Distinct modulatory effects of high-fiber and fermented-food diets on gut microbiota, immune function, transit time, and sleep quality in a citizen science randomized controlled trial

A randomized, placebo-controlled citizen science-based dietary intervention was conducted among 147 healthy adults to evaluate the effects of 8-week high dietary fiber (HDF) and high fermented food (HFF) diets on gut microbiota, immune function, gut transit time, well-being, and sleep quality. The HDF group significantly increased fiber intake (Δ10.3 g/1000 kcal/day) following high dietary fiber recipes with addition of dried chicory root, while the HFF group increased fermented food consumption (+6.3 portions/day), including a fermentation-derived liquid supplement. At the 21-week follow-up, modest improvements in fiber and fermented intake were sustained, compared to baseline. Microbial diversity significantly increased within the HFF and control groups, especially in HFF participants over 50 ( p = 0.04). Compared to CG, HFF showed no difference in microbial diversity, whereas the HDF group showed a significant decrease. The HDF intervention enhanced butyrogenic potential by increasing Anaerostipes, Faecalibacterium , and Bifidobacterium spp., and significantly reduced gastrointestinal transit time ( p = 0.01). The intake of high fiber improved and sustained sleep quality ( p = 0.03). The HFF intervention significantly increased blood immune markers including CD5, CD6 and CD8A (T-cell activation), IL-18R1 (inflammatory signaling) and SIRT2, a longevity-associated deacetylase (Q < 0.05), and induced a modest shift in the gut microbiota of participants over 50 years toward a composition characteristic of younger participants. These findings highlight distinct biological pathways through which dietary fibers and fermented foods modulate host physiology. This is the first randomized controlled nutritional intervention using a citizen science approach that demonstrates the feasibility and scientific value of engaging participants in healthier food choices.