Gut microbes and brain

The human gut harbors a dynamic and complex microbial ecosystem, consisting of 1 kg of bacteria in the average adult, approximately the weight of the human brain.

Gut microbes are part of the unconscious system regulating behavior. We are fundamentally dependent on a myriad of essential neurochemicals produced by microbes. Research has shown that gut bacteria (especially species belonging to Lactobacillus and Bifidobacterium) can influence social behaviour, anxiety, stress and depressive-like behaviour.

We have long believed that “good” immune cells recognise and defend against “bad” invaders. That’s why a large proportion of medicine has been directed at killing microbial enemies.

But a radical shift in understanding the relationship between humans and microorganisms occurred with the discovery that only 50% of the cells in our bodies are human. The rest are microbes, such as bacteria, yeasts (members of the fungus family), viruses, and even insects. Together, these make up the microbiome. These produce proteins that help us digest food and support our immune systems.

Microbiota is an ecological community of microorganisms that are generally a combination of both beneficial “good bacteria” and potentially harmful bacteria. The human gut harbors over 100 trillion microorganisms.

Through the gut-brain axis, these genes influence mood and memory.

Because we have evolved with microorganisms inside us, we now have specialized communities in our guts, on our skin, and in our mouths. Our microbes are understood to be so critical to our existence, many scientists consider us to be symbiotic organisms.

Considering human life as a function of the microbiome and our environment allows us to acknowledge that we may be affected by entities that harbour different evolutionary needs.

Microbes and diet

The food we eat feeds our gut microbes and directly impacts their survival. Within two days of changing diet, our gut species change. Different gut bacteria thrive on different diets. For instance, Prevotellastrains consume carbohydrates while Bacteroidetes prefer some fats, and Candida prefer glucose over protein. So, some species starve and others thrive based on what we eat.

The species in ours guts are also proving to be relevant to health and disease. Prevotella, for instance, has been linked to improved glucose tolerance and is much more prevalent in the guts of hunter-gatherer societies than those in Western societies. The reduction of Prevotella in gut-bacteria in Western populations is thought to partially explain modern epidemics such as diabetes and obesity.

It shouldn’t surprise us then, that microbes can shape our food choices to ensure their own survival. Some metabolites, the small byproducts of microbial digestion, can make us feel hungry, full or crave certain foods. However, the evidence in humans is so far somewhat circumstantial. A study of chocolate-craving and chocolate-indifferent people found different microbial metabolites in their urine, suggesting different bacteria were present in the gut.

Metabolites are important in terms of function, because we know these can send signals to the brain. Signals to regulate eating behaviour are also transmitted via the vagus nerve that runs between the brain and the gut. At least two human studies have shown blocking the vagus nerve induces weight loss in obesity, while stimulating it in rats has led to overeating.

Microbes and behaviour

Much of the work exploring direct microbe-related behaviour has been done in mice and rats. These studies have had some pretty interesting results though. They’ve shown that behaviour can be transferred through poo transplants, that animals bred without any bacteria show unusual social and emotional behaviours, and that serotonin – the brain chemical associated with mood and depression – is produced largely in the gut. Together, these findings indicate a strong evidence base for the fact that the microbiome can affect host behaviour.

The protective value of a whole-food diet for depression also points to the importance of gut microbes for brain health. Mood disorders that can accompany conditions such as irritable bowel syndrome and inflammatory bowel diseases are thought to be related to microbial disruption in the bowel.

Recent research has also suggested the gut microbiome may have a role in the development of autism spectrum disorders (ASD). Research has found people with ASD have significantly higher numbers of Candida species in their intestines, for instance. Although determining causation is complicated, these microbes reduce the absorption of carbohydrates and release ammonia and other toxins that contribute to autistic behaviours.

There are numerous reports of changed gut bacteria in people experiencing mental illness such as schizophrenia and depression, as well as neurological disorders such as Parkinson’s disease.

Infections

We are ecosystems, whose members are intricately balanced by cooperation and competition. Many of our microbes are neither good nor bad. But they become bad because we change the game, giving them the opportunity to be bad.

For example, we are increasingly interfering in the ecosystem by using antibiotics and sanitisers, hormone and immune system treatments, cosmetic and plastic surgery, or biomedical implants and devices such as contact lenses or heart valves.

Although sanitation and nutrition have greatly improved in much of the world, antibiotic overuse has led to the rise of antibiotic resistant bacteria. Antibiotics also change what is in our microbiome. Many women would be familiar with Candida infections (thrush) that flourish after they use antibiotics, for instance.

Biomedical implants, contact lenses and dentures provide warm, moist and nutritious conditions for colonisation by microbes. Increased oestrogen use in birth control pills and other hormone treatments has been shown to promote yeast infection and reduce immune efficiency.

In fact, the hygiene hypothesis argues that infections help build our immune system and the proliferation of sanitising disinfectants in our homes could be contributing to skin allergies and respiratory conditions.

Our diets have also changed rapidly and the flow–on changes to both human and microbial health are apparent. Non–communicable disease epidemics such as obesity and heart disease are clear consequences of highly processed foods and increasingly inactive lifestyles.
Sources: Scientist, Medical press, Psychology Today

Galina Toktalieva

Kyrgyzstan-born author residing in Graz, Austria

You may also like...