We have trillions of bacteria living inside us, with most residing in our gut. Until recently, it has been challenging to explore our gut bugs, but advancements in genetic sequencing have allowed us to discover these bacteria by sequencing their DNA. Over 1000 species of different bacteria exist in your colon, with many contributing to the healthy functioning of your digestive system, immune system, and other physiological processes in the body. There is still much to be learnt about our gut bugs and their various functions, but we have a fair idea of how the microbiome is formed and how it can be altered. Your gut microbiome, which refers to your gut bacteria and all their genetic material, is largely formed during your first three years of life, with method of birth delivery, infant feeding, and exposure to antibiotics being highly influential in determining its composition. After this period, your gut microbiome stays relatively stable, although it can be improved in richness and diversity upon exposure to such factors as probiotics, prebiotics, and certain dietary patterns, with consistent exposure leading to more sustained changes. What are probiotics? Probiotics are live bacteria that are considered to provide health benefits when ingested in adequate amounts. They are designed to help by adding ‘good’ bacteria into the gut, which makes less room for ‘bad’ bacteria, which ideally enables the whole community of the gut microbiome to produce beneficial compounds for the rest of the body to use. Probiotics are found naturally in fermented foods such as yoghurt, kefir, miso and sauerkraut. Previous research using probiotics in supplement (non-food) form has shown that probiotics canhave a beneficial effect on mood. This reinforces the impact of our gut microbiota on mental health outcomes, which is part of what is termed the ‘gut-brain axis‘. However, more research is needed to understand this relationship, and the types of bacteria that are most helpful. What are prebiotics? A prebiotic is a non-digestible type of dietary fibre that feeds your gut bacteria, thereby beneficially influencing activity of the gut microbial population. Prebiotics are also considered to enhance immunity, enhance absorption of minerals, and induce the production of anti-inflammatory compounds such as short-chain fatty acids. Prebiotics are found in specific types of plant-based foods such as asparagus, leeks, garlic, wheat bran, and watermelon. Previous research on prebiotics has used supplements such as powdered inulin, and in such interventions, promising effects on stress hormone levels, emotional processing,and cognitionhave been found. Since prebiotics are found in certain high-fibre plant foods, might adopting a dietary pattern high in such foods lead to improvements in gut microbiome richness and diversity, which may in turn have effects on mood? Or is a probiotic more beneficial? And what about the combination of both, which is known as a synbiotic approach? These research questions have yet to be answered, and here at the Food & Mood Centre and our collaborating team ARCADIA Research Group (UniMelb), we have set out to do so in the Gut Feelings Trial. Gut Feelings The Gut Feelings Trial is an 8-week placebo-controlled randomised trial, currently being conducted at The Melbourne Clinic, Richmond, VIC, that measures whether probiotics (capsules containing several strains of beneficial bacteria) and/or prebiotics (via adherence to a high-prebiotic diet) improve gut microbiota composition, mental health, and cognition. This is the first trial to be conducted in the emerging field of gut-brain research that uses food-based prebiotics, and further, to apply them in synbiotic combination with probiotics. We are currently recruiting participants! Do you live in Melbourne, Australia and want to participate in the Gut Feelings Trial? We are seeking adults aged 18-65 years who are: generally healthy experience low mood, anxiety, or stress, have scope for improvement in their diet. Please note that certain exclusions apply. Visit the Gut Feelings Trial website or Facebook page for more information and to complete our pre-screener survey to see if you are eligible! The outcomes of Gut Feelings will uncover important insights into the interrelationships between food, mood and gut health that we are just starting to understand. A key take-home that we can all apply now is that probiotics and a healthy dietary pattern appear to be nourishing for you and your many gut bugs, so say yes to fermented and high fibre foods as they enable us and our internal ecosystem to thrive. By Tanya Freijy Tanya is a PhD Candidate within ARCADIA Research Group (School of Psychiatry, The University of Melbourne) and the Food and Mood Centre at Deakin University. This article is also posted on MindBodyMicrobiome.