Exercise might save your brain from the nasty effects of cannabis use, regardless of whether you’re a boy or a girl– according to a new study by the researchers Sullivan et al. (2021).

But back up a second. How in the world do people even measure something like that?

The scientists in this study used an MRI machine to measure differences in brain volumes between people who regularly used cannabis and those who didn’t. They were interested in the possible influence of regular aerobic exercise, gender, and cannabis use on brain density as measured by an MRI machine. You can think of an MRI as working a little like a highway police officer’s speed-gun– it scans your brain, but through your skull, at a distance.

Well, you might say, that’s nice that they used MRI, but why should I care? That’s a good question, and we’re glad you asked. The reason is because an MRI produces great spatial resolution, which means that it takes nice photos of the brain compared to other types of brain scans. It is also good at measuring brain density, or just how jam-packed areas of your brain are with thinking cells. This lets researchers know exactly how plump your brain is in certain areas—or its cortical thickness, if you want to be technical about it. So, one reason researchers in a study might use MRI is because they don’t have to crack open a person’s skull to figure out what their brain looks like or how dense it is. Which is sometimes a plus. Therefore, MRI was perfect for the purpose of Sullivan et al.’s research.

But before Sullivan et al. could use the MRI, they needed to find participants. The first step of the study was to round up a bunch of college kids (both girls and guys)—some who used a lot of cannabis, and some that didn’t. Then they got them to abstain from cannabis use for 3 weeks. Since this was a big ask, they made sure to catch any cheaters by looking for traces of drug use in their urine and sweat over the course of the study.

They also gathered info on the exercise levels between the groups and measured their brains with — drum roll, please — an MRI machine. And they got them to do tasks of attention and thinking, too.

Well, that’s all fine and dandy, you might say, but again… why should I care? The reason is because of what they found dear reader; the reason is because of what they found. And what they found, in case you were wondering, was this: that those students who exercised a lot had bigger brains (in certain areas) than those who didn’t, regardless of whether they used cannabis regularly. Students who exercised a lot also did better on tasks of thinking and attention… again, regardless of whether they used cannabis. However, (and this is important), those who exercised and didn’t use cannabis had bigger brains and better levels of attention than those who did exercise and use cannabis . This suggests that exercise might protect your brain from cannabis. These effects were the same for both boys and girls.

But don’t you go lighting that blunt just yet, eager reader—even if you are grinding it out at the gym three times a week. Things get a little more complicated because, for one reason, brain scans like our friend the MRI are like pharmaceutical companies—they don’t always tell the truth.

It’s hard to discern causality with brain scans. In non-science-y words, it’s difficult for researchers to know what exactly is affecting what. For example, in this study, maybe exercise causes increased brain density… or maybe, people who exercise also do something else that causes increased brain density that wasn’t measured by the researchers. The researchers don’t know. We don’t know. And you certainly don’t know either, so don’t be using this blog as an excuse to light up during your next hike.

But one thing we do know for sure is this: there is now some evidence that exercising might be a protecting factor against the harmful consequences of cannabis use on the human brain. And this is where other studies will need to pick up in the future.

References

Sullivan, R. M., Wallace, A. L., Wade, N. E., Swartz, A. M., & Lisdahl, K. M. (2021). Cannabis Use and Brain Volume in Adolescent and Young Adult Cannabis Users: Effects Moderated by Sex and Aerobic Fitness. Journal of the International Neuropsychological Society, 27, 607-620. DOI: 10.1017/S135561772100062X