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Exciting new studies in the field of gut microbiome research have shown that the trillions of microbes that reside in our gut affect our mood and vice versa in what’s now called the gut-brain axis. This two-way communication occurs via the vagus nerve, which runs from the brain stem all the way down throughout our digestive system. The vagus nerve forms part of the enteric nervous system which is located in the gastrointestinal tract and contains 200-600 million neurons. This communication primarily travels from the gut up to the brain. Within the enteric nervous system, intestinal microbes, hormones, immune cells and beneficial metabolites produced by bacteria called short chain fatty acids communicate with the central nervous system which controls our hunger, food preferences, satiety and hunger. Additionally, there is an association between the gut microbiota and many psychological conditions and illnesses such as anxiety, schizophrenia, autism, Alzheimer’s disease and Parkinson’s disease. 

Needless to say a growing body of research is showing that there is a link between our gut health and overall feeling of mental wellbeing. 

Microbes that affect the brain within gut-brain axis research has emerged a new field called the microbiota-gut-brain axis that specializes in studying how specific microbes affect our mood, overall mental health and pain
perception. An imbalanced gut microbiota composition, also called dysbiosis, has been associated with increased stress, and anxiety. 

For example, specific non-beneficial bacteria and parasites have been associated with depression, anxiety, stress, autism-like behavior and memories. Interestingly, several intestinal microbes can produce neurotransmitters, or brain chemicals, such as dopamine, serotonin, norepinephrine, glutamate and gamma aminobutyric acid (GABA). In fact, an estimated 95% of serotonin is produced in our intestines, which regulates digestive function and pain perception. 

Some microorganisms in the GI tract also contain enzymes that can break down neurotransmitters like serotonin when we experience a stressful event like endurance exercise or excess inflammation. Not surprisingly, new evidence shows that stress negatively affects the gut-brain axis. When we experience physical and mental stress, the gastrointestinal tract releases neurotransmitters such as GABA, neuropeptide Y (NPY) and dopamine which can not only affect digestive functions, but also levels of anxiety and immune responses.

IBS patients, who frequently suffer from anxiety and stress, have been found to have an unhealthy gut microbiome and increased intestinal inflammation which reaches the central nervous system. The
stress experienced by these patients has also been found to alter the gut microbiota composition. 

Experiencing stress and the consequent release of the neurotransmitter norepinephrine can also increase susceptibility to suffering a pathogenic infection like Salmonella enterica. 

This could be partly because norepinephrine suppresses the release of antimicrobial immune cells like IgA in the intestines which allows for “bad bugs” to survive in the intestines. Stress may also affect brain development in fetuses and teenagers. Pregnant women who experience stress during critical times of fetal development can cause altered gut-brain communication and metabolism in their babies. Teens going through puberty may experience sex-specific stress that alters the gut microbiota composition which consequently alters hormone levels and neurotransmitter production.

Inflammation in the gut also negatively affects our mood. Gut microbiota related inflammation can consequently cause brain inflammation which has been observed in Alzheimer’s and Parkinson’s disease patients. For example, immune cells like cytokines as well as microbial derived anti-inflammatory short chain fatty acids can exit the gastrointestinal tract and enter the bloodstream and lymphatic system where they can travel to the brain or influence neural messages via the vagus nerve which influence mood and behavior.

Targeting the gut microbiota through diet and supplements is now becoming a therapeutic target in treating neurological disorders associated with an underlying cause of inflammation such as anxiety, depression, schizophrenia, autism and Alzheimer’s disease. Anxiety and depression are becoming more prevalent and are a global health concern. Although few studies have been performed on humans, research has shown that psychobiotics, which are probiotics that improve neuropsychiatric or neurodegenerative disorders, may be a promising therapeutic approach for improving mood with little side effects. For example, probiotics like Bifidobacteria and Lactobacillus strains have been effective in treating depression, anxiety, autism spectrum disorders and improving Alzheimer’s disease symptoms.

Probiotics have also been shown to play a role in neurotransmitter synthesis such as GABA, serotonin, and dopamine which all affect behavior and mood.

Food and your mood

Long-term adherence to a diet seems to have the greatest impact on the gut microbiota composition, although certain nutrients have also been shown to affect the gut-brain axis.

• Mediterranean diet: Following a Mediterranean diet has been shown to decrease inflammation and the risk for depression. This diet has also been shown to increase butyrate levels and possibly the synthesis of neurotransmitters dopamine, serotonin and noradrenaline.
• Ketogenic diet: This very low carbohydrate diet can increase the level of ketones in the blood which can reduce neurodegeneration. This diet may also decrease inflammation and alter the gut microbiota composition which can be protective against epilepsy, autism, and schizophrenia.
• Fibre: Although studies have been mixed about using prebiotics to treat anxiety and depression, eating a high fibre diet, especially resistant starch from tubers and fructo-oligosaccharides (FOS) from bananas and asparagus, has been shown to increase the production of butyrate in the colon, which may have therapeutic effects for neurodegenerative diseases such as Alzheimer’s and Parkinson’s.
• Polyphenols naturally occur in colorful plant based foods ranging from tea to cacao and can be protective against a wide range of diseases. Their anti-inflammatory and antioxidant properties have been shown to reduce oxidative stress in various psychiatric disorders and have a positive effect on the central nervous system. Interestingly, the gut microbiota has been shown to make the beneficial components of polyphenols more bioavailable.
• Vitamin B12: The gut microbiota can synthesize B12 and affect its absorption in the intestines which consequently influences neurological health.
• Omega 3: The omega 3 fatty acids DHA and EPA appear to improve depression, schizophrenia, age-related cognitive decline, overall mental health and possibly Alzheimer’s disease. Omega 3 fatty acids also influence the gut microbiota composition.
• Avoid food additives that are inflammatory and can have a negative effect on the gut microbiome.

Gut health can greatly affect our mental health through various mechanisms. On the other hand, high stress levels and inflammation can have a negative effect on the gut microbiome and consequently neurotransmitter production and overall mental health. Luckily more studies are emerging that show that adhering to a whole foods anti-inflammatory diet like the Mediterranean diet and supplements like probiotics can support and even improve mood and cognitive health.