidence that may be due to greater recognition of the disease or a larger number of people reaching old age.
To try to clarify the picture, researchers examined data from participants who enrolled in the Einstein Aging Study between 1993 and 2015. The cohort comprised 1,348 participants who had completed at least one annual follow-up visit, with an average follow-up time of four years. All participants were older than 70, and about two-thirds were non-Hispanic white. The researchers diagnosed dementia by a clinical exam. A subset of participants donated their brains after death, and 96 percent of those with a dementia diagnosis had some type of extensive brain pathology. For example, in a subgroup diagnosed with Alzheimer's disease, 79 percent had plaques and tangles.
Within each age group, the researchers saw a steady drop in dementia incidence for those born in later years. Among people born before 1920 there were 5.09 cases per 100 person-years. This dropped to 3.11 for people born in the early 1920s, and 1.73 for those born in the late 1920s. The most dramatic shift occurred right at the turn of that decade, when the rate fell to 0.23. Mathematical modeling pegged the best estimate for the change point to July 1929. While the model suggests an abrupt change in dementia rates, the researchers noted that this might partly be the result of small sample size; the post-1929 cohorts totaled only 350 people, with just three cases of dementia among them. "If there were more people in the analysis, the trend might be smoother." Nonetheless, the findings were statistically significant, and the researchers believe the data are picking up a real decline in dementia risk at around this time point.
What might explain it? The researchers found marked decreases in the rates of heart attack and stroke in later birth cohorts, but after adjusting the model to account for this, the drop in dementia incidence in those born after 1929 remained unchanged. While previous epidemiological studies did not specifically examine birth years, those older findings are roughly congruent with the Einstein Aging Study data, reporting the greatest drop in dementia cases after 1990, the authors noted. People born after 1929 would have entered their 60s in that decade. Most cases of late-onset dementia occur after age 60. The Rotterdam Study found a 25 percent decrease in dementia incidence in the 1990s, while the Framingham Heart Study recently reported that incidence dropped starting in the late 1980s and continued to decline into the 2010s.