The Phenomenon of Global Aging
Human life expectancy has consistently been increasing throughout the last two centuries, which has led to an increased proportion of elderly persons worldwide. For example, while in 1990 individuals aged 65 years or older accounted for 6% of the global population, the proportion of individuals aged 65 years or older reached 9% by 2019. Moreover, the process of global aging is projected to continue in the future. Thus, while in 2019 the number of individuals aged 65 years or older globally was 703 million, this number is expected to reach 1.5 billion by 2050.1
Cognitive Changes Associated With Aging
The increasing number of elderly individuals worldwide underscores the need to understand the subtle changes in cognition that occur with normal aging and the more drastic cognitive changes that take place with certain medical conditions. The term cognition is broad and encompasses all processes and activities related to the acquisition of information, its processing, retrieval, and storage using thought, senses, and experience.2 Cognition includes different cognitive domains, such as attention (which reflects the ability to concentrate), memory, processing speed (which reflects the speed with which activities are performed), language/vocabulary, executive functioning (the ability to independently engage in purpose-serving behavior), and visuospatial abilities (the capacity to assess visual and spatial relationships). Notably, different aspects of cognition demonstrate different trajectories with aging.3,4 Moreover, there are interindividual variations in the trajectories of cognitive changes with age as well as in the baseline levels of cognitive functions.
Variable Changes in Different Cognitive Domains With Age
As mentioned earlier, different cognitive domains have different trajectories with age. For example, the processing speed generally slows down with age. Some aspects of memory and attention also decline with age. However, the vocabulary remains stable in elderly individuals. Finally, some of the aspects of executive functioning remain stable with age, whereas other decline.3,4
Different Trajectories of Crystallized and Fluid Intelligence With Age
The concepts of crystalized and fluid intelligence also illustrate the variations of cognitive changes with age. Crystallized intelligence refers to familiar, learnt, and practiced knowledge and abilities, such as general knowledge and vocabulary. Fluid intelligence refers to reasoning and problem-solving on topics that a person is less familiar with. In other words, crystalized intelligence relates to information that an individual has learnt throughout their life, whereas fluid intelligence relates to novel information. Interestingly, crystallized intelligence has been shown to remain stable in later life. Contrarily, many aspects of fluid intelligence peak in the third decade of life, after which they slowly decline.3
Health-Related Factors Associated With Cognitive Decline During Aging
Both genetic (related to genes or heredity) and environmental factors affect cognitive changes with aging. For example, several health-related factors have been associated with more rapid or pronounced cognitive decline during aging, including cardiovascular disorders (diseases of the heart and blood vessels), an elevated body mass index (a potential indicator of people’s fatness), self-rated health, physical function, diabetes, and depression.5
Association of Lifestyle Factors With Cognitive Decline in Aging
Several lifestyle factors, including smoking, use of illicit drugs, heavy alcohol consumption, and sedentary lifestyle have also been associated with poor cognitive function in old age.6
Compensatory Mechanisms Against Cognitive Decline With Aging
Cognitive Reserve
The cognitive reserve is one of the compensatory strategies against cognitive alterations occurring with age. The cognitive reserve can be passive, when it relates to brain volume, neuron number, and synapse number (number of connections among the neurons). However, it can also be active when it refers to neuroplasticity, which reflects the capacity of the brain to build and reorganize connections among the neurons, and which is especially prominent in situations involving learning. Factors that have been associated with the cognitive reserve include baseline intelligence, socioeconomic indicators, educational attainment, and certain mental and physical activities.3
Cognitive Training
In studies of cognitive testing in elderly individuals, cognitive retraining has shown positive effects. The utilized cognitive retraining paradigms have included strategies that promote the memory, processing speed, and reasoning. Even though the findings of these studies are promising, they are still associated with significant challenges. For example, most of these studies show an association between cognitive retraining and a positive cognitive outcome, whereas proving the existence of a causative effect is more challenging.3
Activities That Have Been Associated With Positive Findings in Cognitive Testing in the Elderly
Intellectually Engaging Activities
Participating in discussion groups, reading, computer use, playing musical instruments or board games, and solving puzzles have been associated with better results in cognitive testing in elderly individuals.
Physical Activities
Exercise, dancing, and gardening have also been associated with positive cognitive findings.
Social Engagement
Engaging in social activities, cultural events, and traveling have also been correlated with positive cognitive effects.3
The Effects of Combined Physical and Mental Activities
A combination of mental and physical activities (conducted both simultaneously or subsequently) has shown a pronounced positive association with cognitive outcomes in the elderly. Examples are body-mind exercises and a combination of physical and cognitive training.
The Positive Effect of Mind-Body Exercises
Mind-body exercises, such as yoga, qi-gong, and tai-chi, combine physical movement, breathing, and mental concentration. A meta-analysis (a study that synthesizes and analyzes other independent studies) demonstrated that mind-body exercise has a positive effect on cognition in individuals 60 years and older.7 Moreover, the positive effects of tai-chi on cognitive functions in later life have been confirmed by another independent meta-analysis.8
The Combination of Physical and Cognitive Training
A systematic review (a study that systematically reviews other studies on the same topic) has indicated that a combination of physical and cognitive training may have a more pronounced positive effect on cognitive functions in elderly individuals than physical or cognitive training alone.9 However, it should be noted that the positive effects of combined physical and cognitive training were generally limited to trained cognitive abilities and depended on the specific training parameters, such as the duration, intensity, and frequency of the training.9
Dietary Factors and Cognitive Function in Elderly Individuals
The association between dietary factors and cognitive function in elderly individuals has also been investigated. A meta-analysis found that the Mediterranean diet in combination with olive oil is positively associated with global cognition. Moreover, the Mediterranean diet in combination with olive oil and soy isoflavone supplements showed a positive association with memory.8 However, in both cases the observed effect sizes were small.8
The Role of Social Support and Other Social Factors in Cognitive Functions in Later Life
Social support, including support from friends and marital partners, has demonstrated a protective role against age-related cognitive decline.5,10,11 Moreover, higher attained education has been associated with better cognitive function in elderly individuals.11
Structural and Functional Brain Changes Occurring With Aging
The subtle cognitive changes occurring with aging are paralleled by structural and functional alterations in the brain. For example, the volume of the grey matter (brain substance containing the neuronal cells) decreases with age, and this phenomenon is most prominent in the prefrontal cortex. The decline in grey matter has mainly been attributed to decreased neuronal size and number of connections among neurons (synapses) as well as neuronal death. Moreover, there is a reduction of the white matter (brain substance containing the nerve fibers) with age as well as a decline in the integrity of the white matter. Interestingly, the decline in the white matter is more pronounced than the decline in the grey matter.12
Normal Cognitive Aging Versus Pathological Cognitive Changes
Generally, the cognitive alterations that normally occur with aging are subtle. If cognitive impairments develop that interfere even with the (even complex) functional activities of an individual, a medical workup should be conducted to identify any potential underlying cause. Possible reasons for a pronounced cognitive decline may include dementia, another medical condition, a reaction to a medication, or a visual impairment.3
Complexity of Cognitive Aging
Certain subtle cognitive alterations are an inevitable part of aging. However, these changes do not affect all cognitive domains, and there are large interindividual variations in their trajectories. Moreover, the human brain has “reserves” that assist it in compensating for the negative effects associated with aging. Researchers have identified factors related to cognitive decline during aging, including a number of medical conditions and lifestyle choices. Moreover, certain intellectual and physical activities as well as social and dietary factors have been associated with positive cognitive outcomes in elderly individuals. Even though these data are promising, larger studies that clearly prove causality will be needed to identify the most appropriate activities to promote cognitive health during aging.
References
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