Exercise can boost your memory and thinking skills, says neurologist Scott McGinnis in a press release published by Harvard Medical School, where he works. David Jacobs, a professor of epidemiology and community health at the University of Minnesota, agrees: “For generally healthy people, exercising regularly can enhance brain function over a lifetime — not just after a workout,” he writes in an article published by Scientific American. The list of researchers and health advocates who take the cognitive benefits of physical exercise for granted is long, and numerous studies appear to support this widely held belief.
But a few days ago, Dr. Daniel Sanabria Lucena (Bordeaux, France, 46 years old), a professor at the University of Granada and a researcher with the Mind, Brain and Behavior Center, published a review in the journal Nature Human Behavior that calls this belief into question. Sanabria’s team analyzed 109 studies, involving over 11,000 participants, that found exercise to have a positive effect on cognitive ability. The team discovered various problems with the studies’ methodologies, leading them to conclude that there is no difinitive evidence to support the claim that physical activity has a positive effect on brain performance.
“Our knowledge of this subject is not advanced enough to make recommendations as strong as the ones that are being made,” Saravia explained in a video-call interview, adding that his team “is not the first to say this.” Adele Diamond, a professor of neuroscience at the University of British Columbia in Vancouver, published research findings arguing that “aerobic-exercise and resistance-training interventions have been among the least effective ways to improve executive functions,” which include working memory and the ability to make plans and decisions. “We’re not talking about mental health,” Sanabria clarifies. “That’s a different subject.”
Question. Studies like yours show how difficult it is to provide definitive recommendations for so many aspects of human health. Do you think people can learn to live with that uncertainty?
Answer. First of all, I think there’s a need for more science education in general, starting at school, so that people understand how science works. It’s important to know that researchers have our biases, interests and prejudices. For some, science has become a kind of religion, a source of absolute certainty, but in reality, it doesn’t work that way. People want to know what they have to do to take care of their well-being, including their mental well-being. We want simple answers. But, in most cases, giving health advice is complicated, and, as we saw with the pandemic, we have very little tolerance for uncertainty.
In the classes I’ve taught, I remember talking about two contradictory theories about some phenomenon, and someone asked me: “So, what am I supposed to believe?” I told them that science is not about believing, it’s about generating theories and accumulating evidence, which may be more or less solid and conclusive in favor of one hypothesis or another.
Q. So, as your students might say, what are we supposed to believe about exercise?
A. In this particular case, and based on what we know, what I would say is that if exercising makes you feel good, do it. Because there’s very strong evidence indicating its physical health benefits.
Q. How did all of these studies that led us to believe that exercise had a positive effect on cognition come about?
A. In the past two decades, there’s been a growing interest in the topic, looking for the benefits of physical exercise beyond just physical health. Studies have tried to find a connection between the level of a given group’s physical activity — in some cases with thousands of participants — their level of cardiovascular fitness, and their cognitive performance. These studies show correlations, but correlation is not the same as causation. This is why we have intervention studies, where you randomly assign people to the experimental group, who perform physical exercise, and other people to the control group, which does not perform physical exercise, or which performs an activity that, you assume, is not going to have an impact on cognitive performance. The evidence from these types of studies is what is generally used to prove that physical exercise improves cognition. We analyzed 109 such studies, those that focus on healthy populations, and our conclusion is that the evidence for the supposed cognitive benefits of exercise is far from conclusive. In fact, we think that intervention studies might not actually be the best tool to study the possible effects of regular exercise on cognitive and brain function, and that it would be better to seek evidence from longitudinal studies. If we obtain conclusive evidence that these effects exist, then the question becomes why, but that’s a subject for another interview. And then there’s the fact that, sometimes, results are not published when the desired effect is not achieved.
Q. So, are there results and studies that, because they’re more in line with what’s desired, they’re more sought after and get published more?
A. One paradigmatic example is bilingualism, the cognitive advantages of being bilingual. Between 2000 and 2010, we saw a major upsurge in articles talking about how people who speak more than one language have better cognitive performance than monolinguals. I’ve even seen examples of bilingual schools selling bilingualism as a tool to improve the cognitive ability of their students. But research came out showing that there was publication bias [the tendency for positive results to be published more than null ones], especially by organizations that were especially prolific in the field. On top of that, studies on the topic of bilingualism and cognitive performance, with fairly large samples and null results, have also started to emerge in recent years.
In the case of physical exercise, we’ve done several studies that, at first glance, might show counterintuitive results. One is on the effect of mental fatigue on physical performance. There is literature suggesting that when you do a mentally demanding task right before you do physical exercise, you’re going to perform worse physically than if you do something less demanding before. In sports science, this is taken for granted. But we tried to replicate a classic study along these lines, and got null results, and that’s when we started to question the reliability of the evidence. We looked at the literature and noticed that the studies used very low sample sizes. We did a meta-analysis and found that, indeed, there was a trend of using very small sample groups, which increases the probability of finding false positive results, poor quality studies, and publication bias.
And another line of research we’ve been working on is one that looks for the effects of electrical brain stimulation by direct, low intensity current, to improve physical and sports performance. There was even a company that sold a device that stimulated the brain to improve physical performance. We did an empirical study, trying to replicate previous results and, again, we found null results. We did a meta-analysis, and again we found studies with very low sample sizes, publication bias, inconsistent literature.
That said, the results of our research that I’ve discussed here don’t mean that these effects don’t exist, since the absence of evidence of an effect is not evidence of the absence of an effect. What they indicate is that, with the studies available so far, nothing can be concluded about these phenomena. More and better research is needed.
Q. Does the method for choosing who participates in a study influence the results?
A. It can have a major impact, yes. For example, imagine that you put out a call for participation saying you’re looking for older people for a study that wants to look at the effects of exercise on cognitive and brain performance in the prevention of cognitive decline. Who’s going to sign up? It’s most likely going to be people who have an interest and an expectation that exercise is going to have an effect on their brain. And that group, in many studies, is compared to a so-called “wait-list” group, who go about their normal lives, who do nothing. In medicine, no one buys that approach. You always have to have a placebo group, because you know that expectations about a drug’s effect will already have an effect. Also, in our recent review study on the effects of exercise on cognitive performance, we found that in many studies, people in the experimental group, who receive physical exercise training, tend to start from a lower point in their cognitive performance than people in the control group, who don’t receive physical exercise intervention. Thus, the experimental group has more room for improvement than the control group. The fact that in many studies, this difference between the groups is usually in favor of the experimental group could be another indicator of publication bias.
Q. Do explanations of psychological outcomes focus too much on the effects on the brain, and not enough on the context?
A. One of the dangers with this issue of measuring the effects of something — and this applies to exercise, mindfulness or whatever — is that certain very relevant factors are often overlooked, which are the contextual factors. The best predictor of academic performance and subsequent career success is not cognitive ability, it’s the sociocultural context. It’s whether your parents have money. Certain ways of interpreting results can send subtle messages, which out the focus on individual responsibility. If you’re fat, it’s your fault, and it has nothing to do with being surrounded by junk food; if you don’t exercise and you get sick it’s because of your lack of willpower... I think this kind of messaging is dangerous.
Q. That might be true, but limiting the availability of junk food isn’t incompatible with telling people that their health is partly in their own hands, in choosing to go for a run or trying to buy less processed food.
A. I totally agree, they’re not incompatible. And I don’t want that to be the message of our work. I recommend people exercise, of course. But, above all, if you’re thinking about signing up your son or daughter to practice sports or play chess, do it because you think they’ll like it, not to seek out some beneficial effect on their mind, because the effects, if they exist, are small, and the scientific evidence, at least so far, is not at all conclusive in this regard. And I think it’s important to emphasize, again, that not all responsibility for physical and mental well-being should rest on the individual.
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