One of the hot topics in neuroscience these days is the influence of the gut’s microbiome on the brain. While the microbiome is comprised of all the microorganisms in the body, it’s the bacteria that’s gaining the most focus. So you might ask: does marijuana effect the microbiome? And if so, what affect does it have on brain function?

The bacteria in the gut can have a substantial impact on brain function by modulating the enteric nervous system, which is a layer of nerve cells that line the intestinal tract from the esophagus to the rectum. This enteric nervous system communicates directly with the brain. Research has revealed that this “Gut-Brain Axis” can impact everything from brain development to psychological health by affecting immune function in the brain, producing hormones and neurotransmitters, and directly activating bundles of neurons that travel up to the brain.

Consequently, fecal transplants are gaining in popularity, not just for intestinal problems, but for ones involving the brain including Parkinson’s disease and autism.

By impacting the Gut-Brain Axis, bacteria may influence cognitive functioning in many ways. For one, it could modulate the efficiency of the organelles that serve as the body’s power plants. You may recall from high school biology that they’re called mitochondria. Mitochondria convert the energy from food, in the form of glucose, into a useable from of energy, called ATP. So altering the gut microbiome could negatively impact cognitive function by reducing mitochondrial function.

Frequent use of THC-rich marijuana is also associated with cognitive problems, but it’s unclear if these problems are related to changes in the gut microbiome. A recent study published in the 'Journal of Neuroimmune Pharmacology' investigated the relationship between marijuana use, microbiome composition (assessed from stool samples), mitochondria function, and cognitive performance.

The study found that marijuana users had substantially different microbiomes than non-users, and the microbiomes of marijuana users were associated with reduced mitochondria function. Reduced mitochondria function generally means that less energy is produced for use by cells throughout the brain and body. It’s like going to a gas station and only filling your tank up three-quarters of the way. You just can’t drive as far and so you’ll arrive at your destination slower.

Interestingly, the microbiomes of non-users were comprised largely of a type of bacteria that’s consistent with a plant-based diet, while the bacteria of marijuana users were associated with an animal-based diet. The higher the ratio between plant-diet-based bacteria to animal-diet-based bacteria was associated with better performance on tests of attention, inhibitory control, and cognitive function in marijuana users. Therefore, the further that the marijuana user’s gut microbiome deviated from that of non-users, the worse their mitochondrial function and cognitive performance became.

These findings highlight the importance of diet and gut bacteria on cognitive performance in marijuana users. Fortunately, there are ways of improving your gut microbiome, even if you use marijuana. These include eating lots of fruits and vegetables, or consuming probiotics which you can get in everything from yogurt to kombucha to probiotic armies stuffed into pills.

One caveat is that this study did not assess if or how marijuana changed the microbiome. Future studies are needed to assess the impact of marijuana on the microbiome in a “cause-and-effect” manner, as opposed to simply looking at the “association” as this study employed. Further, future research should focus on identifying whether rescuing the microbiome through probiotic or dietary intervention can improve some of the cognitive deficits associated with frequent marijuana use.

Lastly, it’s unknown if cannabidiol or cannabidiol-rich marijuana could counter the effects of THC-rich marijuana on the gut’s microbiome. Cannabidiol is known to block the actions of THC, as well as directly activate serotonin receptors, which are even more prevalent in the gut than the brain. Either way, the Gut-Brain Axis represents an exciting new way to modulate both the positive and negative effects of marijuana on health.

Josh Kaplan, Ph.D. is a neuroscientist at the University of Washington and freelance science writer specializing in the science of cannabis. Visit neurokaplan.com to learn more.