Effects of a high-prebiotic diet versus probiotic supplements versus synbiotics on adult mental health: The “Gut Feelings” randomised controlled trial

This was a 4-arm randomized controlled trial in non-clinical adults with moderate psychological distress and low prebiotic intake recruited in Melbourne, Australia. Active arms randomized 28 participants to the prebiotic diet, 30 to probiotics, and 32 to synbiotics; week-8 completers were 22, 23, and 23, respectively.

This 8-week randomized trial tested three active regimens against placebo: a high-prebiotic diet targeting at least 7 serves per day of prebiotic-rich foods and at least 5 g/day of prebiotics; a probiotic capsule taken twice daily with food; or the combination of both as a synbiotic. The probiotic product provided 12 billion CFU per capsule and contained Bifidobacterium bifidum Bb-08, Bifidobacterium animalis subsp. lactis HN019, Bifidobacterium longum R0175, Lactobacillus acidophilus La-14, Lactobacillus helveticus R0052, Lactobacillus casei Lc-11, Lactobacillus plantarum Lp-115, and Lactobacillus rhamnosus HN001.

The high-prebiotic diet was the clearest effective intervention and improved mood disturbance and anxiety, while the probiotic arm showed only limited, non-primary benefits and the synbiotic arm was null. For total mood disturbance, the prebiotic diet beat placebo at week 8 by MD = -6.97 (95% CI -13.6, -0.345; p = 0.039), whereas probiotics (MD = -2.17; p = 0.51) and synbiotics (MD = -0.331; p = 0.92) did not. The prebiotic diet also improved perceived stress (MD = -3.20; p = 0.017) and reduced odds of more severe anxiety (OR 0.290; 95% CI 0.102, 0.801; p = 0.018). The authors concluded that a high-prebiotic diet for 8 weeks may reduce mood disturbance and anxiety and improve sleep, but there was no strong evidence that the probiotic combination improved the primary mood outcome, and the synbiotic combination showed no benefit.

The trial was small, single-center, and short duration, which limits power and durability of inference. Outcomes were largely self-reported, with multiple active arms and multiple comparisons, and the sample was heavily female and Caucasian, reducing generalizability. Attrition by week 8 and the possibility of interaction between the diet and probiotic components also complicate interpretation.