The Gut Microbiome: Your Second Brain

The human microbiota within our gut makes up 1-2kg of our body’s total weight, equivalent to roughly the weight of our brain. Many consider our gut microbiota as an organ in its own right due to its effect on our health and it is often referred to as ‘our second brain’. We are exposed to microbiota from the moment we are born and continuously rely on it for survival throughout our life. But what exactly is our gut microbiota and why is it so important?

The Gut Microbiota

The gut microbiota refers to all of the microbes that can be found within our intestine, specifically a small area of the large intestine known as the cecum. While this is not a large proportion of the human body, the microbiome genome within our gut easily trumps a human genome’s mere 23,000 genes, encoding for over 3 million genes. While there are many different types of microbes found within the gut, bacteria are the most studied. In fact, humans have more bacteria cells than human cells. There are roughly 40 trillion bacterial cells in the human body, compared to our own 30 trillion human cells, which includes over 1000 bacterial species. Each species will have a different role within the body, and while some may cause disease, most are incredibly important for our health, suggesting the importance of a diverse microbiota. So, why does our gut have such a huge impact on our overall health?

The Gut-Brain Axis

The gut and brain are connected through what is known as the gut-brain axis. This allows communication between the two through physical and biochemical means. Many of the effects that the gut microbiome can have on our health and happiness is due to the connection through this axis.

There are 500 million neurons within the gut that connect to the brain using the nervous system. The Vagus Nerve (VN) is one of the biggest and is responsible for sending signals bidirectionally between the brain and gut. Studies suggest increased levels of stress reduce signals through the VN which cause problems to the gastrointestinal tract such as an imbalanced gut microbiota (known as dysbiosis). Dysbiosis tends to be prevalent in those with bowel diseases such as irritable bowel syndrome (IBS) and Crohn’s disease and makes these individuals more susceptible to effects of stress as their microbiota is already imbalanced. The VN is also associated with areas of the brain responsible for regulating emotions, therefore, negative signals sent from an imbalanced and unhappy gut to the brain via the VN have been linked to symptoms of anxiety and depression.

Hormones and metabolites play a huge role in the gut-brain axis. It is not just the brain that creates hormones involved in feelings and emotion. In fact, the gut microbiota is responsible for making many of the same important hormones. Amazingly, 95% of serotonin, known to many as the ‘happy hormone’, is made by our gut bacteria. Emotions of fear and anxiety are also controlled by our gut microbes through a neurotransmitter known as gamma-aminobutyric acid (GABA). Short-chain fatty acids (SCFA) are microbial metabolites made from digested fibre and have huge effects on brain function such as learning and memory. Specifically, the SCFA Butyrate is thought to be important in forming the blood-brain barrier, while SCFA propionate is thought to reduce appetite and food intake.

The immune response is another key player in the gut-brain axis. Gut microbiota control what goes into the body and what is excreted, which in turn controls the immune system and inflammation. Some bacteria secrete an inflammatory toxin known as lipopolysaccharide (LPS). Chronic stress can increase gut permeability causing what is known as ‘leaky gut syndrome’. This not only causes dysbiosis but also increases LPS concentration in the blood causing inflammation associated with brain disorders such as severe depression, dementia, and schizophrenia.

The Effect of the Gut Microbiome on Mental and Physical Health

While research is ongoing, microbial imbalance is suspected to be a huge factor in many depressive disorders, as this can cause a loss of microbes that are key for producing metabolites we rely on for happiness. A recent study looked at whether there were specific gut bacteria species which correlated with depression. Firmicutes and Bacteroidetes are the two most common bacterial families found in our gut, representing 90% of our total gut microbiota. Two members of the Firmicutes family, Faecalibaterium and Coprococus, were constantly associated with the indicators showing higher quality of life, and where another member of the same bacteria family, Dialister, and Coprococus were found to be seriously depleted, depressive cases were found. This study also highlighted a better quality of life when dopamine producing microbes were in abundance.

It is not just depression that can be caused by an unhappy gut. Studies have also shown the effect microbial dysbiosis can have towards schizophrenia. Those with schizophrenia have been found to have a much less diverse microbiota and also show an increase in specific bacterial families, to the point where patients with schizophrenia could be easily distinguished from those without, simply by analysing their gut microbiota. Links have also been shown between children with autistic spectrum disorder and an imbalance in certain microbiota species and an overall less diverse gut microbiota.

Unsurprisingly, the imbalance of gut microbiota also has links with obesity. Obesity has been linked to a less diverse microbiota as well as being linked with low fibre intake, which as previously mentioned, is necessary for the production of SCFA that affects appetite and food intake. Obesity, however, is just one of many physical diseases correlated to a low microbiota diversity, including irritable bowel syndrome (IBS), arthritis, diabetes, eczema, and coeliac disease, showing the importance of microbiota diversity and suggesting it is an indication of how we would define a ‘healthy gut’.

Keeping a ‘Healthy Gut’

So, how can we maintain a ‘healthy gut’ and keep our microbiota diverse? Many suggest the use of probiotics and prebiotics. Probiotics are live bacteria and yeast, that when administrated appropriately, are beneficial to health. These are often taken in the form of fermented foods such as yogurts or kefir, or taken as food supplements, and have been shown to improve gut health and brain activity. Prebiotics are usually fibres or microbiota accessible carbs fermented by our gut bacteria to help them grow. While these can be taken as supplements, prebiotics dietary fibres are also found in whole grains, fruit, and veg. The use of these has been shown to help improve stress, anxiety, and depression. Food’s rich in polyphenol, a plant chemical digested by our gut bacteria, can also help contribute towards a ‘healthy gut’. These include foods like green tea, olive oil, coffee, and cocoa – great for caffeine and hot chocolate lovers! Tryptophan rich foods such as turkey, eggs and cheese also have great benefits for our microbiota due to the importance of tryptophan in producing serotonin, as well as oily fish which contains omega3s known to promote good bacteria in the gut and reduce risk of brain disorders.

It is clear that our gut health can impact our body in so many ways, from our mood to disease and fitness. While stress appears to have a large impact on the health of our gut, this is not the only factor determining its health. It is important that we do what we can to keep our gut happy and healthy, because if your gut isn’t, then you won’t be either.

Graphic courtesy of Nahal Sheikh