- And as we grow older, we inevitably start to notice changes in cognitive function.
- Several lines of research, including comparative studies done as early as 1975, support the belief that exercise matters.
- New research shows that exercise may have direct beneficial effects on the brain at the anatomical, cellular, and molecular levels.
By Sehba Husain-Krautter, MD, PhD, and Robert P. Roca, MD, MPH
Time marches on, and we grow older. Short of traveling at the speed of light, nothing can be done to alter that chronological constant. And as we grow older, we inevitably start to notice changes in how we function – how fast we can run; how easily we can balance on one foot; how quickly we can process information; how easily we can learn new things. Or as Bette Davis reputedly put it, “Getting old is not for sissies.”
Of all the above changes, declining cognitive function may be the most worrisome. One of the central questions in the study of aging is the extent to which decline in cognitive functioning is caused by aging itself and the extent to which it is due to diseases or patterns of disuse that are simply correlated with advancing age. If disease and disuse are major contributors, then hopefully we can alter the slope of our functional slide by taking steps to reduce our risk of disease and by using (e.g., exercising) our bodies and brains.
What Can Be Done?
This hope has been the subject of both research and popular writing. Disease prevention is an important opportunity, and there is little debate that brain health is enhanced by efforts (e.g., not smoking; controlling blood pressure) to prevent diseases that can lead to strokes and other types of brain disease.
And then there is exercise. It’s become commonplace to praise the benefits of exercise as a way to preserve and enhance physical and mental functioning as we age. And no one is more eager to accept this notion than the co-authors of this post – a Baby Boomer and a Millennial, both geriatric psychiatrists and avid exercisers.
But is this just wishful thinking, or a way to allay anxiety about growing older by imagining that we can control how we age?
There are several lines of research, including comparative studies done as early as 1975, supporting the belief that exercise matters. One line of research studies large populations looking for correlations between exercise and a variety of medical and functional outcomes. And the message from many (but not all) of these epidemiological studies is that regular physical activity is associated with decreased rates of cognitive decline and, in some studies, reduced risk of dementia. One of the more recent studies followed thousands of older adults without dementia for several years and found that self-reported physical activity – even at a low level – was associated with a lower risk of being diagnosed with dementia during the follow-up period. Further, the higher an individual’s activity level, the lower the risk.
This research is encouraging, but there is a catch: While many of these studies do show that people who report exercising have lower rates of cognitive decline, they don’t prove that exercise was the cause of the better outcomes. The better outcomes might have been the result of other factors, e.g., better diets or better genes. Researchers attempt to correct for these confounders statistically, but such corrections are not foolproof, and one can therefore draw incorrect conclusions about causation.
Or, perhaps, people who are brewing illnesses such as Alzheimer’s disease – which begins affecting the brain long before there are obvious behavioral signs – are less likely to exercise because of subtle brain changes making exercise unpleasant or reducing their motivation to work out. That could also result in a correlation between exercise and brain dysfunction, but the causal arrow would point in the opposite direction.
Exercise Should Start in Childhood
Offering some reassurance in this regard is a recently published Australian study examining cognitive function in midlife among persons whose fitness had been assessed in childhood, long before the potentially confounding effects of subclinical Alzheimer’s disease could appear. It turns out that those with the highest levels of fitness in childhood had the highest levels of global cognition decades later. This result supports the idea that physical fitness may benefit cognitive functioning years down the road.
It would bolster confidence in the benefits of exercise if it could be demonstrated that exercise exerts physiological effects that would be expected to enhance cognition, and in fact, there is a line of research looking at just that. Exercise, of course, has been shown to enhance general health, which would be expected to enhance brain function. More to the point, there is research showing that exercise may have direct beneficial effects on the brain at the anatomical, cellular, and molecular levels.
For example, a 2020 study comparing aerobic exercise to stretching in individuals with mild cognitive impairment showed that 12 months of aerobic training, but not stretching, resulted in a significant increase in blood flow to crucial brain regions. Another study published in the journal Nature in 2021 showed that individuals with cognitive impairment who participated in structured exercise for six months had higher plasma levels of an anti-inflammatory molecule called clusterin. So we can point to physiological pathways – e.g., increased blood flow to the brain and increased anti-inflammatory activity — by which exercise may benefit the brain and preserve cognition.
Even more confidence-building, however, would be prospective randomized trials showing that current exercise leads to measurable improvements in cognitive functioning — and there is a growing body of research along these lines. One recent example is a randomized study of the effects of aerobic training on cognition and a variety of physiological measures relevant to brain functioning in older, sedentary adults. The finding was that those assigned to the exercise group (an average of 100 minutes per week of supervised moderate-intensity physical activity for 12 weeks) showed boosts in cognition and brain-relevant physiological functioning not enjoyed by counterparts in the control group.
Where Does This Leave Us?
We know that many studies have shown statistical correlations between exercise and better cognitive functioning, but we can’t be 100% confident that exercise is the cause of this fortuitous result. That said, there is research showing pathways by which exercise might benefit the brain, and there are randomized clinical trials showing that aerobic exercise over a period of weeks may improve cognition in older adults.
There are certainly many outstanding questions: If exercise is good for cognition, how much and what kinds are best? Is more better? Is there a ceiling above which there is no additional benefit, or even harm? These are just a few of the questions we hope to see answered in the future. But one thing is for certain: Both of us will be going out for a run tomorrow.