The health benefits attributed to physical activity are numerous and well known. Exercise has been associated with a lower incidence in many chronic diseases, such as coronary heart disease [6
], type 2 diabetes [7
], obesity [8
], cancer [9
], bone loss [10
], and high blood pressure [11
]. We have reviewed the effects of physical exercise on cognition.
Higher cardiorespiratory fitness has been related to higher scores on tests of cognitive function [12
]. A meta-analysis of randomized controlled trials examining the relationship between exercise and cognition showed modest improvements in attention, processing speed, executive function, and memory among older adults in the treatment arms [13
]. This is highly relevant for the elderly population, as it suggests that physical activity can serve as a preventative measure against age-related cognitive decline [14
Several large longitudinal studies followed older adults without cognitive impairments at baseline and measured rate of incident dementia to clarify the relationship between physical activity and incident cognitive loss. A large prospective study by Podewils et al. identified an inverse relationship between physical activity and dementia risk [15
]. Compared to no exercise, physical activity has been linked with reduced risks of developing cognitive impairment and dementia [16
] with the risk for dementia being further reduced with increasing levels of physical activity. Larson and colleagues found that persons who exercised three or more times per week had a reduced risk of developing dementia compared to those who exercised less, and the reduction was more marked among those with the poorest physical function at baseline [17
]. These results were corroborated by Buchman et al. who found that participants in the lowest percentiles of physical activity had more than twice the risk of developing dementia than those in the highest percentiles of physical activity [18
]. Furthermore, Lautenschlager et al. demonstrated that these results might be transferable to adults with mild cognitive impairment (MCI), and, thus, at high risk for dementia; participants who underwent exercise training showed modest improvements in cognition after six months [19
]. Physical exercise has, therefore, been recommended as a preventative measure of mild cognitive impairment and dementia [20
There is much less research focusing on the effect of physical activity in AD patients. This may be due to the challenges of implementing an exercise regime while managing the behavioral and emotional disturbances in AD patients, particularly in the later stages of the disease. However, the results in the available literature are promising. Early research involving AD patients in nonrandomized controlled trials showed significant cognitive improvements among participants who underwent cycling training and somatic and isotonic-relaxation exercises [22
Physical exercise may have beneficial effects in AD patients beyond cognition as well. A meta-analysis on 30 randomized controlled trials that employed exercise, behavioral and environmental manipulations in patients with cognitive impairment found exercise had positive effects on strength and cardiovascular fitness in addition to improvements in behavior and cognition [24
]. Further evidence supporting multifaceted positive effects of exercise on AD can be traced to recent randomized controlled trials of physical exercise regimes on AD patients ().
Summary of randomized controlled trials of physical exercise regimes in AD patients.
Compared to controls, patients in the intervention programs showed better physical functioning (functional reach, walking, and mobility). After treatment, these patients also showed improved performance of activities of daily living (ADLs), and less cognitive decline and cognitive improvement in some cases. Physical exercise, therefore, appears to be beneficial for AD patients. While the majority of the studies did not find any differences in depression, one study by Steinberg found increased depression and decreased quality of life in patients who underwent the exercise intervention [31
]. Further research into the effect of physical exercise on mood and quality of life in AD patients is, therefore, required.
When considering the role of exercise on AD, it is important to note that any positive results may be due to a placebo effect, even in randomized controlled trials. However, due to the varied nature of the outcome measures used in these studies, it is unlikely that every intervention group demonstrated significant gains over the controls due to a placebo effect alone. Furthermore, control group members never appeared to show any improvement and often showed higher rates of functional and cognitive decline.
The majority of the reviewed studies employed aerobic exercise as a component to their therapy. Yet, two studies demonstrated that nonaerobic activity, such as strength and flexibility training, might also be beneficial to AD patients [29
]. Studies involving cognitively normal adults and nonaerobic exercises offer insight into the potential effects of nonaerobic activity and cognitive decline. Cassilhas et al. found that participants who underwent resistance training showed significant improvements in memory measures compared to controls [35
]. Liu-Ambrose and colleagues, who used a combination of resistance and balance exercises as their intervention, saw significant improvements in response inhibition in their intervention group while the control group deteriorated [36
]. Resistance training may, therefore, provide protective effects for cognition. The effect of stretch exercises, such as yoga, on cognition has been briefly examined; Oken and colleagues found no cognitive differences between their control and yoga groups after treatment [37
]. However, more research into stretch exercises and cognition is required to draw more definitive conclusions. Future studies may investigate which types of exercises, or combinations of exercises, yield the greatest benefit specifically to AD patients. It is important to mention that some of the aforementioned randomized controlled trials [27
] were conducted on participants recruited from long-term care facilities, and thus were likely to be in the more moderate-advanced stages of the disease. These studies demonstrate not only the feasibility of the exercise programs in AD patients but also that patients can benefit greatly from physical exercise even in moderate-to-severe stages of the disease.
Enhanced neuroplasticity might be underlying the improvements seen. Colcombe and colleagues demonstrated that older adults without dementia who performed aerobic exercises had greater grey and white matter volumes compared to adults who engaged in stretching and toning exercises [38
]. Exercise has also been associated with functional connectivity between brain networks often affected by age, such as the default mode, frontal parietal, and frontal executive networks, in older adults without dementia [39
]. While randomized controlled trials in AD patients examining the relationship between neuroplasticity and exercise are underway, correlational studies examining brain volumes and cardiorespiratory fitness have been done. In AD patients, cardiorespiratory fitness has been associated with brain volume. VO2peak
, peak oxygen consumption, has been positively correlated with greater whole brain volume and white matter volume [40
], notably in the inferior parietal lobule, hippocampal, and parahippocampal regions [41
]. Future results of randomized controlled trials will improve our knowledge in this field of research.
Overall, physical activity offers promising outcomes for cognition and physical health in the elderly population and AD patients.