Fermented foods are increasingly recognized as beneficial for health due to a myriad of demonstrated health benefits. They are also considered to be ‘functional foods’, which means they have additional health benefits beyond supplying basic nutrition. A large array of fermented foods and beverages are becoming more and more widely available including fermented dairy, vegetables, fruits and tea. But did you know that fermented foods may also be good for mental health? What are fermented foods? Fermentation is a natural process driven by microorganisms in the absence of oxygen. The original food becomes more nutritional and valuable once it is fermented. The desirable health effects of fermentation are attributed to the bacterial metabolites/biogenics (vitamins, amino acids) that are generated during the process of fermentation and the live microorganisms in the food. Lactobacillus and Bifidobacteria are the most commonly occurring bacterial types in fermentation and are both recognized as probiotics. (Probiotics are formally defined as microorganisms that contribute to improving host health when administered in adequate quantities). The fermentation process augments the digestibility of food, for instance lactose (milk sugar) in milk is converted to lactic acid when fermented and people with lactose intolerance choose yogurt and cheese over milk to avoid unwelcome side-effects such as flatulence. Increasing the bioavailability of essential nutrients (amino acids), removal of toxins, enhancing aroma and improving the structure of the food are some of the other advantages of fermentation. Originally, fermentation was essential as a natural way of processing and preserving food. Dairy, soy, vegetables and fruits can be kept longer at ambient temperature when fermented. In general, bacterial fermentation produces weak organic acids such as lactic, propionic and butyric acid that decreases the pH of the food, increasing the acidity of the food. This acidifying property of fermentation has been used to eradicate pathogenic bacteria and keep food for longer in shelves. Gut and mental disorders Gut functions extend well beyond the obvious roles in digestion, absorption and assimilation of nutrients. We now know that gut health and the human gut microbiota are involved in many physiological processes, diseases, and in the maintenance of overall health. Mental disorders often go hand-in-hand with compromised gut functions and there is clear evidence of a bidirectional relationship between mental and gut health. At least one study has showed that depressive symptoms can be transferred through fecal transplant. As such, inflammation is also comorbid with mental disorders and people experiencing depression often have higher levels of proinflammatory molecules in their blood. The proinflammatory molecules in blood can damage the gut lining and facilitate the flow of gut contents to the blood which in turn aggravates inflammation around the body. Also, when the gut is inflamed it starts to leak nutrients and bacterial components including bacterial cell wall components to circulation which automatically triggers immune activation and augments inflammatory molecules in the blood. Over time, the increased level of inflammatory molecules in blood can adversely affect brain functions and exacerbate symptoms of mental disorders. Are fermented foods a remedy? In this context, there is accumulating evidence on the therapeutic effects of fermented food to restore imbalance in the gut and mitigate inflammation. Why are fermented foods beneficial? It is mostly due to their probiotic features. Health benefits of probiotic foods include the elimination of pathogens, producing bioactive components beneficial for the host such as vitamin B12, creating a favourable environment for the proliferation of beneficial bugs in the gut, boosting immunity and fostering a ‘healthy’ intestinal balance. In addition, probiotics have shown to mitigate inflammation associated with irritable bowel syndrome. Also, the organic acids produced during the process of fermentation, so-called short-chain fatty acids, have demonstrated anti-inflammatory properties. There are some other mechanisms of action too. For example, fermented foods contain neurotransmitters such as gamma-aminobutyric acid that are essential for good mental health. Also, fermented food contains prebiotics which are produced by the degradation of complex carbohydrates by bacterial enzymes. Prebioticsare food for probiotic bacteria and selectively stimulate the growth of these beneficial bacteria in gut. Prebiotics escape digestion in stomach and become a food source for the bacteria living in the colon. Whilst these mechanisms provide great theoretical reasons to consider fermented foods to be therapeutic, it is important to note that direct evidence in humans is not yet well established. Studies in animals however, do show that administration of fermented foods reduces behaviours indicative of anxiety and depression. Nevertheless, more research is warranted before fermented food can be recommended as an adjunctive treatment for mental disorders. We are learning increasingly more about nutritional psychiatry which tells us more on how food can influence our mental health, emphasizing that food has important functions beyond basic nutrition. Fermented foods contain many important physiological health benefits due to the probiotics, prebiotics and biogenics (bioactive bacterial metabolites) available. However, the potential therapeutic effects of fermented foods need to be investigated further as they may provide safe and novel approaches to difficult-to-treat mental health conditions. Hajara Aslam is a PhD candidate at the Food and Mood Centre with a Masters Degree in Food Science and Technology and special expertise in prebiotics. Her current research interests are focussed on the gut microbiome and mental and physical health.