Cognitive Decline Precedes Physical Decline in Older Adults

Experts generally agree that older adults who remain physically active tend to also remain mentally sharp far into their senior years. This is generally explained in terms of cognitive reserve, which is viewed as a sort of shield against the onslaught of dementia in old age. Habitual physical activity, social engagement, and mental exertion are all believed to confer cognitive reserve.

While this view of cognitive reserve certainly has intuitive appeal, there’s a fundamental problem with the way in which we tend to think of the relationship between psychological and physical health in old age. Specifically, we know that seniors who are physically, socially, and mentally active show few signs of cognitive decay, so we recommend that all seniors engage in more of these activities to avoid dementia. In particular, we often assume that maintaining physical activity actually causes older adults to remain cognitively sharp.

However, to date, no such study has demonstrated this purported causal relationship between physical activity and cognitive reserve. Rather, all studies so far have been correlational in nature, meaning that they’ve established that physical activity and cognitive reserve are related. But they haven’t demonstrated that the former actually causes the latter.

While the assertion that physical activity confers cognitive reserve has intuitive appeal, our intuitions often deceive us, especially in the realm of science. In fact, it could very well be that cognitive decline precedes physical decline in older adults. This is exactly the hypothesis that University of Geneva (Switzerland) psychologist Boris Cheval and colleagues tested in a study recently published in the journal Health Psychology.

The researchers start off with the observation that humans are by nature lazy. We all cut corners and minimize exertions. Nobody ever really has a burning desire to run five miles on the treadmill or lift weights for 45 minutes. Rather, it takes quite a bit of willpower to stick to an exercise regimen, especially in modern society where there are so many sedentary activities that are far more appealing.

Yet, if this assertion is true, then it means that we must have cognitive reserve first in order to push ourselves to engage in physical activities we would have no natural inclination to engage in otherwise. To test this hypothesis, Cheval and colleagues analyzed data from the Survey of Health, Ageing, and Retirement in Europe (SHARE), in which over 100,000 adults from 50 to 90 years of age were measured for levels of cognitive resources and physical activity on five separate occasions over 11 years, between 2004 and 2015. This repeated-measures data set allowed the researchers to analyze which came first—cognitive or physical decline.

Physical activity was measured with a single question, “How often do you engage in activities that require a low or moderate level of energy such as gardening, cleaning the car, or doing a walk?” Respondents indicated their level of physical activity, ranging from “more than once a week” to “hardly ever, or never.”

Cognitive resources were measured in three ways. First, participants engaged in a delayed recall task. That is, they heard a list of ten words and then were asked to repeat back as many as they could remember. This is a standard measure of short-term memory, which is known to be an important aspect of the ability to stay focused on long-term goals. In other words, to accomplish a desired task you need to be able to keep it in mind.

Second, the participants were given a verbal fluency test. For example, they were asked to name as many different animals as possible in 60 seconds. An early sign of dementia is a disruption in vocabulary, whereby patients often struggle for the words they want to use. The verbal fluency test provides an indication of whether the participant is at risk of dementia.

Third, each participant indicated their ultimate level of educational attainment, whether that be high school, college, or graduate school. Ample research has shown that education provides considerable cognitive reserve against dementia in old age.

Across the five measurements over the course of eleven years, the researchers were able to discern a clear pattern. Namely, a decline in cognitive resources preceded a decline in physical activities. This means that physical activity doesn’t necessarily keep you mentally healthy, but rather it’s the reverse.

Older adults who were psychologically fit also tended to keep themselves physically fit. But those who showed declines in cognitive abilities at one point in time also showed reductions in physical activity at later points in time. In other words, those older adults who engaged in challenging mental activities to keep themselves cognitively sharp also had the willpower to keep themselves physically active and thus in good bodily health.

Life is complicated, of course, and the causal relationship between cognitive resources and physical activity isn’t straightforward either. Because the researchers had five measurements for each participant, they could detect long-term patterns in the data. While a decline in cognitive resources at one time predicted a decline in physical activity at the next time, the researchers also detected a reciprocal relationship when they looked at the times after that.

That is to say, cognitive decline comes first, but a subsequent decline in physical activity also leads to further declines in cognitive resources later. In short, these people fall into a vicious cycle in which a reduction in cognitive activity leads to a reduction of physical activity, which further impedes cognitive activity, and so on.

The take-home from this study is clear. If you want to avoid dementia in old age, you need to maintain ample cognitive reserve. You can do this by engaging in mentally challenging tasks, including negotiating complex social relationships. You can also be a life-long learner, thus gaining the benefits that education confers on cognitive reserve. And as a mentally fit senior citizen, you’ll also understand the need to remain physically active for your body’s sake—and have the willpower to stick to a reasonable exercise regimen.