Why gut health is important for your brain and immune system

Dr. Mirjam Bloemendaal
About the Author

Mirjam Bloemendaal, is a neuropsychologist by training and obtained a Ph.D. in cognitive neuroimaging by studying cognitive aging using pharmacological and nutritional interventions. She is interested in the effect of physiological processes on mental health and behavioral performance.

Dr. Jeanette Mostert
About the Author

Dr. Jeanette Mostert specializes in Cognitive Neuroscience and Biological Psychiatry and is the Dissemination Manager for several Horizon2020 projects, including CoCA, PRIME and Eat2BeNice/New Brain Nutrition.

gut health

We recently saw a video (see footnote below) on the CNN website where medical correspondent Dr. Sanjay Gupta explains why gut health is important for immunity. This video is very short and many complex terms are mentioned here. So we thought we’d give you a bit more context about this topic.

What’s a microbiome test?

What we think Dr. Gupta means here is sending in a sample of your feces to a lab ran by either a hospital or company to identify which types of bacteria and how many of these live in your gut. It is certainly interesting to know which bacteria inhabit your gut, but if you do not have a serious intestinal problem (complaints or disorder) you will not learn a lot from such analysis. A hospital will not send in your feces without such reasons.

What you can learn from profiling your gut bacteria is just whether you have a diverse gut microbiome, meaning whether there are many different kinds of bacterial species. If your gut is overgrown or dominated by one type of bacterium, this indicates a bacterial infection. Beyond ruling out overgrowth, we currently cannot learn from this information at an individual level.

This comes back to Dr. Gupta’s statement: as a general rule we can say “the more diverse the microbiome, the better”. The results of this test will not tell you which types of food are better or worse for you. You could follow the effects of dietary patterns by repeating the test, but there is not a health claim that we can make from this on an individual level. You can read more about gut microbiome tests and what these can (and cannot) tell you in this recent article from The Washington post.

How can serotonin from the gut influence the brain?

95% of the body’s serotonin is produced in the intestine. The gastro-intestinal tract is in contact with the brain through various routes, this is called the gut-brain axis. Serotonin coming from the intestines cannot pass through the blood-brain-barrier, which is a closely guarded border between the brain’s blood system and the rest of the body.

So in itself, serotonin from the gut can’t influence the brain. But, there are other compounds, such as kynurenine, that depend on the amount of available serotonin in the gut, and that can pass the blood-brain-barrier. This means that gut-derived serotonin can indirectly affect brain functioning. Another mechanism through which gut serotonin can influence the brain is via the vagal nerve [1]. This is the longest nerve in the body connecting the brain with many organs, including the gut.

Can diet impacts your mood?

Indeed, diet can impact your mood and long-term mental health. This also has to do with the serotonin metabolism in your gut, but here the critical compounds is tryptophan. The molecule tryptophan is converted into serotonin in the gut and in the brain (and tryptophan can also cross the blood-brain-barrier). Humans and other mammals depend on the intake of tryptophan through their diet. If you would eliminate or limit your tryptophan intake from your diet, this will affect the serotonin levels in your brain.

Many cognitive processes supporting amongst others mood and learning depend on optimal levels of serotonin in the brain. With a normal, varied diet you will have enough intake of tryptophan as it is present in many food items, and particularly high in protein-rich foods (i.e., diary, meat, fish, chickpeas, almonds, peanuts).

Can a healthy gut improve immunity?

Dr. Gupta also spoke about “being as prepared as you can be” in terms of prepping your immune system against a covid-19 infection. As a general rule it is true that by living healthy you can keep your immune system in its optimal shape. Conversely, your lifestyle choices may result in a (temporary) sub-optimal immune functioning. Nutrition can impact immune functioning within the course of a couple of hours. For example, in athletes, carbohydrate intake after exercise modulates levels of signalling immune molecules and improves recovery. But this is an intervention after a strenuous activity.

These type of rapid adaptations to the state of your immune system, caused by the nutritional quality of your breakfast as Dr. Gupta proposed, will not influence the progression of your covid-19 infection. Only long-term dietary habits can positively influence the immune response to an infection. Moreover, your diet will not protect you from getting a covid-19 infection, only qualitative protection such as wearing a mouth-nose mask, a covid-19 vaccine or social-distancing will.

Further reading