An exciting new area of research regarding the health of our gastrointestinal (GI) tract (made up of the stomach, intestines and colon) and how it affects our overall physiology and well-being has emerged over the last 10 years. The findings confirm century-old narratives such as ‘you are what you eat’ in a myriad of intricately related ways. The knowledge scientists have gained in this field has led to a novel way to view mental health diseases such as anxiety and depression, as well as neurodegenerative conditions such as Parkinson’s, autism spectrum disorder (ASD) and Alzheimer’s. It turns out, as you will discover in this article, that many of these conditions can be prevented or mediated through the maintenance of a healthy gut microbiome (GMB).
Our intestines contain a mind-blowing amount of bacteria. In the intestine, numbers of bacteria reach 1014 organisms and 1800 different species per gram of intestinal contents (3,5). All together the total weight of the bacteria in our gut is between 1 – 2.5 kilograms. Coincidentally, the brain weighs about 1.3 kilograms (4). While this may seem frightening, the GMB is colonised from birth and forms a lifelong, beneficial relationship with our bodies, producing an incredible amount of essential nutrients, hormones and neurotransmitters that affect the way we think, feel and function (5,6).
The gut is responsible for producing 90% of the serotonin in our body (4,6). Serotonin is a major contributor to feelings of happiness and well-being, so it makes sense to look after the biggest contributor of serotonin in our body from a mental-health perspective. Eating fibre from whole grains and vegetables results in the GMB fermenting this otherwise non-digestible material into beneficial molecules such as short-chain fatty acids (butyrate, acetate and GABA) and tryptophan – essential for serotonin synthesis (4,6,7). These short-chain fatty acids produced exclusively from a healthy GMB do more than just provide us with serotonin, but are also responsible for a tight, protective barrier in our brain and gut that defend against disease and harmful metabolites.
The GMB is the largest immune organ in our body (4), as it lines every nook and fold in our massive GI tract. This prevents pathogenic bacteria and viruses from entering our body every time we eat. However, the GMB also contains a surprising number of harmful bacteria, which lay relatively dormant in our intestines. A diet high in sugars, saturated fats and low in fibre feeds harmful bacteria (known as gram-negative bacteria) and thins the protective layer. This results in increased gut leakiness, where harmful pathogens and metabolites are able to enter the blood stream and create an immune response and inflammation (3,4).
This increase in inflammation creates sickness behaviours commonly observed in anxiety and depression (1,3,4,6). Also, the inflammatory response brought on by an unhealthy diet functions in the opposite of some anti-depressant medications, where the concentration of serotonin is reduced in the connective areas of our nervous system (2,3,4,6). You can read all about the effects of inflammation in this article
Interesting studies performed on mice that were raised without a GMB; achieved through antibiotics or sanitised, ‘germ-free’ conditions, found astounding results related to cognitive function and social interaction. Germ-free mice that did not contain a GMB were found to have lower amounts of brain-derived neurotrophic factor (BDNF) which is responsible for the growth of neurons in our brain. This resulted in a decrease in brain plasticity (the ability for neurons to create new connections in the brain) as well as cognitive performance. Also, these mice expressed lower levels of social interaction with the other mice and acted more depressed in general (5). Social interaction is a vital aspect in the maintenance of good mental health which you can also read about here.
Finally, exciting findings related to stress levels and the GMB in the development of anxiety and depression from birth to adult life have been observed. Early disruption of a child’s GMB has been associated with an increased risk of developing depression/anxiety and autism spectrum disorder (ASD) later in life (3,4,6). Normal vaginal birth, as well as breast feeding, are incredibly pivotal moments in the early development of a child’s GMB, as these are the first instances where a child can come in contact with beneficial bacteria.
The stress levels and mental state of the pregnant mother are able to pre-emptively shape the anxiety-like behaviour in children. Stress levels, diet and inflammation all affect the encoding of the child’s stress response system. A heightened stress-response system in the mother prepares the child for a dangerous world, equips them with higher stress responses and less outgoing behaviours (4). In a relatively safe and harm-free society, a heightened stress-response leads us to fear more abstract and harmless threats, such as social interactions – key components to anxiety and depressive conditions.
Stress has a disruptive effect on the GMB and the GI tract. When we are stressed, the body activates the stress-response ‘fight or flight’ system, which ups inflammation/immune activity in anticipation of damage to our body and increases stress hormones such as cortisol and adrenaline. In response to stress, our stomach increases acidity to kill bacteria and speeds up the evacuation of the contents in our GI tract – leading to discomfort, bloating, diarrhea, all common symptoms of irritable bowel syndrome (IBS). Surprisingly, 50% of individuals with IBS also report anxiety and depressive symptoms (1,4). Furthermore, the addition of stress results in increased cravings for ‘comfort foods’ that are high in sugars and saturated fats, which further propagates this whole inflammatory/stress response and poor diet (4).
It would seem, considering the information that has been discovered, that caring for our gut microbiome through the maintenance of a healthy diet rich in fibre, a diverse range of whole foods and low in processed foods, saturated fat and sugar, would be a risk-free and cost-effective method of mediating neurodegenerative and mental health diseases such as depression, anxiety, ASD, Parkinson’s and Alzheimer’s. The introduction of pro-biotic supplements to increase the number and diversity of beneficial bacteria that promotes healthy functioning of our brain, immune system and mental health can also be interpreted as a simple and effective prevention against these diseases. With a healthy diet and methods of reducing stress (meditation, for example), especially in pregnant and breast-feeding mothers, we can interrupt the spread and development of neurodegenerative and mental health issues.
References:
1. Dinan, T; & Cryan, J; (2013). Melancholic microbes: a link between gut microbiota and depression?. Neurogastroenterology & Motility, 25(9), 713-719.
2. Fröhlich, E; Farzi, A; Mayerhofer, R; Reichmann, F; Jačan, A; Wagner, B; & Kashofer, K; (2016). Cognitive impairment by antibiotic-induced gut dysbiosis: analysis of gut microbiota-brain communication. Brain, behavior, and immunity, 56, 140-155.
3. Lima-Ojeda, J; Rupprecht, R; & Baghai, T; (2017). “I Am I and My Bacterial Circumstances”: Linking gut microbiome, neurodevelopment, and depression. Frontiers in psychiatry, 8, 153.
4. Mayer, E. (2016). The Mind-Gut Connection. New York, NY: HarperCollins.
5. Neufeld, K; Kang, N; Bienenstock, J; & Foster, J; (2011). Reduced anxiety‐like behavior and central neurochemical change in germ‐free mice. Neurogastroenterology & Motility, 23(3), 255-e119.
6. Sampson, T; & Mazmanian, S; (2015). Control of brain development, function, and behavior by the microbiome. Cell host & microbe, 17(5), 565-576.
7. Tillisch, K; Labus, J; Kilpatrick, L; Jiang, Z; Stains, J; Ebrat, B; & Mayer, E; (2013). Consumption of fermented milk product with probiotic modulates brain activity. Gastroenterology, 144(7), 1394-1401.
