Eating to change our brains: The gut-brain axis

Is having a healthy gut the key to your mental health?

Writer: Charlotte Li
Editor: Marta Caldeira

You’re waiting for your date to arrive and your stomach is fluttering. Your “gut” feeling tells you something is wrong, you’re anxious and miserable but what you don’t realise is that this stems from the trillions of bacteria that make up your gut microbiome, and these outnumber even your human cells.

How you feel now is affected by the bacteria in your gut, with the gut and brain being interlinked by the Gut-Brain Axis (GBA). This is influenced by the vagus nerve that runs from the brain all the way to the colon, and innervates a major depression system, the Hypothalamic Pituitary Axis.

From the moment we are born, our microbiome continually changes due to our diet and feeding patterns, determining whether we are more or less prone to disorders such as obesity and even autism.

Gastrointestinal (GI) dysfunction is very common in neurodevelopmental conditions such as Autism Spectrum Disorder (ASD) and schizophrenia. In fact, when researchers treated an ASD mouse model suffering from GI dysfunction with the bacteria Bacteroides fragilis, this not only restored intestinal permeability and metabolite levels. It also alleviated ASD-related behavioural symptoms such as anxious marble-burying, and even produced a better response to social encounters, thus demonstrating a clear relationship between the gut and brain.

Autistic patients have been found to have more species of Clostridia bacteria and higher levels of Propionic Acid (PPA) (a product of Clostridia fermentation) in faeces. When autistic children were put by researchers on a specific antibiotic (Vancomycin) that depletes Clostridia bacteria, their autistic behaviour drastically improved; demonstrating the critical role microbes play in ASD.

Germ-free mice are found to have lower levels of Brain-Derived Neurotrophic Factor (BDNF), a protein important in memory and learning (low levels were linked to depression and anxiety). When compared with normal mice, germ-free mice had noticeable memory defects as well as an exaggerated anxiolytic behaviour. In another experiment by McMaster University, when the faeces of extroverted and introverted mice were exchanged, their anxious behaviours and BDNF levels also changed accordingly. These studies show us that our behaviour and even our memory is dependent on the microbes living in our gut.

Despite what sceptical parents might say, depression is not just “all in our head”. 90% of the neurotransmitter serotonin is produced in our GI tract, and it is the most common target for anti-depressants, regulating our mood and circadian rhythm. Microbes have also been shown to produce neurotransmitters which produces relaxing and anxiolytic effects. Our diets can hence affect the neurotransmitters released, affecting our emotion.

Since now we know that the microbes in our gut affect the development and function of our brain, so what we eat is very important to our mental health. Ancient civilizations swore by drinking sour milk before going to battle, an example of probiotics – fermented food that can stimulate the growth of “good microbes”. These probiotics have been shown to have anti-anxiety and anti-depressive effects that are similar to therapeutic drugs such as Citalopram.

An example of natural probiotics are the antioxidant compounds found in fruits and vegetables. These can reduce neuronal damage induced by oxidative stress, especially in the hippocampus, an area of the brain involved in memory. A “Western” diet has been associated with a smaller hippocampus and a 25% higher risk of depression as compared to eating a “traditional diet”. This is because the latter includes unprocessed food with unrefined sugars, such as lentils and asparagus, which are other natural probiotics. Our “good” gut microbes ferment prebiotic fibres (e.g. garlic and artichoke), producing short-chain fatty acids that strengthen our gut lining as well as our blood-brain barrier, which has also been implicated to control our gene transcription.

Taking care of our microbiome by eating right is one of the most important things you can do for your mental and physical health. Improving cognitive function or treating brain-related disorders with bacteria is not too far-fetched. Professor John Cryan, a microbiome expert from University College Cork, commented that ‘‘microbiome-derived medicine is the future of precision medicine”. This suggests that we might even be en-route to “poop-doping” with faecal transplants for greater performance and mental agility.

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