Back in 2023, researchers pumped the brakes on one of my most cherished beliefs about exercise: the idea that aerobic fitness makes your brain healthier and more resistant to age-related cognitive decline. Exercise, the thinking goes, spurs the production of growth-promoting brain chemicals, or enhances blood flow to the brain, or simply promotes better sleeping habits and healthier social interactions. The idea has been around for decades and is a staple of health journalism. But a major review published in Nature Human Behaviour found that the actual scientific evidence was too weak to conclude that exercise makes you smarter.

So I’m pleased to report a couple of new studies that support the brain benefits of fitness. Both of them rely on VO2 max as a gold-standard objective measure of aerobic fitness, rather than trying to estimate fitness or exercise habits. One finds that higher VO2 max is associated with better cognitive function in older adults; the other finds that it preserves the size of a key brain region as you age. Lately VO2 max has been getting a lot of attention for its ability to predict longevity; the new data bolsters the case that its benefits are in your head too.

The first study is published in the British Journal of Sports Medicine by a large team at multiple universities coordinated by Kirk Erickson at AdventHealth Research Institute. They put 648 adults between the ages of 65 and 80 through a treadmill test to exhaustion to measure VO2 max, plus a battery of cognitive tests to measure episodic memory, processing speed, working memory, executive function/attentional control, and visuospatial function.

There are lots of ways of parsing the data, but the basic result is straightforward: higher VO2 max was associated with better scores for all five elements of cognitive function, with very strong statistical significance. Here’s what the data looked like for each of the five categories:

The use of VO2 max as a measure of fitness here is key. Some of the confusion in previous results may be because they relied on less accurate measures of fitness or exercise levels. This echoes the findings for fitness and longevity. If you ask people how much they exercise, then wait to see how long until they die, the results are often ambiguous. But if you measure their VO2 max, it’s crystal clear: those with higher VO2 max tend to live longer, with no upper limit on how high you want VO2 max to be. This data suggests a similar pattern for cognitive function.

That doesn’t answer all our questions, though. Does higher fitness cause better cognitive function? If so, how? And does it help ward off cognitive decline? The second study, from Rong Zhang and colleagues at the University of Texas Southwestern Medical Center, addresses the latter question by testing 172 adults across a much wider age range, from 22 to 81 years. In addition to VO2 max and cognitive testing, the subjects had MRIs to measure brain size and structure. The hypothesis was that higher VO2 max would be associated with less brain shrinkage with age, which in turn would be associated with better cognitive scores.

The results, which are published in the Journal of Applied Physiology, don’t quite live up to expectations. VO2 max wasn’t associated with overall gray matter volume or cortical thickness (the two measures of brain structure where they expected to see differences), or with overall cognitive performance.

On the plus side, a key area of the brain called the right superior parietal volume tended to be smaller for older adults with lower fitness, but didn’t shrink for older adults with higher fitness. Here’s what that data looked like:

The orange line (and the white circles) shows the gradual decline in size of this brain region with increasing age in the subjects with lower-than-average VO2 max. The blue line (and black circles), on the other hand, is basically flat: there’s no apparent shrinkage with age in subjects with higher-than-average VO2 max.

Interestingly, the size of this brain area was also associated with higher scores on tests of fluid intelligence, which is the ability to think quickly and respond to new information. The implication is that maintaining or improving your fitness might help ward off some of the brain shrinkage and cognitive decline that usually accompanies aging. But when you have to go looking for secondary results—one small brain area instead of the whole brain—the findings aren’t as convincing.

There’s an important caveat, though. The subjects in both these studies were sedentary! In the first study, the average VO2 max was 21.7 ml/kg/min. In the second study, it ranged from about 35 ml/kg/min in the youngest subjects to 23 ml/kg/min in the oldest subjects. These are somewhere between average and below average relative to the general population. It’s less surprising that we don’t see a strong effect of fitness, because (with all due respect) none of them were fit.

The picture is more encouraging when you look at trained athletes. For example, a study back in 2016 compared 16 masters triathletes (average age 53, average VO2 max 55 ml/kg/min, which is exceptional) with 16 healthy active controls (average age 58, average VO2 max 38 ml/kg/min, which is still good). In this case, there were clear differences in brain structure. Overall, the athletes had a thicker cerebral cortex, the outer layer of the brain that’s responsible for cognition. There were also size differences in a bunch of individual brain regions.

Zooming out from the individual studies, the overall picture supports the idea that fitness has an impact on your brain. It’s not an instant fix or a miracle cure, though. If I had to guess, I’d say that any physical activity you do is probably good for your brain (and, of course, your body) on some level. But to get big, measurable benefits, you likely need to exercise in a way that improves—or, as you age, maintains—your VO2 max. In other words, if you want to get smarter, start by getting faster.

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