Impact of Plant-Based Diets on Gut Microbiota in Middle-Aaged Adults

Thesis Abstract

The study addresses the growing concern about how plant-based dietary patterns influence the gut microbiota composition and function in middle-aged adults, a demographic at risk for metabolic and inflammatory disorders linked to dysbiosis. Despite increasing adoption of plant-based diets, evidence linking dietary patterns to microbial diversity, taxa shifts, and metabolite profiles in middle age remains heterogeneous and methodologically fragmented. The aim is to determine how plant-based diets modulate gut microbiota structure, function, and associated metabolic markers, and to identify mediating dietary components. Specific objectives are to (1) characterize habitual plant-based intake using a validated food frequency questionnaire and 24-hour recalls; (2) compare gut microbial alpha and beta diversity, and relative abundances of core and fermentation-associated taxa between high and low adherence groups over a 12-week period; (3) quantify microbial functional potential through shotgun metagenomic sequencing and infer metatranscriptomic activity via computational pathway analysis; (4) assess fecal short-chain fatty acid (SCFA) concentrations and correlate them with microbial profiles; (5) examine associations between dietary fiber types, polyphenols, and microbiota endpoints; and (6) explore relationships between microbiota features and systemic inflammatory and metabolic biomarkers. The methodological design is a longitudinal, prospective cohort with an embedded quasi-experimental component. Participants will be community-dwelling adults aged 40–60 years (n = 240) recruited from urban and suburban clinics, with stratification by baseline BMI (18.5–24.9, 25–29.9, ?30 kg/m2). Data collection comprises dietary assessment (validated FFQ and 3 non-consecutive 24-h recalls at baseline, 6 weeks, and 12 weeks); stool sample collection for 16S rRNA gene sequencing, whole-metagenome sequencing, and targeted metabolomics for SCFAs; fasting blood samples for inflammatory and metabolic panels (CRP, IL-6, TNF-?, lipid profile, glucose, HbA1c); and anthropometric measures. Data analysis will employ mixed-effects models to evaluate longitudinal changes in microbial diversity and taxa abundance, with fixed effects for diet adherence group, time, BMI category, and sex, and random effects for participant ID. Multivariate redundancy analysis will relate microbial composition to dietary variables (fiber subtypes, polyphenol intake) and SCFA concentrations. Differential abundance analyses will use DESeq2 to identify taxa associated with plant-based adherence, and network analysis will map microbe–metabolite interactions. Functional profiling will utilize HUMAnN3 to infer pathways and correlate with SCFA outputs and inflammatory markers. Mediation analyses will test whether SCFA levels mediate the relationship between plant-based intake and inflammatory/metabolic outcomes. The expected findings include higher alpha diversity and enrichment of beneficial taxa (e.g., Bacteroidetes to Firmicutes ratio optimization, increased Faecalibacterium prausnitzii and Roseburia spp.) in higher adherence participants, coupled with enhanced microbial pathways related to complex carbohydrate degradation and butyrate production. SCFA concentrations are anticipated to be higher in this group, with concomitant reductions in systemic inflammatory markers among overweight and obese subgroups. The study contributes to knowledge by integrating dietary patterns with multi-omics microbial data and host biomarkers in a middle-aged population, clarifying mechanistic links between plant-based diets and gut health, and identifying dietary components most predictive of favorable microbial and metabolic profiles. The main conclusion is that sustained plant-based dietary patterns favor a eubiotic gut milieu and metabolic health in middle-aged adults, mediated in part by increased SCFA production. Recommendations include targeted dietary guidance emphasizing soluble and fermentable fibers, whole-food plant sources, and polyphenol-rich foods to optimize gut health, along with considerations for personalized nutrition approaches based on baseline microbiota and BMI. Limitations include potential confounding by unmeasured lifestyle factors and adherence variability; future research should explore longer follow-up periods and intervention trials with randomized design to confirm causality.