Populations of seniors increase in the US
But the incidence of dementia seems to be declining
The rising prevalence of older people in America portends a sharp increase in dementia. But recent studies suggest a silver lining: reduced risk of dementia among younger cohorts of older people. Yet, these studies suffer from design defects. The Framingham Heart Study eliminated many of these defects. Improvements included following individuals for up to 33 years and experts documenting dementia using multiple criteria. This study began in 1948 with 5,205 participants in Framingham, MA. In 1971, 5,214 offspring of the original cohort and their spouses expanded the study.
Starting in 1975, researchers established a dementia-free sub-cohort that included the original and offspring groups. A team of researchers evaluated incident dementia in four non-overlapping periods of five years each. These periods included 1) 1977-1983; 2) 1986-1991, 3) 1992-1998, and 4) 2004-2008. The five-year, sex and age-adjusted rates of dementia declined during all four periods—from 3.6 to 2.8 to 2.2 to 2.0 cases per 100 people. From the first to the fourth period, risk of dementia declined by 44 percent. Interestingly, the reduction in risk occurred only for participants with at least a high school diploma. During the study, some vascular risk factors (obesity, diabetes) increased, while others (stroke, atrial fibrillation, heart failure) decreased. Researchers could not explain the observed decline in dementia risk.
Recent studies suggest that dementia incidence in some high-income countries has dropped since about 1990. A study of participants in the Health and Retirement Study (HRS), a nationally representative sample of US adults over age 50, provides additional evidence. Researchers examined data from 21,057 participants with an average age of 75 years in the 2010 and 2012 waves of HRS. The prevalence of dementia declined from 11.6 percent in 2010 to 8.8 percent in 2012, a 24 percent drop. Why might this have happened? More formal education predicts lower risk of dementia. The average number of years of education increased by nearly one year from 2010 to 2012. On the other hand, the prevalence of self-reported cardiovascular risk factors (obesity, diabetes, heart disease, hypertension, body mass index) increased from 2010 to 2012. These risk factors predict increased risk of dementia. Thus, researchers could not account for the observed decline in dementia risk.
The Alzheimer Cohorts Consortium published a new study of trends in dementia over 27 years. Researchers used data from 49,202 participants aged 65 years and older in seven longitudinal studies conducted in Europe and the US. The incidence of dementia increased greatly with age, from 4 per 1,000 person-years for 65-69 year olds to 69 per 1,000 person-years for 85-89 year olds. But the incidences of dementia and clinical Alzheimer’s disease declined by 13 and 16 percent, respectively, per 10-year calendar decade. The decline in risk of dementia was larger for men (24 percent) than for women (8 percent). If the trend in declining incidence of dementia continues, 15 million few persons in Europe and the US could develop dementia than previously estimated. Given that the human genome has not changed this much over the past 27 years, the reduced incidence of dementia might reflect a net improvement in healthy lifestyle choices. For example, Keep Moving, Eat Better, and Defuse Chronic Stress.
