Secondary plant substances - phytochemicals

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Conclusion:

A varied diet with fresh, unprocessed foods ensures the intake of important health-promoting secondary plant substances. It is striking how numerous secondary plant substances are that at least have an anti-cancer effect, i.e. are anti-carcinogenic or anti-tumor.

In this detailed blog post, we explain the diverse mechanisms of action of secondary plant substances in detail; we also discuss aspects of the history of research and chemical peculiarities. A shorter overview with practical tips for a healthy everyday life is provided in the article Overview: Secondary plant substances - phytochemicals .

Summary

Secondary plant substances (SP) are found in all plants. They give our food color, aroma and taste. As current studies show, a varied and balanced diet with lots of different colored plant foods is the best way to support health. Secondary plant substances have a positive effect on numerous and very different aspects of the human organism. They play an essential role, especially in the defense against diseases and for the microbiome in the human intestine.

The health value of a food ingredient is usually based on the combination of different SPs that work together. If you want to exploit the full potential of secondary plant substances, rely on the greatest possible variety of vegetables, fruits, nuts, seeds, grains and legumes as well as medicinal and aromatic plants.

We should eat plant-based foods as raw and unprocessed as possible, or at least prepare them in a gentle way. Industry and relevant public institutions provide little or no information about a natural diet, perhaps because economic interests are too much in the foreground. As a result, people remain inadequately informed by public bodies. In our 600 or so food descriptions (ingredients for the recipes), we therefore mention the effects of SP wherever possible.

You can find these 600 or so foods directly by searching on each page or as a link in the recipes. Here you can see the active ingredients discussed ( purple ) and foods ( green ) highlighted.

Basics

Plants produce primary and secondary metabolic products. They are created through various biochemical syntheses of primary and secondary metabolism. This is where their name comes from. ^2,4

Primary plant substances, such as carbohydrates, proteins and fats as well as micronutrients, etc. serve the basic processes (growth, development) and are essential for energy metabolism. ^3,4

Secondary plant substances serve to interact with the environment and give the plant a decisive advantage in survival. Secondary substances benefit plants and us in various ways by fulfilling certain ecological functions. Environmental factors such as light, temperature, other living organisms and nutrient availability influence their synthesis by plants.

However, the world of SP is complex: Due to the diversity of secondary plant substances, the classification in the scientific literature does not follow a uniform scheme. Recent research shows that SP are multifunctional; the boundaries between primary and secondary metabolism are fluid and intertwined. Certain SP (or groups of plant substances) or their combinations are often characteristic of individual plant species. ^2,3,4,14

In international usage, SP are also referred to as phytochemicals, phytonutrients or plant secondary metabolites. ^2,4

Functional diversity for plants

Due to their antibiotic, antifungal and antiviral effects, SPs play a crucial role in the interaction of plants with their environment as a defense mechanism against pathogens. Thanks to their physiological properties, secondary metabolites protect plants from external influences such as pathogenic microorganisms. ^1,2,4

Other SPs deter herbivorous insects through an unpleasant smell, repulsive taste or toxic effects. In addition, secondary plant substances act as scents to attract animals such as insects and birds for pollination or seed dispersal. Some secondary metabolites protect plants from UV radiation, oxygen radicals and serve to regulate growth. Secondary plant substances are essential for the survival of a species. ^1,2,4,6,14

Effects and benefits for humans

Naturopathy has been using the effectiveness of secondary plant substances since early human history, specifically in spices, extracts, medicines, incense and dyes, and food. ⁵ Modern science now uses various highly developed, sensitive analytical methods to identify the bioactive substances in secondary plant substances and research their diverse effects on humans. Researchers have so far identified around 100,000 different substances from plant secondary substances, although the number of plants analyzed to date is still relatively small. ^5,11

The spectrum of effects of SP in the human organism is diverse and includes anti-carcinogenic (anti-tumor), antimicrobial, antioxidant, antithrombotic, immunomodulatory, anti-inflammatory, blood pressure regulating, cholesterol-lowering and digestive effects. Evidence for these effects comes from various observational studies in in vitro tests and animal experiments. ^4,10,11

Current studies underline that, in addition to essential nutrients and fiber, secondary plant substances influence numerous processes in the human metabolism and thus make a significant contribution to health and prevent various diseases. The number of secondary plant substances that contain anti-cancer agents is strikingly high. Some secondary plant substances also have a positive effect on the human intestinal microflora because they selectively promote the growth of certain bacteria. This is why they are also called "probiotics". ^5,10,11

Bioavailability and synergistic effects of secondary plant substances

Bioavailability indicates the amount of an ingested substance that is available in the body. The bioavailability of individual SPs is very complex and depends, among other things, on the method of preparation. It is therefore important to consciously select plant-based foods according to their degree of ripeness, origin (wild forms, old varieties), degree of processing, regionality and seasonality. ^6,7

When scientifically researching the health effects of food, the focus is often on individual ingredients. This is often driven by the commercial interests of the pharmaceutical and nutraceutical industries. Plant-based foods usually contain a combination of secondary plant substances and nutrients and are proven to be beneficial to health in this natural matrix.

The interaction of the various components of a food is crucial for a positive health effect, as a large number of biochemical processes also take place simultaneously in the human body. Scientific studies show, for example, that the simultaneous intake of quercetin (found in capers, onions, berries and chives, among other things) together with vitamin C from citrus fruits increases the antioxidant activity in the body. The combination of curcumin (turmeric) with piperine (black pepper) has been proven to increase the bioavailability of curcumin. Together, these substances increase the anti-inflammatory properties and protection against chronic diseases. ²⁸

Secondary plant substances work through the amounts consumed through normal diet. Taking a substance alone in the form of supplements is problematic, as too high a quantity can have a negative effect. ²⁸

General characteristics of secondary plant substances

Secondary plant substances occur specifically in certain taxonomic plant groups in different plant parts such as seeds, flowers, fruits, leaves, stems, bark, rhizomes and roots in rather small amounts. Occurrence and composition vary depending on the plant species, stage of development and environmental conditions. They often show considerable diversity and manifest themselves in a wide range of closely related structures. Compounds with similar structures or those with similar construction principles often have different biological functions. ⁴

Thus, SPs are extremely diverse in terms of their chemical properties. Exact classification therefore requires precise knowledge of often complex, interwoven biochemical biosynthesis pathways.

Glycosides (glycosides) occur in various groups of SP and protect plants from environmental influences. They are structurally diverse and, given their proven bioactivity, important for nutrition and medicine. All glycosides are made up of two parts: an aglycone unit (genin), which is mainly lipophilic, and a sugar component. Natural glycosides occur as saponosides, phenolic compounds, flavonoids, alkaloids, coumarins, anthraquinones, glucosinolates, cyanogen and mustard oil glycosides, among others. ⁴

Main substance groups

Research has so far defined over 80,000 structures of secondary plant substances from higher plants alone. Due to the large number and the diverse structural variations, there are different classification options in the literature, which are based either on physiological or chemical properties.

Taking into account scientific sources ^4,10,15 we summarize SP into the following main substance groups:

• Isoprenoids: monoterpenes and -terpenoids (essential oils, iridoids), sesquiterpenes and -terpenoids (germacranolides, guajanolides), diterpenes and -terpenoids (ginkgolides), triterpenes and -terpenoids (steroids, saponins), tetraterpenes and -terpenoids ( carotenoids )

• Alkaloids: Pyridines, Tropanes, Pyrrolizidines, Indoles, Quinolines, Phenethylamine Alkaloids, Phenyl Alkaloids

• Polyphenols: phenolic acids (hydroxybenzoic and hydroxycinnamic acids), flavonoids (flavonols, flavanols, anthocyanins, flavones, flavanones, flavanonols and isoflavones), coumestans, catechols, lignans, stilbenes, tannins, xanthones and glycosides

• Organic sulphur-containing compounds: including mustard oil glycosides, sulphides

• Other nitrogen-containing compounds: biogenic amines, betalains, cyanogenic glycosides

• Other organic compounds: phenylpropanoids, coumarins, lactones (phthalides), quinones, aldehydes, alcohols, esters, ketones, alkanes, hydroxycarboxylic acids

• Protease inhibitors: phytic acid, chlorophyll, lectins

Our article portrays the most important representatives of the main groups of substances for human nutrition, their occurrence in plant foods including their health-promoting effects.

Classification of secondary plant substances

Isoprenoids

Isoprenoids are a large and extremely diverse group of natural products of secondary plant substances with important properties for medicine, nutrition and industry. They are divided into Monoterpenes and -terpenoids ( essential oils, iridoids ), sesquiterpenes and -terpenoids ( germacranolides, guajanolides ), diterpenes and -terpenoids ( ginkgolides ), triterpenes and -terpenoids ( steroids, saponins ), tetraterpenes and -terpenoids ( carotenoids ).

Numerous in vitro and in vivo studies show positive effects on the heart and circulation (cardiac steroids), an anti-inflammatory effect of sesqui- and triterpenes and terpenoids, and a digestive effect due to bitter substances in mono-, di-, triterpenes and terpenoids. Isoprenoids are the starting material for many natural plant and animal substances. ⁴

The biological functions of isoprenoids are extremely diverse. For example, they serve as color pigments, just as certain carotenoids are essential for the photosynthesis process of plants. Gibberellins, on the other hand, function as hormones, while other isoprenoids act as defense substances, components of membranes, components of signal transduction networks or as light protection substances. ^4,5

Terpenoids exist as a modified class of terpenes with oxygenated hydrocarbons. They have oxidized methyl groups and other functional groups at various positions. Examples of terpenoids are carvacrol (oregano) , linalyl acetate (lavender oil) and diosgenin (yams, fenugreek). Terpenoids have anticarcinogenic, antiallergic, antibacterial and antioxidant effects in the human organism. ⁵

Iridoids are monoterpenoid bitter substances that are mainly found in herbal drugs. Apart from the bitter substance effect in the gastrointestinal tract, they show antioxidant and hepatoprotective properties. The bitter substances of the yellow gentian ( Gentiana lutea ) have a stimulating effect on the T-lymphocytes and the immune system of the intestine. Iridoids are found in valerian root, verbena, motherwort, mullein flowers, chasteberry fruits, devil's claw root and Woodruff herb . ¹⁷

Terpenes and terpenoids have a bitter taste and sticky, sometimes toxic properties that make them repulsive to many herbivores and protect the plant from damage. The sticky and toxic resins also seal plant wounds and prevent infections. Both terpenes and terpenoids can change aromatically and sensorially when exposed to light and oxygen, creating a tallowy, castor-like taste. ^2,4,5

They are found in many spices and medicinal plants, such as basil, mugwort, savory, birch leaves, tarragon, fennel seeds , lady's mantle, cloves, golden nettle, kaffir lime leaves, garlic, bay leaves, marjoram, lemon balm, oregano, parsley, peppermint (Moroccan mint, spearmint), rosemary, sage, chives, star anise and thyme .

They are also found in various fruits such as clementines, limes, olives, oranges , black currants, black elderberries, black pepper, juniper berries and nuts such as pine nuts, hemp nuts and walnuts .

Monoterpenes and -terpenoids

Monoterpenes and -terpenoids are often found in the various parts of coniferous plants, vegetables, fruit and herbs. Many essential oils are structurally monoterpenes and -terpenoids, such as pinene, myrcene, limonene, linalool, terpinene, carvacrol, carvone, eugenol, geraniol and thymol . As fragrances in plants, they attract pollinating insects or ward off predators. They are added to many foods as flavorings, fragrances and natural preservatives or they act as natural insecticides and fungicides. Many monoterpenes, including terpinene, borneol and citral, have antimicrobial effects; myrcene and linalool have anti-inflammatory effects, and nerol, terpinol and alpha-pinene have antioxidant effects. ^5,11

Limonene and carvone are currently intensively studied monoterpenes with anti-tumor properties against stomach, breast and lung cancer. Limonene is the main component of citrus oil and caraway oil . It is also found in numerous plants, e.g. valerian, basil, bitter orange, dill , spruce, ginger, hemp, cardamom, pine, garlic, coriander, bay leaf, mint, nutmeg, parsley, rosemary, celery, fir and juniper . ⁵

Sesquiterpenes and terpenoids

These groups of substances are mainly found in plants in essential oils, where they perform important biological functions: for example, abscisic acid ( birch, maple, mosses ) acts as a phytohormone, rishitin ( tomato, potato ) as phytoalexin, farnesol ( star anise ) as a pheromone, indole ( jasmine ) as a plant sex attractant and cnicin ( blessed maidenhair ) as a bitter substance. Individual representatives are, for example, farnesol, nerolidol ( neroli oil ), caryophyllene ( clove oil ), cedrol and santalol . Important representatives of the strongly bitter sesquiterpene lactones are germacranolides, eudesmanolides, guajanolides and pseudoguajanolides, which are found, for example, in artichokes, wormwood, yarrow, gentian root, centaury, dandelion and Lovage occurs. ¹⁷

Diterpenes and terpenoids

These include forskolin, ginkgolides and retinal (in vitamin A). Forskolin (from the harp tree) is involved in a number of functions in the body. It helps to improve blood circulation and strengthens heart function, among other things. The health effects of ginkgo are mainly based on the ginkgolide content; ginkgolide B in particular increases blood circulation in the brain and can also protect against allergies. ⁴

Cafestol and kahweol are natural diterpenes extracted from coffee beans and are mainly found in unfiltered coffee in the form of fatty esters. Some studies confirm that coffee diterpenes, particularly cafestol, can effectively increase human blood lipid and low-density lipoprotein (LDL) cholesterol levels. This poses a potential risk for the development of some cardiovascular diseases. Other studies show that moderate coffee consumption (3 to 5 cups/day) reduces the risk of cardiovascular disease. ⁹ Studies are often designed to sell a particular food product more often. A well-known example in the past is the advertising of tobacco products, which downplayed the health risks of smoking.

Triterpenes and terpenoids

Limonoids in citrus fruits and other triterpenes and terpenoids have anti-carcinogenic, antiviral, antibacterial and fungicidal effects. A well-known triterpene is squalene, which serves as a precursor for the production of steroids. The substance is a component of plant and fish oils and is found in olive and wheat germ oil, among other things. Squalene is an antioxidant and also has antibacterial, fungicidal and antitumor properties. It serves as a precursor for the formation of steroids. ⁴

Steroids are tetracyclic triterpenes. Sterols, such as cucurbitacins, are derived from steroids. One important sterol is cholesterol, which is important as a building block for biomembranes. Cholesterol is a zoosterol and comes from animal sources. ⁴ Plant sterols (phytosterols) have a similar chemical structure, and around 200 of these have been identified so far. Some well-known examples of phytosterols (phytosterins) arebeta-sitosterol (β-sitosterol ) in canola , pistachios, pine nuts, almonds, soy and avocado ; Campesterol (campesterine) in shiitake and morels as well as stigmasterol (stigmasterine) in dark chocolate , linseed oil, peanut and edamame .

Phytosterols are found in small amounts in fruits and vegetables and are mainly found in seeds, nuts and vegetable oils. Margarine is usually enriched with phytosterols. Phytosterols in hypoxis root ( Hypoxis rooperi ), saw palmetto fruit ( Serenoa repens ) and nettle root ( Urtica dioica ) are used as herbal drugs. ^4,17,27 Phytosterols such as beta-sitosterol have a cholesterol-lowering effect when taken daily by reducing the absorption of cholesterol in the small intestine by up to 15%. Other known effects include antibacterial, antifungal, anti-inflammatory, anticongestive and antitumor properties. ¹⁷

Phytosterols are used to treat allergies, asthma, psoriasis, rheumatoid arthritis, chronic fatigue syndrome, migraines and menstrual cramps, stomach and duodenal ulcers, neurological disorders, prostate problems and to improve hair growth. They also reduce the risk of heart attack and stroke. ²⁷ Epidemiological and experimental studies indicate a protective role of beta-sitosterol in the development of some types of cancer, including breast, colon and prostate cancer. Beta-sitosterol also has an antioxidant effect and reduces the risk of arteriosclerosis. ^11,17

Cucurbitacins are triterpene derivatives derived from lanosterol, a basic substance of sterols, and are found in large numbers in pumpkin plants . They taste bitter and have a local irritating effect on the skin and mucous membranes. They have cytotoxic, carcinogenic, antimicrobial, antifungal and antiparasitic effects. Other secondary ingredients with a triterpenoid structure are: betulinic acid, which mostly comes from various birch species ( Betula ), and boswellic acids, which are found in Indian and African frankincense and have an anti-inflammatory effect. ¹⁷

Saponins are another well-known group of triterpenes with foam-forming properties. The foam-forming ability of saponins is created by the combination of a fat-soluble sapogenin and a water-soluble sugar part. Saponins have a bitter taste. Saponins are used as dammaranes, tirucallanes, etc. and oleananes . Due to their ability to inhibit division rates, DNA synthesis and growth of tumor cell types in the colon, they reduce the risk of developing colon cancer. They also have anti-inflammatory and cholesterol-lowering effects. Saponins of the ginsenosides ( Ginseng ) have immunomodulating, analgesic and sedative effects. ^10,11,17,27

Saponins are found in many different plant species in nutrient-rich tissues such as roots, tubers, leaves, flowers and seeds. They are found in various legumes ( edamame, peas, peanuts, green beans, chickpeas, lentils, alfalfa, mung beans, broad beans, soybeans ), in vegetables ( Aubergine, cassava or manioc, Chinese water chestnut, fennel, jackfruit, potatoes, garlic, chestnuts, asparagus , star fruit, tomatoes ), cereals ( oats ) and pseudocereals ( amaranth, quinoa ). ¹⁷

Due to their foaming effect, saponins are also used as additives in cosmetics and food production. In some herbs, the name refers to their foaming properties, e.g. soapwort, soap root, soap bark and soapberry . Saponins are also found in various medicinal plants, such as birch blossoms, fenugreek seeds, borage, epazote, lady's mantle, ginseng, ground ivy, golden nettle, garlic mustard, chestnuts, lemon balm, moringa, rosemary, sage, ribwort plantain, liquorice and chickweed . ¹⁷

Tetraterpenes and terpenoids

Carotenoids belong to the category of tetraterpenes and are a very extensive group of substances. So far, around 750 carotenoids have been identified. Of these, around 50 have a vitamin A effect. At least 18 different carotenoids have been identified in the human organism. Carotenoids are naturally occurring, fat-soluble, yellow, orange to red pigments. ¹¹

They consist of the two main groups of carotenes (pure hydrocarbons) and xanthophylls (oxygen-containing derivatives). Carotenoids are mainly found in the photosynthetically active tissues of plants and algae and are bound to chromoplasts, which determine the color of numerous flowers and fruits. The most important function of carotenoids in plants is the absorption and transfer of light energy to chlorophyll. Carotenoids are highly sensitive to the effects of oxygen, oxidizing substances and light. ^4,11

Carotenes give fruits and vegetables their yellow-orange or red color (α- and β-carotene), while xanthophylls ( astaxanthin, beta-cryptoxanthin) are found mainly in yellow and dark green foods. Carotenoids such as β-carotene (beta-carotene), lycopene (lycopene), lutein, zeaxanthin, β-cryptoxanthin (beta-cryptoxanthin) and α-carotene (alpha-carotene) are the most common. β-carotene is found in higher concentrations in carrots , sweet potatoes and spinach . Lycopene is characteristic of tomatoes (dried and raw) , guava, watermelon , papaya and grapefruit as well as some medicinal herbs such as rose hip, orange marigold, willowherb and porcini mushrooms - and is more effective as a radical scavenger than other carotenoids. Lutein and zeaxanthin are found in larger quantities in goji berries , dandelion, nettles, nasturtium, marigold, kale, radicchio, spinach, chard, red algae and garden cress . ¹¹

The concentration of carotenoids in plants depends heavily on the variety, season, degree of ripeness, growth, harvest and storage conditions, method of preparation and varies in the different parts of a plant. The outer leaves of cabbage contain over 100 times more lutein and β-carotene than the inner leaves. Carotenoids influence various processes of food intake, distribution, metabolism and excretion in the human organism. Epidemiological studies show that α-carotene, β-carotene, canthaxanthin, lutein and lycopene suppress the formation of cancer cells, i.e. have an anti-carcinogenic effect. ⁴

High concentrations of carotenoids in the blood reduce the risk of cardiovascular disease and damage to the retina. In addition, through their interaction with free radicals, they protect against cell damage that contributes to skin aging, support the immune system and inhibit arteriosclerosis (lowering lipids). β-carotene is converted into vitamin A in the cells of the small intestine wall, which plays an important role in the process of vision. β-carotene and most other carotenoids can be produced purely synthetically and are approved as colorants in food and pharmaceutical products. Fucoxanthin has been shown to have anti-cancer, anti-hypertensive, anti-inflammatory, radioprotective and anti-obesity effects. ^4,5,10,27

Carotenoids are found in various vegetables ( broccoli, chili, kale, kohlrabi, pumpkins, corn, horseradish tree, peppers, spinach, tomatoes, sweet potatoes, asparagus, savoy cabbage ), fruits ( apricots, apples, avocados, bananas, dates, durian, grapefruit, goji berries, guava, coconut, kumquat, limes, tangerines, mangos, melons, oranges, papaya, peach, watermelon, lemon, plum ), berries ( goji berries, sea buckthorn ), legumes ( chickpeas ), nuts ( peanuts, hemp, pumpkin seeds ), cereals ( kamut ), pseudocereals ( quinoa ), herbs ( fenugreek, safflower, nasturtium, garlic mustard, lupins, chives, parsley, willowherb, cow parsley ), spices ( fenugreek leaves, mace, saffron, mustard ) and medicinal plants ( arnica, dandelion, marigold, pansy ), oils (from hemp, olive, oil palm, rapeseed ) and algae ( arame algae, red algae, wakame, laminaria algae ) as well as honey . ²⁷

Main group of alkaloids

The group of alkaloids, with around 12,000 described substances, is the second largest group of plant secondary substances. Alkaloids have antiviral, anti-cancer and analgesic effects in therapeutic applications. Dividing this large class of substances into subgroups is difficult; the main aspects of the classification are biogenesis, structural relationship and botanical origin. ¹⁷

Alkaloids are nitrogen-containing derivatives of amino acids such as ornithine, arginine, lysine, phenylalanine, tyrosine and tryptophan . Most alkaloids contain one or more nitrogen rings as their basic structure. However, there are also a number of non-heterocyclic alkaloids, including the phenethylamine alkaloids, including mescaline, ephedrine, tyramine and adrenaline . ³⁸

Alkaloids with heterocyclic compounds are the pyridine alkaloids, including gentianine (gentian family) and arecoline (betel nut) and the tropane alkaloids such as nicotine, anabasine, nicotelline and anatalline (in the tobacco plant) and cocaine (cocaine bush) as well as atropine (belladonna). Pyrrolizidine alkaloids are found in coltsfoot and comfrey, which in large quantities have a damaging effect on the liver, lungs and kidneys. The indole alkaloids are found in ergot, a sac fungus that can attack cereals and is poisonous. ³⁸

Cinchonine, cinchonidine, quinine and quinidine come from the bark of the cinchona plant and are among the quinoline alkaloids . Isoquinoline alkaloids (including morphine, codeine, taxol, penicillin, salsolin) have antiviral, antifungal, anticarcinogenic, antioxidant, antispasmodic and enzyme-inhibiting effects. ³⁸

Imidazole alkaloids such as pilocarpine have a wide biological and pharmaceutical potential. The indole alkaloids (carbazoles) include different groups, depending on whether they are of terrestrial or marine origin. Important purine alkaloids are caffeine, theophylline and theobromine . Caffeine is found in over 60 plants such as coffee, tea, cocoa or Guarana . Theobromine is the most important alkaloid in coffee, tea and cocoa or chocolate. Piperidine alkaloids ( piperine ) are responsible for the hot taste of pepper .

The alkaloids that do not arise as products of amino acid metabolism and are therefore structurally related to other natural substances include the steroid alkaloids, which include, among others, the toxic solanine (potatoes).

Many plants that contain alkaloids are poisonous and have a bitter taste that protects them from predators. Coffee plants produce caffeine mainly as an insecticide to protect their seedlings from insects; for humans, it has a stimulating effect. Basic medical research is very interested in the molecular mechanism of action of alkaloids with the aim of developing new drugs. Alkaloids include many stimulants, hallucinogens, sedatives and stimulants that primarily act on certain functional centers of the central nervous system.

Plant families rich in alkaloids include the Fabaceae (broom), the Poppy family (poppy, celandine) and the Nightshade family (deadly nightshade, thorn apple, henbane). In phytotherapy, alkaloids are considered to be pure substances that can be dosed. Alkaloids such as taxol (Pacific yew), vinblastine (pink catharanthe) and colchicine (autumn crocus) are used in cancer therapy, for example.

Main group of polyphenols

The group of polyphenols is one of the most important groups of natural substances in the plant kingdom. Phenols are aromatic compounds that carry one or more free OH groups (oxygen and hydrogen) on an aromatic ring system. They are found almost everywhere in food and beverages of plant origin and often in considerable concentrations as bioactive substances such as colorants, flavorings and tannins. These protect the plants from predators, attract pollinating insects or filter UV-B radiation.

Polyphenols include phenolic acids (hydroxybenzoic and hydroxycinnamic acids), flavonoids, tannins, stilbenes, xanthones, glycosides and phytoestrogens (coumestans, lignans, isoflavones). ²⁰

Certain polyphenols such as quercetin (flavonol) are found in almost all plant products (fruits, vegetables, cereals, legumes, fruit juices, tea, wine, infusions, etc.), while others are specific to certain foods (flavanones in citrus fruits, isoflavones in soy, phloridzin in apples). Grapes, apples, pears, cherries and berries contain up to 200-300 mg/100g fresh weight of polyphenols. Oats (177 mg/100g), corn (115 mg/100g) and barley (49 mg/100g) are particularly rich in polyphenols. ²⁰

Phenolic compounds promote our health by reducing the risk of metabolic disorders, including type 2 diabetes, cardiovascular disease, rheumatoid arthritis and inflammatory bowel disease. They also have a range of anti-inflammatory, antioxidant, anti-cancer, anti-aging, antibacterial, anti-allergenic and antiviral properties. Phenolic compounds such as curcumin affect the cell cycle and inhibit the growth of malignant tumors, such as skin tumors. ^18,20

Phenolic acids

Phenolic acids are divided into hydroxybenzoic and hydroxycinnamic acids.

Common hydroxybenzoic acids include salicylic, protocatechin, vanillin and gallic acid . Various studies show the antioxidant and neurobiological effects of gallic acid. Gallic acid is found in strawberries, blueberries, blackberries, grapes, mangoes, plums, walnuts, cashews and hazelnuts, among others. Tea is also an important source and contains around 45 mg/100g of gallic acid. ^21,22 Protocatechin acid is found in brown rice, onions, plums, grapes, gooseberries and almonds, among others. Protocatechin acid has neuroprotective, anti-inflammatory, anti-osteoporotic, anti-tumor and antibacterial effects. ^10,26

Hydroxycinnamic acids include p-coumaric, caffeic, ferulic, chlorogenic, rosemary, chicory, cinnamic and sinapic acids . Caffeic acid is a very common phenolic acid and accounts for between 75% and 100% of the total hydroxycinnamic acid content of most fruits. Caffeic acid exhibits anti-inflammatory, anti-tumor and antioxidant properties. Mulberries, quince and blueberries are good sources of caffeic acid. ¹⁷

Ferulic acid acts as an antioxidant, protects against UV radiation and breast cancer, and is found in numerous fruits, vegetables, and herbs, including bananas, eggplants, citrus fruits, cabbage, and sage. Sinapic acid protects lysosomes, has a preventive effect against lung and prostate cancer, lowers blood sugar levels, and inhibits oxidative stress. Cholorgenic acid has a stimulating, antioxidant, and digestive effect. It is found in medicinal herbs such as arnica flowers, birch leaves, and elderflowers. ¹⁷

Many fruits contain phenolic acids in concentrations of 50-200 mg/100g, including blueberry (200-220 mg/100g), kiwi (60-100 mg/100g), plum (14-115 mg/100g), cherry (18-114 mg/100g), blackberry (8-27 mg/100g), apple (5-60 mg/100g), raspberry (6-10 mg/100g), currant (4-13 mg/100g), strawberry (2-9 mg/100g) and pear (1.5-60 mg/100g). ¹⁷ In cereal grains, the content varies: corn has 31 mg/100g while wheat, rice and oat flour contain about 7-9 mg/100g. ^17,19

Flavonoids

Flavonoids are divided into different classes such as flavonols, flavanols (catechins, proanthocyanids), anthocyanins, flavones, flavanones, flavanonols and isoflavones . Flavones and flavonols occur in plants in various forms, including as glycosides . ^17,12 Apigenin (flavone), kaempferol and quercetin (flavonols) are relatively common flavonoids. Numerous other compounds are derived from them. They are generally present in relatively low concentrations of 1.5–3 mg per 100 g fresh weight. ¹⁶

Flavonoids are found in all higher plants with a wide variety of chemical structures. Around 6,500 different flavonoids are known to date, which occur as yellow and red pigments in many plants and plant-based foods. In plants, flavonoids have a protective function against UV light radiation. ¹⁰

Flavonoids have anti-allergic, anti-inflammatory, antiviral, antimicrobial, antioxidant and anti-carcinogenic effects. Medicinal herbs containing flavonoids and some pure substances that are used as venous agents, cardiovascular agents, diuretics, spasmolytics, liver therapeutics and for gastrointestinal complaints are particularly beneficial for therapeutic purposes. Flavonoids are found in berries, citrus fruits, tea and cocoa . Onions and kale are particularly rich in flavonols. In terms of drinks, red wine and black tea are particularly rich in flavonoids, as are juices. Medicinal herbs that contain large amounts of flavonoids include birch leaves, goldenrod, elderflower, Roman chamomile, linden blossom, broom, milk thistle, hawthorn leaves, marigolds and chamomile flowers . ¹⁷

Flavonols are abundant in onions (35-120 mg/100g), kale (30-60 mg/100g), leeks (3-22.5 mg/100g), tomatoes (1.5-20 mg/100g), broccoli (4-10 mg/100g) and blueberries (3-16 mg/100g). ¹⁷ In general, the flavonoid content in processed foods is only about half as high as in fresh, unprocessed foods due to leaching of the substances. ^10,12,17

Flavanols occur as catechins and proanthocyanidins, especially in abundance in chocolate (46-60 mg/100g), beans (35-55 mg/100g), apricots (10-25 mg/100g), cherries (5-22 mg/100g) and green tea (10-80 mg/100ml). An infusion of green tea contains up to 200 mg of catechins (epicatechin), while black tea provides more proanthocyanidins. The tea epicatechins are stable when exposed to heat as long as the pH value is in the acidic range of around five. Catechins show antibacterial, antiviral and antioxidant effects. ¹⁹

Proanthocyanidins (condensed tannins) are precursors to anthocyanins and are found in abundance in aronia berries . They are responsible for the purple, red or blue colors of flowers, fruits and leaves. They have antioxidant properties and have a positive effect on the intestinal flora. Proanthocyanidins from acacia plants ( Acacia angustissima ) have an intestinal regulating effect. ¹⁰

Anthocyanins are water-soluble plant pigments that color leaves, flowers and fruits pink, red, blue or purple. Anthocyanins, including cyanidin, pelargonidin, peonidin, delphinidin and malvidin , are found in eggplants (750 mg/100g), red grapes (30-750 mg/100g), blueberries (25-500 mg/100g), raisins (130-400 mg/100g), rhubarb (200 mg/100g), blackberries (100-400 mg/100g), strawberries (15-75 mg/100g) and red wine (20-35 mg/100ml). These values increase as the fruit ripens.

Anthocyanins are mainly found in the peel, with the exception of certain red fruits where they are also found in the flesh (cherries and strawberries). ^17,19 The fruit pigments known as anthocyanins have been shown to have antioxidant and anti-inflammatory effects by influencing various biochemical processes in the body. ^10,20

The most common flavones are luteolin and apigenin . Important edible sources of flavones are parsley (25-185 mg/100g) and celery (2-14 mg/100g), as well as oregano, rosemary, green olives, pumpkin, watermelon, bell pepper, honey, beans, chickpeas and peas . The peel of citrus fruits contains large amounts of the following flavones: tangeretin, nobiletin and sinensetin (up to 65 mg per 100 ml of essential oil). Flavones protect against free radicals, cardiovascular diseases and age-related diseases. ^17,27

Flavanones are found in tomatoes and certain aromatic plants such as mint, but in high concentrations only in citrus fruits. The most important are naringenin in grapefruit juice (10-65 mg/100ml), hesperetin in orange juice (21.5-68.5 mg/100ml) and eriodictyol in lemon juice (5-30 mg/100g). Orange juice also contains between 20 and 60 mg of hesperidin per 100 ml and 1.5–8.5 mg of narirutin per 100 ml; a single glass of orange juice can contain between 40 and 140 mg of flavanone glycosides. ²⁰ Since the solid parts of citrus fruits, especially the albedo (the white, spongy part) and the membranes that separate the segments, have a very high flavanone content, the whole fruit can contain up to five times as much flavanone as a glass of orange juice (40-140 mg/200ml). Flavanones protect against cardiovascular disease, inflammation and allergies. ^17,19,27

The flavanols include taxifolin, astilbin and angelin . Astilbin is found in St. John's wort ( Hypericum perforatum ) and grapes . Astilbin has immunomodulatory, anti-inflammatory, antioxidant, antimicrobial and liver-protective effects. ²⁴

Isoflavones are flavonoids that have a structural similarity to the body's own estrogens. Isoflavones, coumestans and lignans are also known as phytoestrogens . They are found in alfalfa, barley, corn, flax and oats, among other things. Phytoestrogens protect plants from damage caused by UV radiation. They influence the growth and stress levels of plants. They also have an effect of reducing fertility in grazing and flying predators. In the human body, phytoestrogens have a hormonal effect similar to the estradiol produced in humans, including the ability to bind to estrogen receptors. ²³

Although the estrogenic effect of isoflavones is much lower than that of the sex hormone estradiol, studies have shown that daily consumption of food rich in isoflavones has positive effects. Phytoestrogens are therefore considered to be potentially beneficial to human health. People who eat a diet based on soy-based foods have a lower incidence of tumors, such as breast and prostate cancer. Isoflavones can alleviate menopausal symptoms that occur during menopause, promote bone health and reduce the risk of cardiovascular disease in women. The daily recommendation is around 40-100 mg/day. ^4,8,17

Isoflavones, including genistein, daidzein, biochanin A and glycitein, are found mainly in legumes. Soy and its processed products are a good source of isoflavones. The fermentation carried out in the production of certain foods such as miso and tempeh leads to the formation of aglycones . The aglycones are very heat-resistant. Soy flour contains between 80 and 180 mg of isoflavones/100g - for comparison: cooked soybeans (20-90 mg/100g), miso (25-90 mg/100g), tofu (8-70 mg/100g), tempeh (43-53 mg/100g) and soy milk (3-17.5 mg/100ml). ¹⁹

Coumestans are substances similar to coumarins and are found as coumestrol especially in vegetable sprouts such as alfalfa, mung bean and soy sprouts, types of alfalfa, clover and cabbage such as Brussels sprouts . ²⁶

Lignans are particularly abundant in oilseeds. Flaxseeds are the richest food source of lignans, containing secoisolariciresinol (up to 37 mg/100g) and small amounts of matairesinol . They are also found in much smaller amounts in algae, legumes ( lentils ), cereals ( wheat ), sesame, sunflowers, peanuts, oily fruits, vegetables such as fennel, broccoli, onions and garlic, and fruits ( pears, plums ).

In food itself, lignans do not have an estrogen-like effect; they are only converted into enterolignans in the large intestine, which include enterolactone and enterodiol. Lignans improve the antioxidant status of both tissue systems and the entire body; they also prevent cancer by limiting cell proliferation through anticarcinogenic effects, improve the immune status of the individual and protect against infectious diseases and arthritis. ¹⁰ Lignans have an anabolic effect (build-up of the body's own substances) and reduce stress, e.g. syringin in the Siberian ginseng root ( Eleutherococcus senticosus ). ^10,17,19

Stilbenes are also phytoestrogens. They are found in small amounts in the human diet. Stilbenes include various compounds such as resveratrol, piceatannol, pinostilbene, batatasin III, oxyresveratrol and thunalbene . ¹⁰

Resveratrol is found in particular in muscat grapes, cranberries, red currants, Japanese knotweed and rhubarb . However, since resveratrol is found in small amounts in food, a protective effect of this molecule is unlikely with normal dietary intake. In recent years, resveratrol has attracted increasing interest as an antioxidant, antidiabetic, anti-obesity agent and intestinal flora improver. The substance supports the healing process of inflammation and is effective in the treatment of cardiovascular and Alzheimer's diseases. It has anti-cancer effects in various types of cancer such as pancreatic, colon, stomach, esophageal, liver, mouth and bile duct cancer. Piceatannol is found in white tea, blueberries, grapes and passion fruit. It is known as an anti-leukemic agent and has similar health effects to resveratrol. Studies show that the stability of piceatannol is greater compared to resveratrol. ^10,17,26

Tannins are divided into ellagitannins and condensed tannins. Ellagitannins release ellagic acid, which is metabolized in the intestine to urolithin. In vitro studies show an antimicrobial effect, in vivo the effects on the intestinal microbiota are still insufficiently researched. Studies show that punicalagins ( pomegranate ) suppress the growth of pathogenic bacteria and promote the growth of beneficial bacteria such as Enterococcus, Bifidobacterium spp. and Lactobacillus . Urolithin A also shows positive effects on the intestinal microbiota. ^10,17

Xanthones, including xanthone glycosides, xanthonolignoids and bisxanthones, are found primarily in the purple mangosteen fruit ( Garcinia mangostana ), but also in other plants. A mangosteen fruit contains more than 70 different xanthones and tastes sweet. Traditional Eastern medicine uses them in teas, ointments and tinctures to treat skin infections, urinary tract infections, dysentery, inflammation, abdominal pain, diarrhea and fever. Numerous studies have shown that mangosteen xanthones have antioxidant, anti-inflammatory, antibacterial, antifungal and especially anti-cancer activities. Studies have shown that α-mangostin and other smaller xanthones such as gartanin, β-mangostin, γ-mangostin, garcinone-C and garcinone-E affect proteins and cell signaling pathways and promote the death of cancer cells. Further studies are needed to draw reliable conclusions and evaluate the anti-cancer effect in humans. ³⁰

Main group organic sulphur-containing compounds

Mustard oil glycosides are sulfur and nitrogen-containing glycosides or glucosinolates that contribute significantly to the typical smell and taste of mustard, horseradish, cabbage, nasturtium, watercress and mustard seeds . The actual active ingredients are the enzymatic degradation products isothiocyanates, thiocyanates and indoles . Animal studies have shown anti-tumor effects against stomach, breast, liver and lung cancer. They also influence the metabolism of the body's own estrogens and thus protect against estrogen-related cancers such as breast and endometrial cancer. Glucosinolates also have antimicrobial effects and lower blood pressure and cholesterol.

Sulfides are sulphur-containing compounds in onion and leek plants - e.g. alliin in garlic, onions, chives, shallots, leeks and wild garlic - and in various brassicas. Enzymatic or thermal decomposition of the main active ingredients produces characteristic odours, typically in garlic. Ajoene is an unsaturated sulphur compound in garlic and onions. The substance has a positive effect on cholesterol levels, also works against parasites and viruses and is more effective against bacteria and fungi than allicin. Ajoene also inhibits the growth of cancer cells. ¹⁰

Sulfides have antimicrobial effects and are protective against various types of cancer, such as stomach cancer. They improve the taste of food and have a positive effect on digestion, the immune system and the formation of blood fats and cholesterol. ¹¹

Main group other nitrogen-containing compounds

Biogenic amines (BA) are nitrogen-containing organic compounds that are commonly found in foods, plants and animals, as well as in microorganisms. They are formed by the decarboxylation of amino acids. The formation of BAs depends on the availability of free amino acids and the presence of microbial activity. Although BAs are important for cellular function and metabolism, high concentrations are toxic and can cause symptoms such as diarrhea, food poisoning, and vomiting. They can also accelerate carcinogenesis. ³⁷

Like anthocyanins and carotenoids, betalains also give plants their colors - in flowers, roots, stems, leaves and fruits. The red-violet betacyanin pigments differ from the yellow betaxanthin pigments. Betalains are suitable as preservatives or additives in the food and cosmetics industry. In studies with animal models, they have shown antioxidant, anti-inflammatory, anti-tumor, lipid-lowering, antidiabetic, anti-obesity and liver-protecting effects. Betalains are found in prickly pears, dragon fruit, beetroot and yellow turnip, quinoa, ulluco tubers, Malabar spinach, Djulis grains ( Chenopodium formosanum or 'Taiwanese Red Quinoa') and the copao fruit. ²⁹

Cyanogenic glycosides serve as a defense against herbivores and release poisonous hydrogen cyanide after tissue damage. Natural cyanogenic glycosides include prunasin (cherry laurel) and sambunigrin (elderberries). A high content of cyanogenic glycosides can be toxic to humans and requires appropriate preparation methods to remove the toxic substances before consumption (for example in cassava). Plants with a moderate content of cyanogenic glycosides are almonds, whose kernels contain hydrogen cyanide ( amygdalin ). The use of amygdalin (also vitamin B ₁₇ ) in cancer therapy remains controversial due to a lack of scientific evidence. ³⁵

The structural and chemical relationship of many nitrogen-containing secondary plant substances is still unclear. This includes a number of non-proteinogenic amino acids.

Main group other organic compounds

These include many different substances such as phenylpropanoids, coumarins, lactones, quinones, aldehydes, alcohols, esters, ketones, alkanes (see also the Click For "Essential Oils") and hydroxycarboxylic acids.

Anethole ( anise oil ), cinnamaldehyde ( cinnamon bark ), eugenol ( cloves ), myristicin ( nutmeg ), safrole ( sassafras oil ) and vanillin ( vanilla ) belong to the phenylpropanoids . In studies, anethole shows an anti-cancer effect. Myristicin has anti-proliferation and anti-inflammatory properties. Safrole has antidiabetic, antimicrobial, analgesic and antifungal activities. Vanillin shows antimicrobial, antioxidant and anticarcinogenic effects. ⁴

Coumarins have antibacterial, neuroprotective, anti-edematous, anti-tumor, anti-inflammatory, circulation-promoting, immune-stimulating and sedative effects. Coumarin is a white, crystalline powder with a hay-like, sweet scent with a nutty undertone. It is found as a fragrance component in perfumes, soaps and detergents. Various coumarin derivatives, including dihydrofuranocoumarin, furanocoumarin, pyranocoumarin, biscoumarin and phenylcoumarin are natural types of coumarin. Furanocoumarins are present in tonka beans, angelica, yarrow, lovage, garden rue, burnet root, buckwheat, masterwort and sweet clover, among others. ^17,26

Hydroxycarboxylic acids include malic, quinic, tartaric, citric, mandelic and lactic acid . They have an astringent effect and are very acidic. As a result, they promote secretion in the gastrointestinal tract. ^17,19

Main group of protease inhibitors

Protease inhibitors consist of either protein molecules or non-proteins and are found in various plant organs and cell compartments. PIs influence plant growth, development and responses to the environment. They play a role in seed germination, seedling growth, protection against herbivores, cell homeostasis and in the regulation of protein and nitrogen turnover. ³⁴

Phytic acid occurs as an important phosphorus reservoir in the outer layers of whole grains (such as corn, wheat, barley and rye), pulses (such as soy) and oilseeds (such as pumpkin seeds). It primarily inhibits the bioavailability of minerals. Since it also influences the activity of protein-splitting enzymes and thus the digestive process, phytic acid is also counted as a protease inhibitor - depending on the classification criteria. In plant-based foods, it is present in the form of phytin, a calcium-magnesium salt. If consumed in large quantities, phytin can impair the absorption of minerals in the human body through poorly soluble bonds. Iron, zinc, calcium, copper and magnesium are particularly affected. Soaking, grinding, cooking, germination, fermentation and extensive chewing of the food reduce this negative effect. Recent research shows that phytin has a variety of positive health effects, such as antioxidant, antimicrobial, antidiabetic, neuroprotective, bone-protective, anti-inflammatory and anti-cancer properties. ^25,33,34

Chlorophyll is the most important pigment in plants, algae and certain bacteria. It absorbs light energy during photosynthesis and converts it into chemical energy. Chlorophyll is found in abundance in green fruits and vegetables and various salads; it is most commonly present as chlorophyll a. Other forms are chlorophyll b, chlorophyll c, chlorophyll d and chlorophyll e.

Some studies attribute antioxidant, anticarcinogenic (colon and liver cancer) and antimutagenic activities to chlorophyll. Chlorophyll binds hydrocarbons, aflatoxins and other hydrophobic molecules linked to cancer. The total chlorophyll content in green lettuce varieties is higher than in red varieties. ^31,32

Lectins are glycoproteins found in the seeds of many legumes (such as soybeans), in castor oil, mistletoe and in nettle root. Lectins can cause toxic reactions when consumed raw. Studies have shown that they have antimicrobial, antibacterial, antifungal and antiviral activities. Lectins from legumes have antimicrobial and antifungal properties against Candida albicans . Mistletoe lectins have an immunomodulatory effect by attaching themselves to the membrane of lymphocytes. Studies have shown that lectins from nettle root have a positive effect on prostaglandin metabolism. ^17,36

Memory:

Despite the complex topic, in practice the following applies: Prefer a varied, plant-based and seasonally oriented diet with organic foods that are as unprocessed as possible. This way you can benefit optimally from the many positive effects of secondary plant substances. Since secondary plant substances are often found in the outer layers, peel vegetables and fruit carefully. As a vegan or omnivore, read the article Vegans often eat unhealthily. Avoidable nutritional errors .

Many of our newly edited food descriptions provide you with specific information about which of the substances mentioned are most prominently present in the ingredient discussed.