A randomised controlled trial of the effects of dietary prebiotic foods and probiotic supplements on adult mental health and cognition: The "Gut Feelings" trial

INTERVENTION: Group A (probiotic group; 8‐week duration): Twice‐daily oral capsule of Bioceuticals MoodBiotic, containing 8 probiotic strains delivering 12 billion colony forming units (CFUs) + no change to existing diet. Each oral capsule will contain the following probiotic strains: Bifidobacterium bifidum (Bb‐06): 2 billion colony forming units (CFU) Bifidobacterium animalis subsp. lactis: 1 billion CFU Bifidobacterium longum (Rosell‐175): 1 billion CFU Lactobacillus acidophilus (La‐14): 2 billion CFU Lactobacillus helveticus (Rosell‐52): 2 billion CFU Lactobacillus casei (Lc‐11): 2 billion CFU Lactobacillus plantarum (Lp‐115): 1 billion CFU Lactobacillus rhamnosus (HN001): 1 billion CFU Group B (prebiotic group; 8‐week duration): High‐prebiotic diet + twice‐daily oral placebo (inactive) capsule. Dietary intervention involves eating 7 or more serves of prebiotic‐rich foods per day. Prebiotic‐rich foods are plant foods containing relatively high levels of fructans and/or galactooligosaccharides, as reported in the food‐science literature. Some examples are asparagus, garlic, onion, oats, whole wheat, chickpeas, and watermelon. The remainder of the diet is subject to the participant's discretion. An online educational video (8‐minute duration), prepared and delivered by an Accredited Practising Dietitian/nutrition lecturer will be shown to participants at their baseline session and a hard‐copy handout provided. Content of video/handout: information on the high‐prebiotic diet, suitable foods and how to introduce them into the diet, and recipe suggestions. The video and handout were specifically designed for the current study. Participants are responsible for purchasing and incorporating high‐prebiotic foods into their diet. A hamper of shelf‐stable high‐prebiotic foods will be provided to participants at baseline, as a 'starter kit'. Group C (synbiotic group; 8‐week duration): Combination of Group A (twice‐daily oral probiotic capsule) + Group B (high‐prebiotic diet) interventions For all groups, supplement compliance will be measured by returned capsule count at 8‐weeks. Adherence to allocated diet will be measured by completion of a purpose‐built online dietary screener at Weeks 0, 2, 4, 6, and 8. CONDITION: Anxiety Depression Stress PRIMARY OUTCOME: Psychological distress (POMS 2‐SF total score) SECONDARY OUTCOME: 16S rRNA stool sequencing Anxiety (BAI) Bowel health (ROME IV) Cognition (processing speed, attention, learning, working memory) (Cogstate Brief Battery) Depression (BDI‐II) Diet quality (CNAQ) Health‐related quality of life (SF‐36) Polyphenol intake (Phenol‐explorer 3.6) Sleep (LSEQ) Stress (PSS) Well‐being (WHO‐5) INCLUSION CRITERIA: ‐ Prebiotic fibre intake of <3 grams/day, as measured by a purpose‐built Dietary Screener, to ensure that there is sufficient scope for improvement in diet ‐ Subclinical levels of anxiety and/or depression. Eligible participants must score 16‐26 on the Kessler Psychological Distress Scale (K10) ‐ Free of psychiatric medications for > 4 weeks