We identified three distinct groups based on global cognitive function trajectories over 8 years: cognitive maintainers, minor cognitive decliners, and major cognitive decliners. In contrast to the majority of past research, this study systematically assessed factors that predict maintenance or successful cognitive aging by examining these trajectories of cognitive function in a biracial cohort of elderly men and women.
A valuable outcome is the identification of several predictive factors of successful cognitive aging that are potentially modifiable. Behavioral factors including exercise, smoking, and volunteering are modifiable factors that may be used in strategies to ensure optimal cognitive function in old age. Volunteerism in the elderly, which was higher among cognitive maintainers, has been shown to reduce mortality12
and increase one's sense of well-being.13
It is suggested that benefits of volunteering stem from increased social networks, available recourses, and emotional gratification.14
Cognitive maintainers in the present study were also less likely to live alone, emphasizing the potential importance of social interaction in old age.
Maintainers were more likely to engage in moderate to vigorous exercise compared to minor cognitive decliners. In terms of cognitive function, aerobic exercise training is shown to increase brain volume in once sedentary older adults between the ages of 60 and 79 years.15
Furthermore, prospective and case-control studies report enhancing effects of physical fitness on cognitive function in late life.16–18
These findings, along with the present study, emphasize the importance of physical function on cognitive maintenance in later years.
It is hypothesized that the brain may experience accelerated aging due to metabolic dysregulation including chronic inflammation, oxidative stress, and glucose dysregulation.19–21
Interestingly, prevention programs that incorporate lifestyle changes including cessation of smoking, exercise, and proper nutrition may modify biologic factors,22
which in turn may maintain cognitive function in late life.23,24
Surprisingly, biologic markers did not significantly differentiate cognitive maintainers from minor decliners. The impact of biologic markers may have been diminished due to the nature of the two cognitive groups or may have been overpowered by other factors that were investigated such as weekly moderate/vigorous exercise.
In addition to lifestyle changes, prevention programs have recently focused on building one's cognitive reserve capacity. Cognitive reserve denotes resilience to the clinical presentation of age-related neurodegeneration due to the maximization of alternative neuronal networks or cognitive strategies.25
Cognitive reserve is reportedly determined by innate intellectual ability and by external experiences, including educational attainment,26
which was found to be higher in the cognitive maintainer group. Indeed, previous research shows that lower levels of education and literacy are associated with greater cognitive impairment and increased risk of dementia.27,28
It is important to note that education and literacy level independently predicted cognitive maintenance in the final adjusted model, indicating separate roles of these two factors on maintenance of cognitive function. Effects of education and literacy may explain the present association between race and cognitive maintenance. Although education and literacy were statistically controlled for in the present study, we cannot eliminate the residual confounding that may also contribute to cognitive reserve. Importantly, however, studies show that one can enhance cognitive reserve in old age.29,30
Cognitive training not only improves cognitive function31,32
but may also result in more efficient neuronal activation.33
Also, such cognitive training appears to produce long-term effects and not simply transient stimulus-response associations.34,35
It is interesting to note that predictors of major decline may slightly differ from predictors of cognitive maintenance. For example, behavioral factors such as working/volunteering, engagement in moderate to vigorous exercise, and not smoking seem to be more predictive of maintenance than of major cognitive decline. These differences in predictive value have important implication in defining prevention vs treatment programs in the context of cognitive aging.
An important strength of this study was the ability to define cognitive groups based on participants' cognitive trajectory over several years. In contrast to past cross-sectional definitions largely suggesting an absence of clinical cognitive impairment, this method defines successful aging based on longitudinal performance. While our primary goal was to examine factors that differentiate cognitive maintenance from typical age-related (or minor) cognitive decline, we were able to confirm factors that determine major cognitive decline, such as presence of the APOE
4 gene. Also, the Health ABC study provided access to a wide range of baseline characteristics, enabling examination of factors from various domains. Finally, since the cohort included black and white men and women, the findings are generalizable to the general population of well-functioning elders. A potential limitation of the present investigation is selection bias. Because the cohort roughly represents relatively well-functioning persons aged 70 to 79 years, the proportion of cognitive maintainers may be higher than what would be found in a less well-functioning sample. Another limitation is that we only focused on global cognitive function in defining our three cognitive groups. It is well established that not all cognitive functions decline at similar rates with age. Thus, future studies should use a combination of cognitive tests when defining cognitive trajectories in an elderly population.
Overall, these results suggest that a subset of the aging population may be characterized as successful agers based on maintenance of cognitive function over several years. With an anticipated exponential expansion of the population that will develop dementia and the resulting costs to society,36
it is increasingly important to investigate and understand the mechanisms that promote optimal cognitive function in old age. The present study demonstrates that cognitive maintainers display a unique profile consisting of factors, some of which are modifiable, that differentiate them from the majority of elderly who experience age-related typical cognitive decline.
Further research is needed to replicate the present findings and expand on the potential predictors of successful cognitive aging. Also, future studies should incorporate imaging techniques to assess the correlation between cognitive maintenance and brain function/morphology over time. Elucidating predictors of optimal cognitive function will enable the development of strategies that may prevent onset or slow the progression of dementia in late life. Such models will not only reduce the projected costs to society, but will further ensure increased quality of life in later years.