A ketogenic Mediterranean diet (KEMEPHY or similar Modified Mediterranean-Ketogenic Diet/MM KD variants) that incorporates fermented vegetables tends to modestly lower the Firmicutes-to-Bacteroidetes (F/B) ratio while preserving or enhancing overall gut microbiome diversity and enriching beneficial short-chain fatty acid (SCFA)-producing taxa. This contrasts with some standard high-fat ketogenic diets (which can sometimes raise F/B or reduce diversity due to very low fiber) but benefits from the Mediterranean components (olive oil, avocados, nuts, fish, low-glycemic vegetables, polyphenols, and fiber from allowed plants) plus probiotic/prebiotic effects of fermented foods (e.g., kimchi, sauerkraut, kefir, yogurt, fermented cheeses).0
Direct Evidence on F/B Ratio and Microbiome Shifts
The most targeted human data come from a 2022 randomized controlled trial (30 days, parallel-arm) in 16 semi-professional male soccer athletes. The KEMEPHY diet was very low-carb (<30 g/day), high-fat (~64% from quality mono-/polyunsaturated sources), adequate protein, and explicitly included fermented products (kimchi, kefir, whole yogurt, fermented cheese) plus phytoextracts, berries, seeds, and MCT oil.045
F/B ratio: Decreased in the KEMEPHY group (pre: 1.11 [IQR 1.07–1.23] → post: 0.99 [0.73–1.15]) vs. a slight increase in the Western diet control (pre: 1.07 [0.99–1.67] → post: 1.16 [0.94–1.23]). Authors noted the intervention specifically increased Bacteroidetes and lowered Firmicutes phylum relative to controls (time × group interaction p > 0.05 but directional benefit highlighted).0
Other microbiome effects: Stable alpha-diversity (no loss of richness/evenness, unlike some pure keto diets); decreased Actinobacteriota (including Bifidobacterium); increased SCFA-producers (Butyricimonas, Odoribacter, Ruminococcus). Changes correlated with carb restriction, higher healthy fat intake, and reductions in body weight, fat mass, and visceral adipose tissue.03
Related MMKD trials (e.g., in mild cognitive impairment/MCI subjects) show favorable modulation: increased Akkermansia, Christensenellaceae, and SCFA shifts (↑ butyrate/propionate), with correlations to improved cerebrospinal fluid Alzheimer’s biomarkers. Baseline MCI often shows slightly higher Firmicutes/lower Bacteroidetes; MMKD helps normalize beneficial taxa without major dysbiosis.5062
Fermented vegetables alone (or in keto-compatible amounts) boost microbiome diversity and SCFA producers via probiotics (Lactobacillus spp.) and prebiotic fibers, counteracting potential high-fat drawbacks and supporting the net F/B-lowering or stabilizing effect seen in KEMEPHY-style protocols.307
Caveats: Effects are promising but from small/short-term studies; F/B ratio is an imperfect biomarker with high inter-individual variability. Longer trials in broader populations are limited, and responses depend on baseline microbiome, adherence, and exact fermented intake.
Benefits of Lowering the Firmicutes-to-Bacteroidetes Ratio
A higher F/B ratio is frequently linked to dysbiosis in obesity and metabolic syndrome (Firmicutes more efficient at calorie extraction from food). Lowering it (as seen with this diet) promotes leaner microbial metabolism, enhanced SCFA production (butyrate/propionate for signaling and barrier support), reduced endotoxemia, and anti-inflammatory effects.2918
Metabolic health: Reduced excess energy harvest (2–10% fewer calories absorbed), aiding weight/fat loss, improved insulin sensitivity (lower HOMA-IR, fasting glucose/insulin), and reduced type 2 diabetes risk. SCFAs enhance satiety hormones (GLP-1/PYY), regulate hepatic metabolism, and improve lipid profiles.2919
Decrease in inflammation: Strengthens gut barrier (SCFAs upregulate tight junctions), lowers lipopolysaccharide (LPS) translocation and systemic low-grade inflammation (e.g., reduced TNF-α, IL-6). Fermented foods in the diet further decrease 19 inflammatory proteins (including IL-6).2930
