Age-related cognitive decline is pretty much inevitable, especially now that people are generally living longer. While this decline in fluid intelligence (e.g., reasoning abilities) is complemented by an increase in crystallized intelligence (e.g., factual knowledge) (see figure below), fluid intelligence helps us deal with many aspects of our daily lives (such as processing new information and solving problems), so it’s worth preventing its decline as much as possible. Thankfully, there are ways to do this. Continual learning and mental engagement (i.e., effortful mental engagement, not simple passive learning) is an excellent way to keep one’s mind sharp. But, perhaps one of the best ways is by doing something that also benefits us in other ways: exercise.
I recently re-listened to a great podcast where author-researcher-doctor John Ratey discusses the positive effects that exercise has on our brains. He mentions findings of how exercise stimulates neurogenesis (i.e., the creation of new nerve cells—you know, the thing we used to think our brains couldn’t do past childhood), promotes brain plasticity (something else we used to think didn’t happen in the adult brain), and can help strengthen current brain cells. It largely does this through the release of neurotropins, chemicals that promote the development of new nerve cells from stem cells and/or help strengthen existing ones. The key player is a neurotropin called BDNF (brain-derived neurotrophic factor), which, he says, is basically a stress hormone, since it can also be released during other forms of stress (e.g., the effortful mental engagement mentioned earlier, hence why that, too, can prevent the onset of cognitive decline)—though exercise seems to be the clear winner when it comes to stimulating the release of BDNF.
So what does a stress hormone have to do with cognitive health? Well, it’s just like physical fitness. Let’s take weightlifting as an example: when you lift weights, you’re basically telling your body that you’re going to be using your muscles more, so it responds by building more muscle. (Basically, our bodies are really smart; they learn to do what we train them to do.) However, there’s a catch: if all you ever do is lift weights and you never give your body time to rest, recover, and rebuild, you’ll only wear it out—and without out putting on the desired muscle mass. It’s the same for our minds. Things like exercise and mental engagement are a form of stress and signal to our brain that we’re going to be using it more. (True, some forms of aerobic exercise are not very mentally stimulating, but because our brains are sensorimotor machines, even cognitively simple but physically demanding activities engage our brains sufficiently so as to stress them. This also makes sense given what we know about embodied cognition: because some of our higher cognitive functions are rooted in more physical, sensorimotor processes and their underling brain areas, developing these sensorimotor areas and processes could, by extension, develop the cognitive functions which rely on them.) However, as with weights, if you never give yourself a recovery period, you’ll only wear yourself out. That’s the difference between acute stress and chronic stress.
When stress becomes chronic, sustained over weeks, months, or years (e.g., as with chronic depression), our bodies and/or minds never get a break and the stress can become physically toxic: our bodies can wear out from fatigue and our brain cells can deteriorate from the toxic buildup of chemicals (chemicals that normally are needed and within safe levels, but, with no opportunity to get cleared out, can build to unhealthy levels). However, with acute stress, the brain and body still respond much the same way as with chronic stress (e.g., the same chemicals are released), but there is time for recovery and rebuilding—and training to respond (again, our bodies learn to do what we train them to do—they’re wicked smart). Thus, the next time a stressor comes along, we’re better able to respond to it. This is the stress inoculation theory. Much like how a vaccine inoculates us to pathogens (i.e., we’re exposed to a weakened or dead form of the pathogen that allows us to safely develop an immune response to it such that if we ever were to be exposed to it later, our immune system could respond effectively and destroy the unwanted invader), a little bit of stress trains our body’s stress response system to better deal with future stress. And apparently, part of the better way to handle stress is with heightened cognitive function. Which totally makes sense: the higher our cognitive functioning, the better able we will be to deal with problems and stressors (which can come in the form of problems) that come our way.
However, apparently not all forms of exercise are created equal. In a recent study, Nokia and colleagues found that sustained aerobic exercise had the greatest positive effect on neurogenesis (at least for those that were genetically more responsive to it), followed by high-intensity interval training (HIIT, which, by the way, is one of the best forms of exercise for burning fat); weight training had no significant effect on neurogenesis relative to being sedentary. (Which makes sense: yes, weightlifting is hard; but it doesn’t stress our entire body as does cardio, which very much engages and taxes our cardiovascular and respiratory systems in addition to the muscular system. Sorry, lifters! But never fear, there are plenty of other health and functional benefits of resistance training.) So for all my fellow runners, the news is good for us! But, thankfully, the results also show that any cardio, even if it’s in intervals, is still good. So by all means, keep doing the resistance training; just don’t skip your cardio days.
Finally, the benefits of exercise extend beyond simply possibly preventing or delaying the onset of cognitive decline. It can also improve our overall mood and mental health. Exercise releases key neurotransmitters, and because our bodies change based on how we train them, this can change the pathways and brain areas associated with these neurotransmitters and can change how our bodies respond to them. Among these key neurotransmitters are norepinephrine (NE), dopamine (DA), and serotonin (ST). NE and DA are hugely important in our executive functions (see my earlier posts on shower thoughts and task-switching), which makes it unsurprising that their systems are dysfunctional in, for example, ADHD, and, therefore, unsurprising that exercise can be a great remedy for ADHD. (So, when you tell kids to do something physically exerting to “burn of steam”, they’re actually not so much burning off excess energy as they are enhancing the functionality of their neurochemistry.) And given that ST is a key player in mood disorders, such as anxiety and depression, it makes sense that engaging the ST system can lead to an overall enhancement in mood. Plus, who doesn’t just love running and feel great when they’re doing it? (Okay, I know not everyone does. In my experience, you love it or you hate it. If you couldn’t tell, I’m in the “love it” camp. Give me an open trail and a cool summer’s day and I’ll be a happy camper!—well, a happy runner.)
But why is exercise so special? Maybe you’ve heard about the positive effect environmental enrichment can have on neurogenesis, such as shown in this 1999 study (which I think might be one of the seminal studies on the matter) which showed that voluntary running and an enriched environment both resulted in greater survival of new brain cells. John Ratey addresses this matter, noting that nothing else comes close to exercise in terms of BDNF release, because exercise releases it all over the brain. Further, environmental enrichment itself does not necessarily or immediately promote neurogenesis; rather, it does so indirectly: it provides an environment rich with options, such that, presumably, the person (or, in the case of the 1999 study, rats) finds something stimulating in which to engage. And, assuming that that stimulation is taxing enough, either cognitively or physically, it can be enough to initiate a stress response, releasing BDNF and, ultimately, leading to greater cognitive health.
So, whatever the activity, get out there and stress yourself. But know that the biggest bang for your buck will be some good, old-fashioned cardio—not just because it has the greatest effect on releasing the key chemicals in the brain, but because it also improves your overall physical fitness. I mean, as great as Sudoku puzzles might be for enhancing cognitive fitness, they do nothing for physical fitness (unless used as a rest day activity or done while on a treadmill or stationary bike). So do something that’s good for your body and brain—for your physical and psychical fitness—and get some exercise. Maybe I’ll see you on the running trail someday—or, shameless plug, in one of my group fitness classes. I’ll be there enjoying the exercise, and all the more because I know because I know it’s good for my brain, too.
D. R. Meriwether