An easy-to-understand guide to the chemical messages created when food, microbes and human biology meet

Your gut microbes do far more than occupy space or help finish the digestion of food. They transform fibre, resistant starch, amino acids, bile acids and plant compounds into a remarkable collection of small molecules. Scientists call many of these molecules microbial metabolites.

These compounds help explain how organisms living mainly in the colon can influence events at the intestinal surface and, in some cases, contribute to signals reaching the liver and wider circulation. Microbes do not speak with words. They communicate through chemistry.

That does not mean every microbial message is beneficial, or that a single food can switch one desirable metabolite on. The effect of a metabolite depends on what it is, how much is produced, where it acts, what the host does with it and the wider dietary and health context. The real story is not “good microbes make good chemicals”. It is a continuous exchange between food, microbial communities and human cells.

This partnership is part of the broader idea explored in The Human Holobiont Explained: Why Humans and Microbes Function as One Ecosystem.

 

What Is a Microbial Metabolite?

Metabolism is the collection of chemical reactions that keeps a living organism functioning. A metabolite is a small molecule involved in, or produced by, those reactions. Gut microbes make metabolites as they obtain energy, build cellular components, compete with neighbours and interact with their environment.

Some molecules are produced directly by microbes. Others begin as human or dietary compounds and are changed by microbial enzymes. A third group is best understood as co-metabolites: microbes perform one part of the chemistry and the human body performs another.

 

Where Are These Metabolites Made?

The colon is a major site because it receives material that was not fully digested or absorbed earlier. Dietary fibre and resistant starch are important examples. The environment is low in oxygen and home to dense microbial communities with an enormous collective library of enzymes that humans do not possess.

Production is not uniform. Conditions differ along the colon, and the available substrate changes after meals and as material moves through the bowel. A stool sample offers useful information, but it does not capture every interaction occurring at the mucosal surface or every metabolite that has already been absorbed.

From Food to Chemical Message

·   Food is chewed, digested and absorbed through the upper digestive tract.

·   Fibre, resistant starch and other compounds that escape earlier digestion reach the large intestine.

·   Microbial enzymes break down or transform those substrates.

·   Metabolites may be used by other microbes, interact with intestinal cells, leave in stool or be absorbed.

·   Absorbed compounds first meet the liver through the portal circulation, where many are modified or cleared before reaching the wider body.

The liver’s position matters. It acts like a biochemical checkpoint between the intestine and systemic circulation, which is why the gut–liver connection is central to understanding microbial metabolism.

Explore that relationship in The Gut–Liver Connection: Why Digestive Health Influences Overall Wellbeing.

The Major Families of Microbial Metabolites

There is no single “microbial metabolite”. The term covers chemically and biologically different families. Even within one family, individual compounds can have different effects. This is why claims about increasing “microbial metabolites” in general are too broad to be meaningful.

Short-Chain Fatty Acids: The Best-Known Example

Acetate, propionate and butyrate are produced when microbial communities ferment certain carbohydrates. Butyrate is an important energy source for many colonocytes. SCFAs can also interact with receptors and cellular pathways involved in intestinal, immune and metabolic biology.

They are related, but not interchangeable. They are produced in different proportions, used differently by gut and host tissues and found at very different concentrations in the colon and blood. Measuring one stool value also does not provide a simple score of gut health: a low stool concentration might reflect lower production, greater absorption or both.

For the detailed guide, read Short-Chain Fatty Acids Explained: How Your Gut Microbes Turn Fibre into Health-Supporting Compounds.

Bile Acids: A Two-Way Conversation

The liver makes primary bile acids from cholesterol and releases them into the digestive tract to help handle dietary fats. Most are reabsorbed, but those reaching the colon can be modified by microbial enzymes into a wider pool of bile-acid compounds.

Bile acids, in turn, influence which microbes can thrive. They also interact with human receptors involved in metabolic and immune signalling. This makes the bile-acid pool a genuine two-way conversation: human biology shapes microbial ecology, and microbial activity reshapes human molecules.

Tryptophan Metabolites: Several Pathways, Not One

Tryptophan is an essential amino acid. Human cells and gut microbes can direct it through different metabolic routes. Some bacteria produce indole and related compounds, while human pathways include serotonin and kynurenine metabolism. These routes interact with intestinal, immune and neural biology, but the details vary by compound and much of the whole-body evidence remains under investigation.

A common misunderstanding is that gut microbes simply “make serotonin for the brain”. Most serotonin is produced in the gut, but circulating serotonin does not freely cross the blood–brain barrier. Gut–brain communication is more complex, involving neural, immune, endocrine and metabolic routes.

The wider network is explained in The Gut–Brain Axis Explained: The Communication Network Linking Digestion and Brain Health.

Polyphenol-Derived Metabolites

Polyphenols are diverse plant compounds found in foods such as berries, apples, cocoa, tea, herbs, spices, nuts and many vegetables. Some are absorbed in the small intestine, while others reach the colon and are transformed by microbes into smaller compounds.

This helps explain why the food matrix matters. Two foods can contain related polyphenols yet deliver them within different combinations of fibre, carbohydrate, fat, protein and other phytochemicals. The microbes present in an individual gut also influence which transformations occur.

Read more in The Food Matrix Explained: Why Whole Foods Matter.

Amino-Acid Fermentation and Other Compounds

Microbes can ferment amino acids as well as carbohydrates. This produces a mixed collection that includes branched-chain fatty acids, ammonia, phenols, indoles and sulphur-containing compounds. Some participate in normal ecology and signalling; others may become undesirable at higher concentrations or under particular conditions.

This is an important correction to the idea that every metabolite produced by a diverse microbiome must be beneficial. Biological systems work through balance, location and dose. Microbial chemistry is neither automatically good nor automatically bad.

TMAO: Why Association Is Not the Same as Causation

Some microbes convert dietary compounds such as choline and carnitine into trimethylamine. The liver can then convert trimethylamine into TMAO. Higher blood TMAO has been associated with cardiovascular risk in several observational studies, but interpretation is complicated by kidney function, diet, host metabolism and other factors. Human evidence has not established a simple rule that a particular food raises TMAO and therefore causes disease.

 

Cross-Feeding: One Microbe’s Output Becomes Another’s Input

The most memorable part of microbial metabolism may be that production is rarely a solo act. One organism can break a complex fibre into smaller fragments. Another can consume those fragments and release lactate or acetate. A third can use those products to make butyrate. The final metabolite may therefore reflect a food chain of microscopic handovers.

This process is called cross-feeding. It means that microbial function cannot be predicted simply by counting one “good” species. Community relationships, available food, gut conditions and the metabolic abilities of neighbouring organisms all matter.

See the handovers in detail in Cross-Feeding Explained: How Gut Bacteria Work Together to Support a Healthy Microbiome.

 

How Chemical Messages Reach Human Cells

A metabolite does not need to travel to the brain or circulate at a high concentration to matter. Some of the most important interactions happen locally, only micrometres away from where the compound was produced.

The Intestinal Surface

Metabolites can interact with epithelial cells, mucus, receptors and immune cells in the intestinal environment. Some are used as fuel. Some change gene activity or enzyme function. Some influence microbial competitors. The intestinal barrier is therefore not merely a wall; it is a sensing and responding interface.

The Portal Vein and Liver

Compounds absorbed from the intestine generally travel first to the liver through the portal vein. The liver may use, transform, package or clear them. Consequently, the molecule measured in blood may not be identical to the one originally produced in the colon.

The Immune System

Microbial metabolites can be detected by receptors on intestinal and immune cells. SCFAs, bile-acid derivatives and tryptophan metabolites are among the families being studied for their roles in immune regulation and barrier function. This does not mean that a food or supplement can “boost” immunity through one metabolite. Immune biology is adaptive, highly regulated and influenced by the whole environment.

Continue with The Gut-Brain-Immune Connection: How Your Gut Influences Whole-Body Health.

The Gut–Brain Network

Gut–brain communication uses multiple routes: the enteric nervous system, vagal pathways, immune mediators, hormones and metabolites. Some microbial compounds may affect intestinal cells or nerves locally; others may influence precursor availability or systemic signalling. Much of the mechanistic evidence comes from cell and animal studies, while human research is growing but remains less definitive.

Muscle, Mitochondria and Metabolism

Researchers are also exploring associations between the microbiome, microbial metabolites, insulin sensitivity, skeletal muscle and mitochondrial function. These are plausible systems-biology connections, not a licence to claim that changing one microbe will directly improve energy or muscle. Human metabolism is shaped by diet, physical activity, sleep, hormones, genetics, medications and health status as well as the microbiome.

For the broader systems view, read The Gut–Mitochondria–Brain Connection: How Cellular Energy Links Digestion, Brain Function & Whole-Body Health.

What Does the Evidence Actually Say?

Microbiome studies differ in diet, population, sample type, laboratory method and analysis. Many measure genes that suggest what microbes could do rather than metabolites showing what they actually did. Metabolomics adds functional information, but a snapshot still cannot capture every fluctuation across meals, days and intestinal locations.

This is why the field is moving towards integrated approaches that combine dietary records, microbial genes, metabolites and human clinical measures. The closer research gets to the full system, the less convincing simple “one bug, one metabolite, one outcome” stories become.

How Food Shapes the Metabolic Possibilities

Food changes the substrates entering the colon, but it does not dictate the result alone. Two people can eat the same meal and produce different metabolite patterns because their microbial communities, gut transit, digestion, genetics, liver metabolism, medications and recent diets differ.

Still, a varied whole-food dietary pattern is a practical way to offer microbial communities a wider range of substrates. The aim is not to maximise every metabolite. It is to support a flexible ecosystem within an eating pattern that also meets human nutritional needs.

·   Vegetables and fruit provide different fibres and plant compounds.

·   Legumes contribute fermentable carbohydrate, resistant starch, plant protein and micronutrients.

·   Whole grains such as oats and barley provide distinct fibres where they suit the individual.

·   Nuts and seeds add fibre, fats, minerals and polyphenols.

·   Herbs, spices, tea, cocoa and colourful plants broaden polyphenol exposure.

·   Cooked and cooled potato, rice or pasta can contain more resistant starch than when freshly cooked, although the amount varies.

·   Fermented foods introduce microbial products and, in some cases, live microorganisms; they are not interchangeable with fibre-rich foods.

For practical variety, explore Food Diversity Explained: Why Variety Matters.

For the wider ecosystem, read Microbiome Diversity Explained: Why Variety Is One of the Best Things You Can Feed Your Gut.

Increase Variety Gradually

A sudden jump in fermentable fibre can increase gas, bloating or changes in bowel habits. Gradual changes allow time to learn what feels comfortable. People with persistent digestive symptoms, diagnosed gastrointestinal conditions or medically prescribed diets may need individual advice from an accredited practising dietitian or their healthcare team.

Lifestyle Changes the Context

Diet is important, but microbial metabolism occurs inside a living person. Physical activity, sleep, stress, gut transit, age, environment and medications can all influence the ecosystem or the way the host responds. Antibiotics and other medicines should be used as clinically appropriate; microbiome concerns are not a reason to stop prescribed treatment.

Where Bone Broth Fits

Bone broth is not a source of dietary fibre and should not be described as a direct way to increase beneficial microbial metabolites. Its practical role is culinary: it can help turn vegetables, legumes, whole grains, herbs and spices into soups, stews, curries and sauces that are enjoyable and easy to repeat.

In that setting, broth is one component of a diverse meal—not a replacement for the plant foods that provide fermentable substrates. The distinction keeps the microbiome message grounded in the overall dietary pattern.

For ideas, browse the collection of nourishing recipes.

A Simple Food-First Framework

·   Start with what you already eat and add one extra plant food rather than rebuilding the whole week.

·   Rotate colours and plant families across the week instead of chasing a perfect number each day.

·   Include legumes regularly if tolerated, beginning with a comfortable portion.

·   Choose whole grains where appropriate and vary the type rather than relying on only one.

·   Use herbs, spices, nuts and seeds to create diversity without making meals complicated.

·   Build soups, salads, stir-fries and tray bakes from several ingredients so variety happens naturally.

·   Pay attention to comfort, bowel habits and individual tolerance; more is not always better overnight.

 

Frequently Asked Questions

Are microbial metabolites the same as postbiotics?

Not exactly. “Microbial metabolite” is a broad scientific description. Postbiotic definitions generally refer to preparations of inanimate microorganisms and/or their components that confer a health benefit; isolated metabolites alone do not automatically meet that definition.

Are all microbial metabolites beneficial?

No. Some support normal gut function, some may be neutral in a particular context and others can become less desirable depending on concentration, location and host health.

Are short-chain fatty acids microbial metabolites?

Yes. Acetate, propionate and butyrate are among the best-studied metabolites produced during microbial fermentation of certain carbohydrates.

Can microbial metabolites reach the brain?

Some compounds or their host-modified products may enter circulation, but gut–brain communication also occurs indirectly through nerves, immune signals, hormones and changes at the intestinal surface. Human causality remains an active area of research.

Does eating more fibre always produce more beneficial metabolites?

Not in a perfectly predictable way. Fibre type, dose, microbial community, transit time and individual tolerance all influence the response. Increasing variety gradually is usually more practical than maximising one fibre.

Do probiotics produce metabolites?

Some strains can produce or influence metabolites, but effects are strain-specific and depend on the surrounding ecosystem and diet. A generic probiotic claim cannot be applied to every product.

Can a stool test tell me whether my microbial metabolites are healthy?

Stool testing can provide selected information, but it cannot capture every metabolite produced, absorbed or transformed throughout the gut and body. Results need cautious interpretation.

Is gas a microbial metabolite?

Yes. Hydrogen, carbon dioxide and methane can be produced through microbial metabolism. Gas is normal, although excessive or troublesome symptoms deserve appropriate assessment.

How can I support microbial metabolism through food?

Eat a varied pattern that includes vegetables, fruit, legumes, whole grains where suitable, nuts, seeds, herbs and spices. Increase fermentable foods gradually and choose amounts that suit your digestion.

Continue Exploring

·   Short-Chain Fatty Acids Explained: How Your Gut Microbes Turn Fibre into Health-Supporting Compounds

·   Cross-Feeding Explained: How Gut Bacteria Work Together to Support a Healthy Microbiome

·   The Gut Ecosystem: Why No Single Food or Supplement Can Do It All

·   Building a Healthy Gut: Why Diversity Matters More Than Any Superfood

·   The Food Matrix Explained: Why Whole Foods Matter

·   The Gut–Brain Axis Explained: The Communication Network Linking Digestion and Brain Health

·   The Complete Guide to Gut Biotics.

References and Further Reading

·   Postler TS, Ghosh S. Understanding the Holobiont: How Microbial Metabolites Affect Human Health and Shape the Immune System.

·   Koh A, De Vadder F, Kovatcheva-Datchary P, Bäckhed F. From Dietary Fiber to Host Physiology: Short-Chain Fatty Acids as Key Bacterial Metabolites.

·   Agus A, Clément K, Sokol H. Gut microbiota-derived metabolites as central regulators in metabolic disorders.

·   Agus A, Planchais J, Sokol H. Gut Microbiota Regulation of Tryptophan Metabolism in Health and Disease.

·   Winston JA, Theriot CM. Diversification of host bile acids by members of the gut microbiota.

·   Silva YP, Bernardi A, Frozza RL. The Role of Short-Chain Fatty Acids From Gut Microbiota in Gut-Brain Communication.

·   Canyelles M et al. Gut Microbiota-Derived TMAO: A Causal Factor Promoting Atherosclerotic Cardiovascular Disease?

·   Australian Dietary Guidelines.

Final Thoughts

Microbial metabolites reveal that digestion does not end when human enzymes have finished their work. Food continues its journey through an ecosystem whose inhabitants take molecules apart, exchange the pieces and build something new.

Some of those products feed neighbouring microbes. Some act at the intestinal surface. Some are transformed by the liver. Some appear to join wider signalling networks, while others are simply removed. The message is not that every metabolite controls the body. It is that human biology and microbial biology are continually meeting through chemistry.

That is the memorable shift: your microbiome is not a passenger, and food is not a set of isolated nutrients. Every meal supplies possibilities. Over time, a varied, balanced dietary pattern gives this complex partnership more useful material to work with—without promising that one ingredient can conduct the entire conversation.