Indole is a small natural molecule that bacteria in your gut make from the food you eat. It works as a chemical signal, letting those bacteria communicate with your gut lining, your immune system and possibly organs farther away. The same compound gives stool part of its smell and, in tiny amounts, gives jasmine its scent.1Office for Science and Society. Scents and Sensibilities Most claims about its wider benefits come from mice and lab dishes, so it is a promising messenger rather than a proven health fix.
Where Indole Comes From
Chemically, indole is two linked rings of carbon atoms, one of which has a nitrogen atom in it.2Wikipedia. Indole That same ring sits inside tryptophan and inside serotonin, a messenger in the brain and gut.3Wikipedia. Indole Many related molecules, known together as indoles, share this core with small changes.4Wikipedia. Indole
Tryptophan is an amino acid, a building block of protein found in foods like poultry, eggs, cheese and beans. A wide range of gut bacteria convert it straight into indole in a single step.5PubMed Central. Dietary fibre directs microbial tryptophan metabolism via metabolic interactions in the gut microbiota E. coli, a common gut bacterium, is one well-known maker, and how much it produces depends on how much tryptophan reaches it.6PubMed. Indole production by the tryptophanase TnaA in Escherichia coli is determined by the amount of exogenous tryptophan So what you eat sets part of how much raw material your gut bacteria have to work with.
The journey starts with a meal. Most protein is digested and absorbed in the small intestine, but some tryptophan escapes and reaches the large intestine, where most of your bacteria live. There they break it down and release indole and related molecules. Some of these are absorbed into the blood and travel around the body. Bacteria turn tryptophan into several related compounds, and different bacteria make different versions, so your mix depends on which microbes you carry. Two people eating the same meal may make different amounts of indole, because their microbiomes differ.
How Indole Protects the Gut Lining
Your gut lining is a single layer of cells held together by protein seals that stop bacteria and toxins from leaking through. They are not a solid wall but selective gates, letting water and nutrients pass while holding back bacteria and their fragments. Mucus sits on top as a second barrier, keeping most gut bacteria a little distance from the cells so the immune system isn’t constantly provoked.
Indole acts on both layers. It switches on genes that build the seals and the protective mucus layer.7PubMed Central. Dual Role of Indoles Derived From Intestinal Microbiota on Human Health In human gut cells grown in a dish, normal gut-level amounts of indole made the cell layer tighter and boosted mucus genes.8PubMed Central. The bacterial signal indole increases epithelial-cell tight-junction resistance and attenuates indicators of inflammation Mice raised without any bacteria had about 27 times less indole in their stool and weaker seals.9PubMed Central. Regulation of Intestinal Barrier Function by Microbial Metabolites Giving those mice indole capsules restored their seals.10PubMed Central. Regulation of Intestinal Barrier Function by Microbial Metabolites
These mice lack many bacterial products besides indole, so the weak seals can’t be blamed on indole alone. The capsule experiment is the stronger clue: adding back one molecule restored the seals, which points to indole as one real contributor, not the only one.
When seals loosen, the lining is often called “leaky,” and more bacterial material can slip into the body and stir inflammation. Indole is one of several bacterial signals that help keep the barrier intact.
Indole also steers the immune system toward calm. It helps immune cells release IL-22, a signal that helps repair and protect the gut lining.11PubMed Central. Dual Role of Indoles Derived From Intestinal Microbiota on Human Health In mice, a related indole made by Lactobacillus bacteria raised IL-22 and helped keep out the yeast Candida while protecting against inflammation.12PubMed. Tryptophan catabolites from microbiota engage aryl hydrocarbon receptor and balance mucosal reactivity via interleukin-22 In mice, a related compound from vegetables like broccoli eased colitis, a gut inflammation, by raising IL-22.13PubMed Central. Indole-3-carbinol prevents colitis and associated microbial dysbiosis in an IL-22–dependent manner
Indole and Harmful Bacteria
Many bacteria count their neighbours using chemical signals and only launch attacks once there are enough of them. Indole interferes with this counting by making a key signal-reading protein fold wrongly and fall apart.14PubMed. Indole inhibits bacterial quorum sensing signal transmission by interfering with quorum sensing regulator folding It also helps block disease-causing E. coli strains from settling in the gut.15PubMed Central. Dual Role of Indoles Derived From Intestinal Microbiota on Human Health
In lab tests, indole weakened Listeria, a food-poisoning bacterium, cutting its ability to form sticky colonies and move.16PubMed. Indole signaling decreases biofilm formation and related virulence of Listeria monocytogenes Several indole relatives blocked colony-building in Serratia, a hospital-infection bacterium.17PubMed Central. Antibiofilm and Antivirulence Properties of Indoles Against Serratia marcescens
The colonies in question are biofilms: slimy, packed communities stuck to a surface. Bacteria in biofilms are harder for the immune system and antibiotics to clear, so stopping biofilms is one way to make bacteria less harmful without killing them outright.
Unlike antibiotics, which kill bacteria or stop them growing, indole in these experiments mostly disrupted coordination. It did not wipe the bacteria out, so it weakens them rather than curing an infection.
These effects were seen on bacteria in dishes, not in human infections. They suggest a healthy, indole-making microbiome may help crowd out some invaders, but they are not a reason to treat food poisoning with indole or supplements.
Beyond the Gut, Brain, Liver and Ageing
Many indole effects run through one receptor, called AhR, a sensor found in immune and other cells across the body.18PubMed. The indole-brain connection: neuroimmune mechanisms and therapy A receptor is a protein that acts like a lock, responding to molecules that fit it and then changing what the cell does. AhR sits inside cells, and once an indole binds, it moves to the nucleus and alters which genes are switched on. Because many cell types carry AhR, one bacterial molecule can influence the gut, skin and immune cells, which explains why indole’s effects look so wide.
Skin fits this picture. Many bacteria on human skin also make indoles that switch on the same sensor.19PubMed. Activation of the aryl hydrocarbon receptor via indole derivatives is a common feature in skin bacterial isolates
The brain results come from animals. In mice, indoles from gut bacteria helped new nerve cells form in a memory area of the brain.20PubMed Central. Tryptophan-metabolizing gut microbes regulate adult neurogenesis via the aryl hydrocarbon receptor In mice bred to develop Alzheimer’s-like disease, indoles reduced brain inflammation.21PubMed. Microbiota-derived metabolite Indoles induced aryl hydrocarbon receptor activation and inhibited neuroinflammation in APP/PS1 mice A study that included unmedicated teenagers with depression, alongside mice, linked indole signalling to brain inflammation balance and new nerve cell growth.22PubMed. Novel insight into the effects of the metabolites from gut microbiota in both unmedicated depressive adolescents and mice: key roles of indole derivatives-aryl hydrocarbon receptor signaling in neurogenesis and microglial homeostasis IPA, short for indole-3-propionic acid, is an indole relative that easily passes into the brain and is being studied for protecting nerve cells.23PubMed Central. Microbiota-Derived Tryptophan Metabolite Indole-3-Propionic Acid-Emerging Role in Neuroprotection Whether it can delay dementia in people has not been tested.
The liver is another target. Blood from the gut goes straight to the liver first, so the liver meets gut molecules early, and some molecules then circulate on to the brain. In mice fed a high-fat diet, indole by mouth reduced fat build-up and inflammation in the liver.24PubMed. Indole alleviates nonalcoholic fatty liver disease in an ACE2-dependent manner In one mouse study, the effect was similar to metformin, a common diabetes drug.25PubMed Central. Indole Alleviates Diet-induced Hepatic Steatosis and Inflammation in a Manner Involving Myeloid Cell PFKFB3 IPA improved grip strength and bone structure in old mice and extended life in fruit flies, though only in some strains and one sex.26PubMed. Microbiota-derived indole-3-propionic acid extends lifespan in Drosophila and improves muscle and bone health in mice Doses in studies are often far above what a gut makes naturally, and many promising animal findings never hold up in human trials.
Indole and Blood Sugar
GLP-1 is a gut hormone that tells the body to release insulin after eating, and drugs like semaglutide copy it. When food reaches the intestine, specialized cells in the gut lining release it into the blood. It prompts the pancreas to release insulin in proportion to the meal and slows stomach emptying, which helps you feel full. Because those cells sit in the gut lining, they are in direct contact with whatever bacteria produce, including indole.
Short exposure to indole made mouse gut cells release more GLP-1.27PubMed Central. Bacterial Metabolite Indole Modulates Incretin Secretion from Intestinal Enteroendocrine L Cells Longer exposure reduced release, because indole slowed the cells’ energy production.28PubMed Central. Bacterial Metabolite Indole Modulates Incretin Secretion from Intestinal Enteroendocrine L Cells In diabetic mice, indole improved blood sugar control and increased the number of GLP-1-making cells.29PubMed Central. The microbial tryptophan metabolite indole acts on the gastrointestinal tract to improve glucose homeostasis in a mouse model of diabetes by enhancing GLP-1 secretion and L cell differentiation
The same molecule can help or hinder depending on timing and amount, which makes simple advice like “more indole is better” unreliable. It is one reason indole is not a diabetes treatment.
When Indole Becomes a Problem
Indole absorbed from the gut goes to the liver, which chemically modifies it into indoxyl sulfate. The kidneys normally filter this into urine, so in healthy people it does not accumulate. In the blood it is carried largely attached to proteins, which makes it hard to remove by dialysis.
When kidneys fail, it builds up. In rats with failing kidneys, blood indoxyl sulfate and stool indole both rose as disease advanced.30PubMed. Disorder of gut amino acids metabolism during CKD progression is related with gut microbiota dysbiosis and metagenome change Indoxyl sulfate is also being studied as a blood marker of an unbalanced microbiome.31PubMed. Biomarkers of human gut microbiota diversity and dysbiosis
So indole has a dual role. Inside the gut it is mostly protective, but once processed and stuck in the blood it can become a toxin. This matters mostly for people with kidney disease. They may hear their doctor mention “uremic toxins,” waste products that build up, and indoxyl sulfate is one of them. Diet and medicines are managed by the kidney team, not by cutting indole on your own.
Can Diet Change How Much Indole You Make
Tryptophan and fiber work together. In mice, extra tryptophan in the diet raised indole levels.32Animal Nutriomics. Interactive effects of dietary tryptophan and inulin on gut microbiota–mediated tryptophan metabolism and intestinal health Adding inulin, a fiber found in onions, garlic and chicory, shifted bacteria to make more IPA and less indole.33Animal Nutriomics. Interactive effects of dietary tryptophan and inulin on gut microbiota–mediated tryptophan metabolism and intestinal health That combination improved signs of a healthy gut lining.34Animal Nutriomics. Interactive effects of dietary tryptophan and inulin on gut microbiota–mediated tryptophan metabolism and intestinal health35PubMed Central. Dietary fibre directs microbial tryptophan metabolism via metabolic interactions in the gut microbiota In mice with a severe abdominal infection, tryptophan supplements kept indole levels up and were linked to better survival.36sisna.org. Oral tryptophan supplementation enhances indole production by the gut microbiota and promotes survival from peritonitis
In practice, that means eating a mix of protein foods that supply tryptophan and pairing them with fiber from vegetables, legumes, onions and whole grains. Variety feeds a more varied microbiome. No human trial yet shows the best amounts. Tryptophan and indole-3-carbinol supplements exist, but the evidence comes mostly from animals. Supplements can interact with medicines, for example tryptophan with antidepressants, so check with a doctor first. Food is the lower-risk route.
Why Indole Smells Like Both Poop and Jasmine
At high concentrations indole smells of feces and mothballs, and it is one of the compounds in stool.37Office for Science and Society. Scents and Sensibilities Very diluted, it smells floral. It occurs in jasmine and orange blossom and is added to perfumes.38Office for Science and Society. Scents and Sensibilities
The nose reads strength as well as type of smell. Smell receptors respond in a way that depends on how many molecules are present, so concentration changes the pattern of receptors activated. At high levels the brain reads that pattern as fecal; at low levels it reads a flowery note.
Indole in Plants and Medicines
Plants use an indole called auxin as a key growth hormone.39Journal of Plant Biology and Crop Research. What Is Indole? Its Role in the Gut and Beyond It tells plants where to grow, for example bending shoots toward light and prompting roots to form.
Chemists made indole for the first time in 1866, from the dye indigo.40tiankunchemical.com. The Story of Indigo And Bayer Its shape now sits at the core of the migraine drugs sumatriptan and zolmitriptan.41Beilstein Journal of Organic Chemistry. An overview of the key routes to the best selling 5-membered ring heterocyclic pharmaceuticals Newly designed indole-based compounds reduced inflammation in mice, including in lung and bowel inflammation models.42Journal of Medicinal Chemistry. Design, Synthesis, and Structure–Activity Relationship Study of Novel Indole-2-carboxamide Derivatives as Anti-inflammatory Agents for the Treatment of Sepsis43Journal of Molecular Structure. Development of new azole-based indole/indoline derivatives for the treatment of inflammatory bowel disease by targeting MAPK/NF-κB signaling pathways
The indole core shows up in many medicines because its shape fits body receptors. Experimental compounds are years from pharmacy shelves, and a drug with an indole core is not the same as the indole your bacteria make.