Alzheimer’s & Dementia: Exercise and Dementia — Does Physical Activity Provide Cognitive Benefits?
The World Health Organization recommends regular physical exercise—both aerobic and strength training—for older individuals as a means of reducing cognitive decline.1 However, studies on the effects of exercise on cognitive function in individuals with dementia have produced mixed results. While some research indicates a positive effect, other studies have failed to find clear benefits. Thus, the question remains: Is exercise actually effective at slowing down cognitive decline in individuals with dementia?
Evidence in Favor: Cognitive Benefits in Dementia
A weakness of most randomized controlled trials showing a cognitive benefit of exercise in dementia is that the study populations have been small. One 2016 trial, however, tested a moderate- to high-intensity exercise intervention in a larger sample: 200 community-dwelling patients with mild Alzheimer’s disease (AD). Participants were randomized to either a supervised exercise group (one-hour sessions three times per week for four months) or to a control group. The study found no effect on cognition in the exercise group as a whole; however, in an exploratory analysis, the researchers found a possible beneficial impact on cognition among those who were most consistent in attending exercise sessions and who exercised at the greatest intensity, suggesting a dose-response relationship between exercise and cognition.8
Evidence Against: No Cognitive Benefits in Dementia
In the case of the Swedish trial, the researchers hypothesized that the lack of benefit could be due to the fact that the exercise intervention focused on strength training rather than aerobic exercise. But a 2018 randomized control trial from British researchers produced no better results with aerobic exercise. In this large, carefully designed trial of almost 500 participants with mild to moderate dementia, participants were assigned to either an exercise group (which included both aerobic exercise and strength training) or to a usual-care group. Not only did exercise fail to produce cognitive benefits, but those in the exercise group actually demonstrated slightly worse cognition at the end of 12 months than did those in the usual care group.10
Mixed Results From Meta-Analyses and Systematic Reviews
Specifically, a 2015 systematic review of 17 randomized controlled trials found only very limited benefits of exercise in dementia—namely, researchers concluded that exercise programs may improve ability to do activities of daily living in dementia, but that exercise provides no benefits for cognition, neuropsychiatric symptoms, or depression.11
By contrast, however, two other recent meta-analyses reached the opposite conclusion and have affirmed the benefits of exercise—especially aerobic exercise—for cognition in dementia. The first of these meta-analyses, published in 2016, found that exercise has a positive benefit on cognition in both AD and other dementias and that both high-frequency and low-frequency exercise programs are beneficial.12
The second meta-analysis with a positive result, published in 2018, included 19 randomized controlled trials involving patients with AD as well as those at high risk of AD. This meta-analysis found that exercise interventions appear to slow cognitive decline in both groups—in those who have AD as well as in those at risk of the disease.13
Resolving the Inconsistencies
Not only are there a limited number of studies, but many of those that are available have been small and of relatively poor methodological quality. In fact, the authors of the 2015 systematic review that found no cognitive benefits of exercise in dementia explicitly noted that there was considerable unexplained heterogeneity in the analysis, and that the quality of the evidence was “very low.”11
With respect to meta-analyses in particular, Panza, a doctoral candidate in the department of kinesiology and the Human Performance Laboratory at the University of Connecticut, notes a major weakness of several analyses that have found a lack of impact of exercise on cognition: Namely, they have included mixed samples of people with multiple types of dementia (AD, vascular dementia, and other types of dementia) and analyzed them all together as one sample, rather than examining each group separately. “This is an issue because there are several physiological differences among the different types, and as a result, exercise may be affecting each type of dementia differently,” Panza says. Additionally, previous meta-analyses usually have failed to examine moderators such as age and gender. It’s important to examine moderators, he says, “because it gives you valuable information on which variables may be influencing the impact that the exercise is having on cognitive function.”
To address these limitations of previous research, Panza and his coauthors adhered to high-quality methodological reporting standards in their own 2018 meta-analysis, suggesting that their group’s finding of a positive cognitive benefit of exercise in dementia may carry more weight than the negative findings of some previous analyses. In their study, Panza and his colleagues also conducted within-group analyses (in which they compared cognitive changes both before and after the intervention for both the exercise and control groups), rather than merely conducting a between-group analysis as had previous meta-analyses. The within-group analysis allowed the group to take into account the cognitive decline that occurs naturally with untreated disease in the control group, and this analysis revealed the novel finding that exercise could improve cognition among controls. Overall, then, the Panza’s meta-analysis offers important support to the hypothesis that exercise can indeed slow cognitive decline in dementia.
Exercise in Midlife Protects Against Dementia
Some research suggests that exercise may have especially significant benefits for individuals at highest genetic risk for dementia. A 2014 study, for instance, examined a group of 97 cognitively normal adults and compared how high vs low levels of physical activity correlated with each group’s hippocampal volume over the following 18 months. Researchers found that exercise had no apparent impact on hippocampal volume in those without genetic risk. But in those at genetic risk (that is, carriers of the APOE-E4 allele), low levels of physical activity were associated with a decline in hippocampal volume. This same group of less-active, higher-risk individuals was also more likely to show both cognitive and functional decline over the study period.16
According to Stephen Rao, PhD, Ralph and Luci Schey Endowed Chair at the Cleveland Clinic Lou Ruvo Center for Brain Health, a main mechanism by which exercise is thought to affect dementia risk is by affecting inflammation. “What exercise seems to be doing is reducing the amount of inflammation that ultimately is a very important factor in the progression of the disease. The disease is going on for 10 to 15 years prior to its diagnosis. So anything you can do to alter processes like inflammation can make a big dent in the rate of progression of the disease.”
To be clear, not all research shows a protective benefit of exercise against dementia: One 2018 systematic review and meta-analysis found that randomized controlled trials on exercise for dementia prevention are limited, but that the existing evidence does not show any significant effect of exercise in terms of reducing dementia risk.17
However, several other meta-analyses have come to the opposite conclusion. A 2011 meta-analysis of 15 prospective studies that included a total of more than 33,000 subjects without dementia concluded that all levels of physical exercise, from low to high, offer a significant and consistent protective effect (-35% or greater) against cognitive decline.18 Similarly, a 2016 meta-analysis of 10 high-quality prospective observational cohort studies found that those who were more active had a 35% to 40% lower chance of developing AD than did those who were less active.19
Implications for Providers
Rao likewise acknowledges the unknowns, but he too affirms that exercise appears to be an important means of reducing dementia risk. “Exercise is key. It’s never too late. Providers should really encourage their patients to exercise, within reason, within their level of fitness.”
— Jamie Santa Cruz is a health and medical journalist based in Parker, Colorado.
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