Bitter substances in food are important
July 15, 2025
Plants contain many active substances, and it is not an exaggeration to say that many of the chemical substances we use in modern medicine today were originally found in plants known in traditional herbal medicine.
When the physician Hippocrates is quoted as saying “let thy medicine be thy food and thy food be thy medicine“, it can be understood that the particularly active plant substances from herbal medicine actually belong to larger families of active plant substances that are found in all plants. Therefore, plants in our diet can generally contribute to our health. Let’s look at some of the mechanisms of action.
Most of the chronic diseases that afflict people who eat a “Western” diet like the Danish one are neurological diseases such as dementia, metabolic diseases such as Type 2 diabetes and cardiovascular diseases such as atherosclerosis. There is increasing evidence that these diseases are caused by two things: Chronic oxidative stress and chronic inflammation, both of which can cause problems in the gut and in other organs of the body.
Until about three decades ago, our understanding of the gut was focused on the body’s own ability to digest food, and the bacteria in the gut were largely considered “stowaways”. Today, we are aware that the bacterial composition of the gut (the microbiota) contributes greatly to the health and disease of the entire body.
It is therefore interesting that the active plant substances found in the diet can change the composition of the intestinal bacteria in a healthier direction. In addition, many plant substances are strong antioxidants that can act both in the intestine and throughout the body when absorbed from the intestine, and many plant substances actively inhibit inflammatory reactions in both the intestine and the body’s cells.
Among the families of active plant substances that we come into contact with on a daily basis are allicins from onions, flavonoids from the colored substances in fruits and vegetables, glycosinolates from cabbage, tannins in virtually all fruits and vegetables, and essential oils from all the fragrant herbs and spices.
Plants produce all these active plant substances to protect themselves against disease-causing bacteria, viruses, fungi, etc., and they have a similar effect when we eat them, where they can act in the intestine and further, when absorbed from the intestine, can act throughout the body.
The most widespread group of active plant substances are tannins, and the following article describes some of the most important functions that tannins can contribute to our physical and mental health.
Klaus Sall
Biologist, Cand. Scient.
The importance of tannins for our health
We are all familiar with the slightly bitter brown skin that surrounds walnut kernels. The skin is high in tannins, and it is a good example of how plants protect themselves from attack by fungi and bacteria. Potatoes and carrots also deposit tannins in their skins to prevent their tubers and roots from being eaten by fungi and parasites – and apples, pears, plums, grapes, blueberries and other fruits use both tannins and the colorful substances called flavonoids in their skins as part of their protection against attacks from, for example, viruses and fungi.
Tannins were originally described as polyphenols that can precipitate protein dissolved in water. They are called polyphenols because they are made up of many smaller phenols, and tannins are usually divided into two main groups: hydrolyzable tannins and condensed tannins.
The illustration shows a few examples of how tannins can be structured, and their high content of OH groups, which forms the basis for their antioxidant effect.
The figure shows on the left the structure of a hydrolyzed tannin with a sugar molecule in the middle and phenolic groups on the right and left bonded together over oxygen O. On the right a small section of a condensable tannin, which is usually very large molecules with the same phenolic group bonded over and over again directly from carbon to carbon, which makes this type of tannins very stable.
Tannins can help change the intestinal environment in a healthier direction. Hydrolyzable tannins are broken down into the phenols they are made up of, such as Gallic Acid and Ellagic Acid. Ellagic Acid is partially broken down into various urolithins and, like Gallic Acid, they can be easily absorbed from the intestine and affect cells throughout the body with their antioxidant effect.
Tannins are by far the most widespread of the special plant substances that give many medicinal herbs their effect, and tannins therefore play an important role in those plants that have traditionally been used to treat, for example, diarrhea and various infections. In our diet, tannins are primarily found in, for example, walnuts and other nuts, peas, beans, pomegranate (juice), cranberries and tea.
In addition to being known as powerful antioxidants, tannins and their building blocks are also valued for their antibacterial, anti-inflammatory, prebiotic, and astringent properties. While this article attempts to describe some of the properties of tannins individually, it is clear that these properties are closely intertwined when they work in the body.
Anti-bacterial
Tannins can inhibit several well-known harmful bacteria such as Clostridium difficile (Clostridioides difficile) which can cause severe watery diarrhea, which most often affects children and the elderly, but also inhibit Clostridium perfringens, which can cause bloody diarrhea. There is also documentation that tannins inhibit Heliobacter pylori, which can cause stomach ulcers, and Staphylococcus aureus, which can infect wounds or cause inflammation in our mucous membranes.
Most bacteria that we live with peacefully get their energy by fermenting starch and sugar into lactic acid and other acids. But some bacterial species prefer to get their energy by fermenting protein – or rather – amino acids, which are the building blocks of protein.
When bacteria break down amino acids, they can only take a small bite of the amino acid, after which they excrete the rest as waste products. The partial breakdown is completely similar to the bacteria that need sugar to get their energy, but in the case of the sugar-fermenting bacteria, we value the waste products in the form of, for example, lactic acid. The waste products from the protein-fermenting bacteria, on the other hand, are capable of creating inflammation in our intestines. These are waste products such as ammonia (NH3), hydrogen sulfide (H2S), cadaverine, putrescine, skatole, acetone and trimethylamine (TMA), among others.
Although the protein-fermenting bacteria are not necessarily actual disease-causing bacteria themselves, their waste products, and especially ammonia, can create an environment where disease-causing bacteria thrive, while the growth of the lactic acid-producing bacteria is reduced.
The illustration shows how a bacteria breaks down the amino acid tryptophan, after which it excretes the rest: ammonia and skaltole.
One theory behind the inhibitory effect of tannins on amino acid fermenting bacteria is that the tannins bind to and inhibit the enzymes that the bacteria secrete to release amino acids from protein. This lowers their metabolism, numbers and their production of waste products so that the lactic acid-producing bacteria can once again dominate the environment in the gut.
Anti-inflammatory
When we talk about inflammation in everyday life, we usually think of infections by bacteria or viruses, but a large part of the inflammatory reactions in the body are purely chemical. Most of the chronic diseases that dominate in Western countries are linked to chronic inflammation in the body. These are diseases such as atherosclerosis and other cardiovascular diseases, type 2 diabetes, various forms of dementia plus of course the autoimmune diseases such as rheumatoid arthritis, psoriasis and gastrointestinal problems such as irritable bowel syndrome and/or colon.
Inflammation is a cascade of reactions of many different chemical substances and cells that clean up all kinds of “disorder” in the body’s cells and tissues. An inflammation should ideally end with some other substances and cells dampening the reaction, all the way down to normal. As the list of chronic diseases above shows, this dampening does not always occur to a sufficient extent. In this regard, diet can play an important role in the regulation of inflammation in cells and tissues, and many studies show that a diet with a higher content of tannins, etc. is associated with lower levels of inflammation in animals and humans.
Tannins and their degradation products such as gallic acid, ellagic acid and urolithins are interesting because they each have a well-documented anti-inflammatory effect in the intestine if/when they are absorbed into the body. Tannins inhibit several inflammatory processes in the body and in particular they inhibit the production of nuclear factor-κB (NF-κB). NF-κB mediates a cascade of pro-inflammatory cytokines from immune cells called macrophages in stage 1 (M1). When tannins inhibit the activation of NF-κB, it leads to reduced production of cytokines that cause inflammation such as tumor necrosis factor-α (TNF-α), interleukin-1β, (IL-1β) and interleukin-6 (IL6) so that the inflammatory cascade is attenuated.
There is increasing evidence that tannins also actively suppress inflammation more directly by promoting the transition of macrophages to an anti-inflammatory stage (M2), where the macrophages suppress and repair cell damage, and that tannins also stimulate the production of interleukin-10 (IL10), which also actively suppresses inflammation.
Antioxidant
The latest research in the field of aging indicates that the ability of cells to passivate free radicals decreases as our bodies age, and the body develops what is often referred to as oxidative stress. Oxidative stress therefore appears to be one of the main reasons why cells and tissues age. Conversely, a number of studies have shown that tannins in food, with their antioxidant effect, can support the body’s ability to passivate free radicals, thereby stopping or reducing oxidative stress.
Free radicals arise because electrons have a very strong tendency to exist as pairs, but in the cell oxygen compounds are constantly formed, where one electron is “alone”. These unpaired electrons will attract an electron from other molecules with great force. When the free radicals pull an electron from other molecules, they can damage, for example, enzymes, fatty acids in cell walls, DNA and other important molecules in the cell. Free radicals are typically oxygen compounds such as O2-, O-, H2O2, OH* or ONOO-, but can also be metal compounds such as iron’s ferrite ion Fe3-, where the number of electrons is not in balance.
Each of the OH groups shown in the tannin illustration below can release an electron and thereby act as antioxidants that can passivate the free radicals that can arise during digestion or metabolism in the cells.
Gallic acid on the left and ellagic acid on the right.The mitochondria of all our cells constantly produce lots of free radicals, but also enzymes and antioxidants that can neutralize them. In young healthy cells and healthy tissue, the balance between the free radicals and the cell’s ability to quickly passivate them is at an appropriate level.
The problem with free radicals is not that they exist, but that the balance between free radicals and antioxidants can become unbalanced, so that more free radicals are formed than the cells have the ability to passivate again. This can, as a kind of chain reaction, cause a kind of chemical inflammation in, for example, the intestines and in the body’s organs and tissues to become permanent, and tannins can help break this chain.
Prebiotic effect
It is difficult to point out some bacteria as “good” and some as “bad”, because the different species of bacteria have a very large variation. However, results from many scientific studies in animals and humans have shown that certain species – genera and/or entire families of bacteria are linked to a healthy gut, while others are linked to various diseases. Tannins have been shown in many studies to have a positive effect on the composition of the gut microbiota, and that they therefore function as prebiotics.
Prebiotics are defined as the part of our diet that we cannot digest ourselves, but which, for example, promotes the growth of beneficial bacteria in the intestine, thereby promoting our own health. The healthy effect may be partly due to some bacteria in the intestine actively fighting bacteria that cause inflammation in the intestine, but may also be due to the “good” bacteria producing substances that are absorbed from the intestine and that the body benefits from, or the “good” bacteria inhibiting the “bad” bacteria’s production of substances that have a negative effect on the body.
It has been known for many years that bacteria in the gut can make an important contribution to the body’s need for vitamin B12 and other B vitamins, and that Bifidobacteria produce butyric acid, which is important for the well-being of the colon tissue. More recently, it has been documented that a significant portion of the body’s serotonin is produced by gut bacteria, which thus contributes to one of the important hormones that strengthen our good mood, and there is increasing evidence that there are several substances produced in the gut that can affect both our mental state, immune system and physiology in a negative or positive direction.
Over the past 20 years, a number of articles have been published on the effect of hydrolyzable and condensed tannins on the gut microbiota, and also the effect of the substances that the bacteria produce when they break down the tannins. While tannins are large molecules that largely only act in the gut, the substances that the tannins break down into can be absorbed from the gut, and thus act throughout the body.
So far, only a few genera and species of bacteria have been discovered that can break down tannins, such as species within the Ellagibacter, Enterococcus, Gordonibacter and Streptococcus genera. In addition, there are a number of bacteria linked to the wider food chain. These are bacteria that thrive better because they can utilize the substances that tannins break down into. When we eat more tannins over a longer period of time, species within the Lactobacillus, Bifidobacterium, Akkermansia, Roseburia and Faecalibacterium genera are promoted, all of which are genera of bacteria that are linked to healthy intestinal function.
When tannins promote colonization in the intestine by Lactobacillus and Bifidobacter, it helps to ensure a slightly more stable, acidic environment in the intestine, which can inhibit a number of pathogenic bacteria. Bifidobacter can also produce the substance nicin, which directly inhibits several different pathogenic bacteria from the Clostridia genus.
Hydrolyzable tannins are broken down by bacteria to, among other things, ellagic acid, which is further broken down to various forms of urolithin. It is not often that one experiences researchers being directly enthusiastic about their research results, but those are the researchers who investigate the physiological effects of urolithins. One of the reasons is that it appears that urolithin both inhibits type 2 diabetes and the development of cancer, and at the same time protects and activates the energy production of mitochondria, thereby also strengthening our muscles.
Similarly, very positive effects have been seen with gallic acid, another breakdown product of tannins. A study on mice with artificially induced intestinal inflammation showed a very positive effect on intestinal health at a dosage of up to 50 mg/kg, which in humans would correspond to a dosage of 3 g for a 60 kg adult.
Astringent
Tannins are commonly known for their astringent properties, which give a tightening sensation in the mouth. The scientific explanation for this effect is still not entirely clear, but can probably be linked to a similar effect that tannins have in the intestine, where they quickly have an inhibitory effect on the transport of water across the intestine, thereby reducing symptoms of diarrhea.
Diarrhea can be caused by many different inflammatory reactions and infections, but what they have in common is that they usually cause fluid and salts from the body to flow into the intestine through the microscopic passage between the cells of the intestine – a passage called a tight junction.
As part of the gut’s response to inflammation in the gut, the gap between the cells becomes larger and loses some of its ability to filter which substances can pass from the body into the gut and vice versa from the gut into the body. Laboratory experiments have shown that tannins pull the cells closer together, and that they can thereby contribute significantly to stopping this uncontrolled transport. Animal experiments have shown that rabbits, pigs, cows and chickens have less diarrhea when tannins are added to the feed – and something similar applies to humans.
The illustration shows four intestinal cells, where the green arrow shows a healthy reaction, where only small molecules of up to about 1 nm (nanometer) can pass. The blue arrow shows an inflamed cell, where larger molecules of up to 10 nm can pass. On the right, the red arrow at an intestinal cell that is undergoing degradation, where everything can pass. Adapted from Zuo, Kuo and Turner, 2020.
The hydrolysable tannins can be broken down in the intestine to the phenols they are made of, and it is therefore both the tannins and their breakdown products that can help change the function of the intestine as a barrier in a healthier direction. Ellagic acid is partially broken down to urolithin A and B, which, like gallic acid, are easily absorbed from the intestine, and thus they can also affect the cells throughout the body with their astringent effect.
In traditional herbal medicine, the astringent effect of tannin extracts is used, among other things, to treat hemorrhoids and to stop bleeding.
Eat more greens
For the past 70 years or so, plant breeders have fairly systematically selected plant varieties with lower and lower levels of most types of plant substances such as alkaloids, flavonoids, saponins, tannins and essential oils, as they have been perceived as anti-nutrients. However, when plants are attacked by various enemies, they increase their production of these substances, which means that the plants produce them with an important purpose, which is to fight bacteria, viruses and other enemies.
When we eat plant substances such as tannins, essential oils, etc., the intestinal contents become more complex, and therefore also the intestinal microbiota, and harmful bacteria find it more difficult to dominate the intestinal environment.
Most of the diseases that characterize old age in the Western world are caused by oxidative stress and chronic inflammation, both of which can be reduced with higher levels of plant substances in our diet.
So – eat more greens and use more spices!
Klaus Sall
Biologist, Cand. Scient.
References and further reading:
Centonze, M. et al. 2025. The Antiaging Potential of Dietary Plant-Based Polyphenols: A Review on Their Role in Cellular Senescence Modulation. Nutrients, 17(10), p. 1716. Available at:
https://doi.org/10.3390/
Cosme, F. et al. 2025. A Comprehensive Review of Bioactive Tannins in Foods and Beverages: Functional Properties, Health Benefits, and Sensory Qualities. Molecules, 30(4), p. 800. Available at: https:/
https://doi.org/10.3390/
He, Q. et al. 2023. Tannins amount determines whether tannase-containing bacteria are probiotic or pathogenic in IBD. Life Science Alliance, 6(5). Available at:
https://doi.org/10.26508/lsa.
Molino, S. et al. 2025. Improving Irritable Bowel Syndrome (IBS) Symptoms and Quality of Life with Quebracho and Chestnut Tannin-Based Supplementation: A Single-Centre, Randomised, Double-Blind, Placebo-Controlled Clinical Trial. Nutrients, 17(3), p. 552. Available at:
https://doi.org/10.3390/
Ozogul, Y. et al. 2025. Tannins for food preservation and human health: A review of current knowledge. Applied Food Research, 5(1), p. 100738. Available at:
https://doi.org/10.1016/j.
Raya-Morquecho, E.M. et al. 2025. Ellagitannins and Their Derivatives: A Review on the Metabolization, Absorption, and Some Benefits Related to Intestinal Health. Microbiology Research, 16(6), p. 113. Available at:
https://doi.org/10.3390/
Wang, Y.-H. et al. 2025. The improvement effect of ellagic acid and urolithins on metabolic diseases: Pharmacology and mechanism. Food & Medicine Homology. Available at:
https://doi.org/10.26599/FMH.
Om antioxidanter og frie radikaler:
Sikder, M.M. et al. 2025. Reactive Oxygen Species: Role in Pathophysiology, and Mechanism of Endogenous and Dietary Antioxidants during Oxidative Stress. Chonnam Medical Journal, 61(1), p. 32. Available at:
https://doi.org/10.4068/cmj.
Note
The drawings we make for organic molecules that living organisms build, such as sugars, tannins, and other molecules, are very simplified. The two drawings on the right show the same molecule, one showing all atoms, while the other only the essential ones. Here are the rules for understanding these “drawings” of molecules:
- Every time a line changes direction or ends in “nothing”, it is because there is a carbon C
- Each carbon C always bonds with 4 bonds.
- Hvis der ikke er vist 4 bindinger ved et C, er alle de bindinger, der ikke er vist, en binding til brint H.
Plant substances – an overview
Alkaloids
Alkaloids are quite diverse, often small basic molecules that always contain one or more nitrogen atoms. They are found, for example, in stimulants such as caffeine in coffee, nicotine in tobacco, quinine in tonic water and several of the flavorings in chili and bell peppers, but also solanine found in green potatoes. In the field of medicines, morphine is known, and in the field of poisons, strychnine.
Allicins
Allicins are substances with one or more sulfur in the carbon chain, and they are best known from onions and garlic. Like many other of these plant substances, they are very biologically active and the cells therefore store them in small “sacs”, from which they are released if the cell is attacked, while at the same time they are converted into even more reactive molecules.
Flavonoids
Flavonoids are a broad group of phenols built on approximately the same structure as seen below. They are antioxidants, anti-inflammatory and anticarcinogenic. A well-known flavonoid is resveratrol which is found in blue grapes and they are also found in abundance in berries such as elderberries, blackcurrants, beets and green tea. They often give fruits and vegetables their characteristic colors.
Glycosinolates
Glycosinolates give cabbage its characteristic aroma and taste. They consist of a sugar molecule and a sulfur group. They are known for their anti-carcinogenic and anti-inflammatory effects. They are most commonly found in cabbages such as broccoli and kale.
Saponins
Saponins are molecules that can foam when shaken in water. They are made up of a group of 2-5 sugar molecules attached to a group that is either a tri-terpene or a steroid molecule. Saponins protect plants by, for example, destroying the cell membrane of fungi. Saponins are often found in small amounts in the roots and green leaves and stems of plants and are best known from ginseng, aloe vera, but are also found in, for example, green tomatoes and potatoes.
Tannins
Tannins are also called tannic acids or polyphenols, and they are probably the most common plant substances. Tannins are strong antioxidants, they are also anti-inflammatory and have a very broad effect against viruses, bacteria, fungi and nematodes. Tannins are divided into condensed tannins and hydrolysable tannins. Hydrolysable tannins are built around a central sugar molecule, with the small phenol groups bound by ester bonds. The hydrolysable tannins are found especially in pomegranate (juice), walnuts and in red wine, which has been aged in barrels of oak or chestnut wood. Condensed tannins are mainly composed of many flavonoids. The condensed tannins are found especially in peas, beans, black and green tea.
Terpenes / essential oils
Terpenes are often called essential oils and are a term for many quite different small molecules that have in common that they evaporate easily (fragrance) and are soluble in alcohol and oil without themselves being oils or fatty acids.
Terpenes are known from the scent of, for example, dill, lavender, mint, oregano, peppermint, rosemary, orange, cinnamon and cloves, but can also be purchased as pure oils or extracts such as cinnamon oil, tea tree oil or clove oil (eugenol).
