“Listen to your gut” – we often hear this phrase and even suggest it to those who find themselves torn apart between decisions. As with many proverbs and sayings, we utter this phrase out of tradition, taking its meaning quite metaphorically. In fact, how could your digestive system help with any life changing decision? More so, how do you listen to it? Surprisingly enough, it seems like this saying has more truth to it than meets the eye.
Scientists have discovered that our guts are comprised not only of bacteria, but also of a huge neural network of around a 100 million neurons. In other words, our digestive system is also a nervous system, containing more neurons than the spinal cord and as many as inside a cat’s brain! [1-3] It is for this reason that scientists have started referring to the gut as “the second brain”. Unlike our “first brain” though, the gut does not have cognitive abilities like thinking, reasoning and decision making. [1,2] Writing love letters and overthinking are realms reserved for the brain in our heads. What our second brain, or enteric nervous system (ENS), is responsible for is psychological stress responses. Recall the gittery “butterflies” in your stomach before an exam? Or the pit in your stomach after receiving devastating news? Both reactions are innate parts of our psychological stress responses, being communicated from the brain below to the brain above. [1,3] Our gut senses environmental threats, then influences our response to them.  Indeed, new research findings show that the ENS is responsible for much of our physical, mental and emotional well-being. [2,4] Gut feelings, it seems, are not that metaphorical after all.
But that’s not the end of it. One may pose the question; if our digestive system is also a nervous system, can the food we consume play any part in how we feel? If you asked yourself that question, 10 points to you, because you’re on the right track. This area of research – “Nutrional Psychiatry” – is relatively new. However, findings are revealing that gut bacteria play a big role in mood regulation. This may seem very strange, yet when analysing the brain-gut connection it’s good to be aware that 90% of serotonin (“the happy hormone”) is produced in the gastrointestinal tract. [5,6,7] In fact, the most common side-effects caused by antidepressants tend to be gut-related, including; nausea, diarrhea and gastrointestinal problems.  Since the vast majority of serotonin is produced in the gut, which is lined with a hundred million neurons, it is quite reasonable that our digestive system plays a role in mood regulation. 
The American Gut Project, the largest study on the human microbiome as of yet, compared the gut environment of participants with no psychological issues, PTSD, schizophrenia, depression and bipolar disorder. It resulted that participants who reported similar mental health issues shared more bacteria in common, supporting the link between mental health and gut bacteria drawn by previous research.  For instance, a study found a variety of gut bacteria lacking in people suffering from depression. Yet whether the lack of such bacteria is the cause or result of depression hasn’t been established. [9, 10]
The food we eat alters our gut environment positively or negatively. It is no news that processed foods and chemical additivities increase the chances for diseases due to the high amounts of sugars, hydrogenated fats, flavour enhancers and colourings. Unfortunately, the Western diet is embedded with such toxic foods like canned foods, salted meat products, soft drinks, sugary snacks, instant noodles and so forth. When ingesting these products, we are not only flirting with physical ailments like obesity, diabetes, asthma and IBS. The bad bacteria disrupting the gut environment may also be inviting depressive moods. 
A study published very recently suggests that a healthy, balanced diet away from inflammation-producing foods can be protective against depression. Like any engine, the brain functions best when fuelled with high-quality foods which nourish it and protect it from oxidative stress. Diets high in refined sugars have been shown to impair brain function and aggravate symptoms of mood disorders. While comparative studies on probiotic intake (supplements rich in good bacteria) found lowered levels of anxiety and stress in participants who consumed the probiotics.  Nonetheless, researchers prompt altering one’s diet before attempting gut modifying-therapies to improve one’s mood. It is suggested to consume fresh vegetables and fruits rather than juices or frozen produce, fibre such as whole grains and legumes, including probiotic-rich and fermented foods and reducing sugar and red-meat intake.  One paper has illustrated an “Antidepressant Food Scale”, a nutrient profiling system outlining dietary recommendations concerning mental health. 
With this being said, a healthier diet can aid in increasing feelings of well-being, but this does not mean that one can eat his way of psychological issues and disorders. It is neither advisable nor feasible to use food as one’s only source of treatment for depressive moods, especially in cases of severe depression, severe anxiety and suicidal thoughts. However, revising one’s diet is a positive contributing step towards maintaining a healthier mind and body.
- Cytowic RE. The Pit In Your Stomach is Actually Your Second Brain. Psychology Today. Sussex Publishers; 2017.
- Hadhazy A. Think Twice: How the Gut’s “Second Brain” Influences Mood and Well-Being. Scientific American. 2010.
- Mosley M. The second brain in our stomachs. BBC News. BBC; 2012.
- Young E. Gut instincts: The secrets of your second brain. New Scientist. 2012.
- Naidoo U. Gut feelings: How food affects your mood. 2019.
- Jenkins T, Nguyen J, Polglaze K, Bertrand P. Influence of Tryptophan and Serotonin on Mood and Cognition with a Possible Role of the Gut-Brain Axis. Nutrients. 2016;8(1):56.
- Selhub E. Nutritional psychiatry: Your brain on food. Harvard Health Blog. 2018.
- Sandoiu A. ‘Largest’ microbiome study weighs in on our gut health. 2018.
- Pennisi E. Evidence mounts that gut bacteria can influence mood, prevent depression. 2019.
- Wilson C. People with depression are less likely to have certain gut bacteria. 2019.
- Lassale C, Batty GD, Baghdadli A, Jacka F, Sánchez-Villegas A, Kivimäki M, et al. Healthy dietary indices and risk of depressive outcomes: a systematic review and meta-analysis of observational studies. Molecular Psychiatry. 2018;24(7):965–86.
- Lachance LR, Ramsey D. Antidepressant foods: An evidence-based nutrient profiling system for depression. World Journal of Psychiatry. 2018;8(3):97–104.