Are you irritated, sad, depressed, or anxious? These can be related to your poor gut health.
Our brain and gut are connected in a complex, bi-directional, symbiotic relationship. This communication network is known as the gut-brain axis.
The human gut houses trillions of bacteria, commonly known as gut microbiota that plays a key role in affecting mood and mental health, as explained by numerous studies. However, this area of research is relatively new and gained a lot of attention over the past years.
This article in the gut health series discusses different ways in which gut, brain and mental health are connected.
A first-of-its-kind review study, from King’s College, London, dated 15 September, analysed 59 case-control studies and found that people experiencing depression, anxiety, bipolar disorder, and psychosis significantly lacked faecalibacterium and coprococcus, two gut microbiome that is anti-inflammatory in nature. Additionally, people who were living with these conditions were carrying higher levels of eggerthella, a bacterium that causes inflammation.
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Vagus nerve-gut-brain interaction
The vagus nerve is an important component of the autonomic nervous system that connects our gut and brain and helps with digestion, swallowing, breathing, appetite, etc. This nerve sends signals both to the brain and the gut, modulates the brain-gut axis, and defines the brain’s response to inflammatory diseases such as irritable bowel syndrome or Crohn’s disease. The vagus nerve-gut connection explains why stress may affect our digestion, but also why digestive issues may affect our mood. The vagus nerve plays an important role in modulating cortisol, the stress hormone. In a study, mice were fed gut-friendly probiotic food, causing them to report a reduction in the level of stress hormones in their bodies. This effect of probiotics was not observed when their vagus nerve was removed.
Neurotransmitters are chemicals that control feelings or emotions. Serotonin, a neurotransmitter that controls happiness and the body’s circadian rhythm, is produced in large quantities by the gut. Gut microbes also produce a gamma-aminobutyric acid (GABA), another neurotransmitter that participates in the controlling mechanism of fear and anxiety. A study demonstrated feeding probiotic food to lab rats increased the production of GABA and helped in reducing anxiety and depression.
A 2019 review provides the latest evidence on glutamate, a neurotransmitter or neuromodulator in the gut-brain axis, explaining how it influences taste, visceral sensitivity and motility of the digestive system, and also affects brain functions such as stress response, mood and behaviour.
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Gut bacteria’s control from appetite to depression
Gut microbiota produces short-chain fatty acids (SCFAs) such as butyrate, propionate, acetate by fermenting dietary fibre that has a potential role in controlling appetite, influence gut environment, and physiology of the colon. Butyrate is an essential SCFA that keeps the gut happy, maintains the gut lining, reduces inflammation, and helps form and maintain the blood-brain barrier, while propionate is found to reduce appetite and block reward-based mechanisms in high-energy foods.
Dysbiosis – a condition defined by an imbalance in gut microbiome culture — leads to the growth of opportunistic bacteria in the gut, followed by the onset of inflammation that is associated with a range of psychological disorders including depression and Alzheimer’s disease.
A review of 26 studies found a strong link between gut microbiota, brain-gut axis, depression and how an individual thinks. The authors reiterated the significant role of gut microbiomes in ensuring mental wellbeing.
Lipopolysaccharide is a bacterial toxin that causes inflammation if leaked in excess from the gut into the blood. Lipopolysaccharide-induced inflammation is associated with serious brain conditions such as dementia, schizophrenia, and depression as explained in a review by John R. Kelly and colleagues.
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Diet and the gut-brain axis
Probiotics or live bacteria culture of Bifidobacterium, Lactobacillus, and Lactococcus species is one of the most beneficial brain foods. Probiotics that benefit the brain are known as ‘psychobiotics’. Renowned researcher and scholar Timothy G. Dinan defines a psychobiotic as a “live organism that, when ingested in adequate amounts, produces a health benefit in patients suffering from psychiatric illnesses.” These probiotics produce brain-friendly compounds like GABA, serotonin, butyrate and maintain a healthy brain-gut axis.
An eight-week-long randomised control trial reported a reduction in depression scores among participants who were supplemented with probiotic cultures.
Probiotic like Bifidobacterium longum NCC3001 was found to be beneficial for people living with irritable bowel syndrome and depression at the same time.
Prebiotics are the type of soluble dietary fibres that are food sources of gut microbiota. These fibres are broken down into neuro-supportive SCFAs by the gut microbiomes. Forty-five healthy volunteers reported a significant reduction in the stress hormone, cortisol after taking galactooligosaccharides, a prebiotic supplementation for three weeks.
There are also food groups that help in maintaining a healthy brain-gut axis. Probiotics, derived from fermented foods such as Greek yoghurt, sauerkraut and kefir play a big role. Prebiotics, obtained from resistant starch and high fibre foods like whole grains, whole fruits, green leafy vegetables, nuts, seeds, legumes, oats, quinoa and millets are also beneficial. Omega 3 fatty acids found in food such as fatty fish, walnuts and seeds are also equally important.
There is no doubt that the gut-brain connection is an important determinant of the overall wellbeing of an individual. The gastrointestinal system is sensitive to feelings and emotions. Anxiety, depression, anger, stress can affect gut health to a great extent. Also, poor gut health, leaky gut, and gut illnesses such as irritable bowel syndrome can trigger the brain to produce negative emotions, and sometimes even serious psychological illnesses such as schizophrenia or Alzheimer’s disease. A diverse, probiotic-prebiotic-rich diet is crucial to maintain gut microbiome balance, prevent dysbiosis and inflammation.
Subhasree Ray is Doctoral Scholar (Ketogenic Diet), certified diabetes educator, and a clinical and public health nutritionist. She tweets @DrSubhasree. Views are personal.
This is the second of a three-part series on gut health. The first article focused on the relationship between the gut and weight gain. The third will explain the relationship between gut microbiota and the immune system, and provide detailed dietary guidelines.