Contrary to previous research, a new study has found that exercising in midlife is not associated with delaying the onset of cognitive decline and dementia — including Alzheimer’s disease — later in life.
Needless to say, this finding is disappointing. For, as the authors of the study point out, there is some “biological plausibility” behind the idea of an association between physical activity and long-term cognitive health — primarily because of exercise’s beneficial effect on the cardiovascular system, including improving blood flow to the brain.
The study, which was published last week in the Journal of Alzheimer’s Disease, does include a bit of encouraging news, however. It found that older people who report that they are engaged in regular exercise tend to score better on cognitive tests two years later than their non-exercising peers.
That finding supports other research that suggests exercise can help people keep cognitively fit in the short term.
“This study reminds us that physical activity has all sorts of benefits for people, including promoting cardiovascular health, managing optimal weight levels and maintaining bones and muscle mass,” said Alden Gross, the study’s lead author and an epidemiologist at Johns Hopkins University’s Bloomberg School of Public Health, in a released statement. “Unfortunately, it is too early for us to say the same about exercise and Alzheimer’s, especially as a possible long-term preventive measure.”
A more rigorous study design
Why the findings of this study are so compelling is that it was a longitudinal study — it followed hundreds of adults for more than three decades. Previous studies that have suggested exercising in mid-life may help stave off dementia were frequently cross-sectional studies, which analyze data collected from people only at a single point in time. Both studies are observational (and thus cannot prove causation), but a longitudinal study helps to significantly reduce the problem of reverse causation — in this case, the possibility that as people develop dementia, they reduce their physical activity.
For their study, Gross and his colleagues used data collected from 646 mostly male participants in the Johns Hopkins Precursors study, which has been called “the granddaddy” of longitudinal health studies. The participants were initially enrolled in the study between 1948 and 1964, while they were medical students at Johns Hopkins. Each had answered detailed health questionnaires, including about their weekly physical activity, annually since graduation. (After 1978, one of the questions asked if they regularly exercised to a sweat — an attempt to determine the intensity of their exercise regimen.) Each had also undergone five different kinds of cognitive testing in either 2005 or 2008.
As the researchers explain in their study, the participants’ homogeneity — all students at a selective medical school — is one of the study’s strengths, for it helped to eliminate confounding factors that are also known to affect the risk of dementia, particularly education and socioeconomic status. (Of course, the homogeneity of the participants is also one of the study’s limitations, for it means that the findings may not apply to broader, more diverse populations.)
Using the cognitive tests and medical records, Gross and his colleagues were able to determine the cognitive fitness of the participants, including the onset of dementia, through 2011. During the study’s time period, 28 participants, or 4.5 percent, developed Alzheimer’s disease.
An analysis of the data found no correlation, however, between the participants’ reported physical activity at midlife and their risk of dementia — or their scores in later life on the cognitive tests. Nor did they find any relationship between the participants’ pattern of exercise over their lifetime and their cognitive health in later life (although most of the participants did not change their exercise habits much over the decades).
The analysis did reveal, however, that participants who reported higher levels of physical activity within two years of the cognitive testing tended to score higher on those tests.
One major limitation of this study is the relatively low number of participants who went on to develop dementia, which, as the study’s authors point out, may have limited the ability of the study to detect an association with exercise habits. Also, the participants self-reported their physical activity, and such reports can be unreliable.
The study’s findings do support, however, a 2015 Cochrane review that found no link between aerobic exercise and cognitive performance among cognitively healthy older adults, even when cardiovascular health was improved.
Such findings should not, however, lead anyone to stop or curb their exercise regimen. Regular physical activity has been shown repeatedly to be one of the best preventive “medicines” against a host of chronic and age-related illnesses, including heart disease and type 2 diabetes.
All this study suggests is that Alzheimer’s disease may not be one of them.
“These findings have implications for intervention work moving forward,” said Gross. “We still need to focus on causes and mechanisms of Alzheimer’s and dementia, since we don’t yet know which preventive measures may or may not work.”
“For now, when I speak in the community about Alzheimer’s, I find that people take some relief in understanding that there wasn’t anything that anyone might have done to avoid a loved one developing Alzheimer’s,” he added. “Of course, the goal for researchers is to identify factors that may help older people maintain their cognitive function into their later years. More long-term studies like the Precursors study are needed.”
For more information: You’ll find an abstract of the study on the website of the Journal of Alzheimer’s Disease, but the full paper is behind a paywall.