The microbiome conversation has passed through several phases. First, diversity — the more diverse your gut bacteria, the healthier you are. Then probiotics — take specific bacteria to restore or augment the ecology. Then prebiotics — feed the bacteria to sustain the ecology. The emerging frontier is postbiotics — the compounds that bacteria produce, which may be the primary mechanism through which the microbiome influences health far beyond the gut.
The postbiotic concept reframes the question from “which bacteria should I have” to “which metabolites should my bacteria be producing, and are they producing them?” This matters clinically because two people can have similar bacterial diversity and similar species composition but dramatically different metabolite production — depending on the dietary substrates available to ferment, the enzymatic capacity of their specific strains, and the interaction between species in the ecological community.
The Key Postbiotics and What They Do
Short-Chain Fatty Acids (SCFAs)
Butyrate, propionate, and acetate — the primary SCFA postbiotics. Covered in detail in the SCFA and butyrate post. Butyrate fuels colonocytes, regulates tight junction gene expression, and suppresses inflammation through histone deacetylase inhibition. Propionate regulates liver glucose production and promotes satiety. Acetate is used as an energy substrate by peripheral tissues. Together they are the most clinically significant postbiotic category and the ones most consistently depleted by modern diet and antibiotic use.
Urolithins
Urolithins A and B are produced when specific gut bacteria metabolise ellagitannins — polyphenols from pomegranates, berries, walnuts, and oak-aged red wine. They activate mitophagy — the selective removal of damaged mitochondria — which is fundamental to cellular energy production quality. Urolithin A also activates AMPK and sirtuin pathways involved in longevity. Critically, only around 40% of people have the gut bacteria to produce urolithins efficiently. Those who don’t receive minimal benefit from ellagitannin-rich foods. Urolithin A is now available as a direct supplement (Mitopure) for those without the producing bacteria.
Equol
A metabolite produced when certain gut bacteria convert daidzein — a soy isoflavone — to equol, which has significantly greater oestrogenic activity than the parent compound. Only 30–50% of people are equol producers, depending on their gut ecology. In postmenopausal women, equol production is associated with reduced hot flushes and better hormonal outcomes from soy-based dietary approaches. The variation in response to soy isoflavones between individuals is largely explained by equol-producer status — which is determined by the gut bacteria present, not by genetics.
Indole-3-Propionic Acid (IPA)
A tryptophan metabolite produced by specific gut bacteria — one of the most potent antioxidants produced endogenously. IPA crosses the blood-brain barrier, where it acts as a neuroprotective antioxidant. It activates the pregnane X receptor (PXR), which regulates gut barrier integrity — IPA production directly supports tight junction function. Plasma IPA levels are consistently lower in people with Alzheimer’s, type 2 diabetes, and obesity. Tryptophan-rich foods (poultry, eggs, dairy, seeds) provide the substrate; gut ecology determines whether it is converted to IPA or to the less beneficial kynurenine pathway metabolites.
Secondary Bile Acids
Primary bile acids (cholic and chenodeoxycholic) produced by the liver are converted by specific gut bacteria to secondary bile acids (deoxycholic and lithocholic). Secondary bile acids act as signalling molecules — activating FXR and TGR5 receptors in the gut, liver, and systemic tissues to regulate glucose metabolism, energy expenditure, and immune function. Adequate secondary bile acid production requires both the bile acid substrate (from the liver) and the specific bacterial populations capable of the conversion. Disrupted gut ecology produces an altered bile acid profile with downstream metabolic consequences.
GABA and Neurotransmitter Precursors
Certain Lactobacillus and Bifidobacterium species produce gamma-aminobutyric acid (GABA) — the primary inhibitory neurotransmitter — through glutamate decarboxylase activity. These bacteria also modulate tryptophan availability for serotonin synthesis. The gut-brain axis operates in large part through these bacterial metabolites signalling via the vagus nerve and the enteroendocrine system. The observation that specific probiotic strains reduce anxiety is likely mediated through their GABA-producing activity rather than through the bacteria themselves surviving to the brain.
Fermented Foods — The Distinct Role That Supplements Don’t Replicate
Fermented foods are not equivalent to probiotic supplements. They deliver a different product — live microorganisms alongside their fermentation metabolites, in a food matrix with a different digestive fate than a capsule. The health benefits of fermented foods are partly from the bacteria they contain, partly from the postbiotic metabolites produced during fermentation, and partly from the food matrix itself. Separating these contributions is difficult, which is why the evidence for fermented foods is often less cleanly mechanistic than the evidence for specific probiotic strains.
A landmark 2021 Stanford study by Sonnenburg and Gardner compared a high-fibre diet to a high-fermented-food diet over ten weeks. The high-fermented-food group showed significantly increased gut microbiome diversity and decreased inflammatory markers. The high-fibre group showed increased SCFA production but variable microbiome changes — suggesting that fibre feeds existing bacteria, while fermented foods introduce new ecological diversity. Both are beneficial, through different mechanisms, and neither replaces the other.
Kefir
Among the most clinically studied fermented foods. Contains 30–50 distinct microbial species including Lactobacillus kefiri (unique to kefir), yeasts (Saccharomyces cerevisiae, Kluyveromyces marxianus), and multiple Lactobacillus and Bifidobacterium strains. The kefiran polysaccharide produced during fermentation has independent immunomodulatory effects. Evidence for: reducing inflammatory markers, improving lactose tolerance (the fermentation converts much of the lactose), and improving IBS symptoms. Water kefir is an alternative for dairy-intolerant individuals — different microbial community, less studied but still beneficial.
Kimchi and Sauerkraut
Traditional lacto-fermented vegetables providing live Lactobacillus species (particularly L. plantarum and L. brevis in kimchi) alongside the prebiotic fibre substrate from the vegetables themselves. The fermentation process produces lactic acid, acetic acid, and antimicrobial compounds that suppress pathogenic bacteria. Kimchi additionally provides capsaicin, allicin, and a range of phytochemicals from the vegetable matrix. Must be unpasteurised to contain live cultures — most commercial sauerkraut is pasteurised and provides the fermentation metabolites (lactic acid) without living bacteria.
Live Yoghurt
The most widely consumed fermented food. Must specify “live cultures” — many commercial yoghurts are heat-treated after fermentation, destroying the bacteria. The Streptococcus thermophilus and Lactobacillus bulgaricus in standard yoghurt are transient — they do not colonise the gut long-term but contribute metabolites during transit. Greek yoghurt provides higher protein, concentrated whey proteins, and a more concentrated live culture. Coconut yoghurt with added live cultures provides an alternative for dairy-intolerant individuals, though the live culture variety and density is more variable.
Kombucha
Produced by a SCOBY (symbiotic culture of bacteria and yeasts) fermenting tea. Provides organic acids (acetic, glucuronic, gluconic), B vitamins, and live cultures. The glucuronic acid content has led to claims about liver detoxification support — the evidence is limited but mechanistically plausible as glucuronic acid is used in Phase Two hepatic conjugation. Variable microbial content depending on fermentation conditions. Unpasteurised commercial kombucha retains live cultures; homebrew provides the highest bacterial density. Sugar content varies — choose lower-sugar varieties as the fermentation consumes sugar but residual amounts remain.
Miso, Tempeh, Natto
Traditional Asian fermented foods with distinct fermentation organisms. Miso (fermented with Aspergillus oryzae) provides digestive enzymes and umami compounds alongside fermentation metabolites. Tempeh (fermented with Rhizopus oligosporus) significantly improves the digestibility of soy protein and phytic acid content — antinutrient reduction is one of the primary benefits of fermentation. Natto (fermented with Bacillus subtilis) provides nattokinase (a fibrinolytic enzyme with cardiovascular applications) and vitamin K2 as menaquinone-7 — the most bioavailable K2 form. Natto is the best dietary source of MK-7, which activates matrix Gla protein for arterial calcium regulation.
Aged Cheeses
Hard aged cheeses (gouda, manchego, comté, gruyère) provide vitamin K2 as menaquinone-4 (MK-4) alongside the fermentation cultures. MK-4 activates osteocalcin for bone mineral metabolism and matrix Gla protein for vascular calcification prevention. The live culture content of aged cheese varies — some provide measurable L. plantarum and other Lactobacillus species. The fat matrix of cheese dramatically slows transit through the gut, potentially extending the time available for fermentation metabolites to contact the gut wall.
The Marketing Reality — What Fermented Foods Are Not
The fermented food category has suffered the same overselling that affects every area of nutrition that attracts wellness marketing. The current iteration is functional mushroom coffees and fermented supplement blends making claims that the evidence does not support. The clinical reality is more modest and more interesting.
Fermented foods are genuinely beneficial for most people with intact gut ecology and no specific contraindications. They contribute microbial diversity, fermentation metabolites, and in some cases specific nutrients (K2 from natto, urolithins from pomegranate polyphenols in people with producing bacteria) that are not reliably obtainable from other dietary sources.
They are not appropriate as a primary intervention in people with SIBO — the fermentation that occurs in the small intestine from live cultures and organic acids can worsen symptoms. They are not appropriate in people with significant histamine intolerance — fermented foods are high in histamine and biogenic amines that can trigger histamine reactions in susceptible individuals (particularly those with DAO enzyme deficiency).
The question is not whether fermented foods are healthy. They are. The question is whether they are appropriate at this stage, for this person, with this gut ecology. Someone with SIBO actively worsening on fermented foods is not failing to be healthy — they are responding predictably to a genuinely contraindicated food category. Fix the SIBO. Then introduce fermented foods gradually as the ecology restores.
SCFAs first. The most clinically significant postbiotics are the SCFAs — butyrate, propionate, acetate. Support their production through prebiotic fibre diversity and the SCFA-producing species identified on GI-MAP. If F. prausnitzii is significantly depleted, supplemental sodium butyrate bridges the gap while the ecology restores.
Urolithins if the ecology supports it. Pomegranate, berries, and walnuts provide ellagitannin substrate. If the specific producing bacteria are present (GI-MAP Gordonibacter), the conversion occurs. If not, Urolithin A supplement (Mitopure) provides the metabolite directly.
Introduce fermented foods gradually. Start with one small serving of a single fermented food. Wait 48 hours to assess any histamine or fermentation-related reaction. If well-tolerated, add variety. Daily small servings of two to three different fermented foods provides the diversity of benefit without the risk of large doses in an ecology that may not yet be ready for them.
Unpasteurised = live cultures. Pasteurised fermented foods provide fermentation metabolites and improved food matrix but no living bacteria. Check labels: “live cultures” or “contains live and active cultures” indicates unpasteurised or post-fermentation culture addition.
The microbiome series ends where functional medicine begins — with the recognition that the body is an ecosystem, not a machine. The bacteria, the substrates they ferment, the metabolites they produce, and the immune and metabolic signals those metabolites generate are integrated into a system of extraordinary complexity and remarkable clinical significance. The tools available — GI-MAP to identify what is present and depleted, targeted probiotics and prebiotics to restore and support, postbiotics and fermented foods to augment the metabolite production that drives systemic health — are not a replacement for the dietary diversity and lifestyle conditions that a healthy microbiome fundamentally depends on. They are the clinical layer that helps restore what modern life has depleted, to the point where the ecology can sustain itself. That is the goal. Everything in this series is in service of it.