There are plenty of reasons to get off your duff and exercise—but is improving your brain one of them? The U.S. Centers for Disease Control and Prevention touts exercise as a way to “boost brain health,” while the World Health Organization suggests that about 2 hours of moderate activity or 75 minutes of vigorous activity per week can help improve thinking and memory skills.
But new research reveals a more complex picture. One recent review of the literature suggests the studies tying exercise to brain health may have important limitations, including small sample sizes. Other studies suggest there is no one-size-fits-all approach to exercising as a way to boost cognition or prevent age-related cognitive decline. Still others indicate exercise may actually be harmful in people with certain medical conditions. Here’s the latest on what we know.
What is the science linking exercise and improved brain function?
Many studies correlate participants’ self-reported exercise with scores on cognitive tests, or track the effects of randomizing participants into groups that either exercise or remain sedentary. They typically find that the more physical activity a person does, the better their cognition. This result holds for healthy people, stroke survivors, and those with other neurological conditions such as Alzheimer’s disease.
A study published earlier this year relied on genetic data to explore the effects of exercise. A team led by sports scientist Boris Cheval at the University of Geneva grouped about 350,000 people in the United Kingdom according to genetic variants associated with more or less physical activity. Those with an apparent genetic predisposition to be more active also tended to perform better on a set of cognitive tests, the researchers concluded in Scientific Reports.
Other studies have focused on age-related cognitive decline. Research published in February in the Journal of Neurology, Neurosurgery & Psychiatry tracked more than 1400 people for 30 years, showing that more physical activity was associated with better cognitive performance at age 69.
But other studies suggest the amount of exercise matters. A randomized trial published in January in Alzheimer's Research & Therapy showed improved cognition only in older adults who upped their physical activity by at least 35%.
How is exercise thought to act on the brain?
Scientists have chalked up many of the positive effects of exercise to a protein called brain-derived neurotrophic factor (BDNF), a molecule that protects neurons and is thought to support learning and memory. Unfortunately, BDNF is not a molecule that “you can just infuse into the brain,” says Travis Gibbons, a neuroscientist at the University of British Columbia. “You can expect to see a ton of benefits, but you need to produce it yourself.”
Gibbons and others think this could be done via exercise; in January, his team reported in The Journal of Physiology that cycling increased BDNF levels in the blood of 12 young, healthy adults. This BDNF boost could explain the stronger neural connections other researchers have reported in people who undergo exercise in clinical trials.
Another possibility is that exercise improves blood flow to the brain, as researchers found in a study of moderate aerobic exercise in older adults, published last month in Cerebral Cortex. More blood flow may increase the flow of nutrients to the brain, and, in turn, improve cognition.
Exercise can also influence cognition indirectly, by improving general well-being, notes Daniel Sanabria Lucena, a neuroscientist at the University of Granada. “The [exercise] intervention itself has a lot of components,” he says, including socializing with others and being in the great outdoors, which might themselves have cognitive benefits.
What has made some researchers doubt the evidence linking exercise and cognition?
Sanabria Lucena and colleagues recently conducted an “umbrella review,” analyzing 24 existing meta-analyses, which in turn encompassed 109 randomized controlled trials linking exercise to cognition in healthy people. Most of these reported a positive effect. But the review, published in Nature Human Behaviour in March, found that many of the original studies had limitations, including small numbers of participants.
Another issue, the researchers say, is that the control groups in different studies had widely varying levels of physical activity. The discrepancy has led to inconsistent findings about the size of the effect exercise has on cognition. The field also likely suffers from a bias toward publishing positive results, the authors note. And they found that the meta-analyses had limitations of their own, such as a failure to include all relevant published studies.
Those limitations may have led investigators to overestimate the beneficial effects of exercise on the brain, Sanabria Lucena says. After 25 years, what do we know about the causal link between exercise and cognition? he asks. “Nothing.”
The review shows that “the encouraging results reported in most meta-analyses in the field may be more related to specific methodological choices and in many cases shortcomings of those meta-analyses, rather than the true effects of exercise,” says Amit Lampit, a clinical neuroscientist at the University of Melbourne who was not involved in the work. It suggests that “exercise is not a universally effective cognitive intervention” in healthy people, he adds. His own group recently published a paper in the Journal of Neurology reviewing studies on people with Huntington disease, a neurodegenerative disorder that attacks brain cells; it concludes that the effects of exercise on cognitive function in this population may be negligible.
“It’s true that at the formal level, it would be great to have additional evidence of causality,” Cheval says of the new results. “But in my view, we should continue to consider, based on all the evidence, that engaging in physical activity will improve your cognitive function.”
Can exercise be harmful in some groups?
Some studies link exercise to worse cognitive outcomes in people who have or are predisposed to certain medical conditions. For example, Pamela Shaw, a psychologist at the University of Sheffield, and her team examined studies published between 2009 and 2021 on the relationship between exercise and the neurodegenerative condition amyotrophic lateral sclerosis (ALS). Their analysis, published last month in the journal Brain, showed that vigorous exercise was linked with higher risk of ALS.
A 2021 study looked at the roughly 10% of patients with ALS who have a mutation in a gene called C9ORF72, the most common genetic risk factor for ALS. In this group, higher rates of exercise were associated with an earlier onset of the disease. The reasons aren’t clear, Shaw says, but for some people already at genetic risk of ALS, strenuous physical activity may “tip the disease into manifesting itself.
In encephalomyelitis/chronic fatigue syndrome, exercise is thought to potentially worsen symptoms, and an early trial supporting exercise as a treatment generated controversy in the field. Similarly, some physicians initially recommended exercise for patients with Long Covid, but studies have revealed that physical activity can lead to more fatigue and worsen respiratory symptoms.
What does the uncertainty in this field mean for exercise recommendations?
The evidence that physical exercise helps the heart is convincing enough for scientists and physicians to recommend it as a strategy for staving off cardiovascular disease and other conditions. Despite her findings in people with ALS, Shaw says, “We don’t want to put the general population off from doing exercise.”
To definitively link exercise to brain health, “we need more research, better practices, and probably a different approach,” Sanabria Lucena says. For example, he recommends that future studies track participants’ cognition long term.
“Exercise is an important part of health,” Lampit says. But when it comes to brain health, he says, “Our job going forward should be to find where it can and cannot be effective.”
