Phenolics: Definition, Sources, Structure, Compounds, Main Classes and Incidence on Health

Phenol is widely used in pharmacy, medicine, and biology in its molecular form.

Phenol is also known as carbolic acid or phenolic acid.

In recent years, fruits and vegetables have received considerable interest depending on the type, number, and mode of action of different components, called ” phytochemicals, “for their presumed role in preventing various chronic diseases, including cancers and cardiovascular diseases.


Plants are rich sources of available dietary micronutrients, fibers, and phytochemicals, such as ascorbic acid, carotenoids, and phenolic compounds, which, individually or in combination, can be beneficial to health as they have demonstrated antioxidant activity.


Phenolics possess an aromatic ring bearing one or more hydroxyl groups. Their structures can vary from a simple phenolic molecule to a high molecular weight complex of a polymer.

They are generalized groups of substances in flowering plants, which occur in all vegetative organs: in flowers and fruits, roots and seeds.

Despite this structural diversity, groups of compounds are often referred to as “polyphenols.”


They are secondary metabolites derived from plants’ pentose phosphate and phenylpropanoid pathways.

The growth conditions, stage of maturity, and post-harvest practices determine the quantity and quality of polyphenols present in plant foods.

Phenolic compounds

Phenolic compounds can be classified into several types.

In addition to being responsible for the color (such as yellow, orange, red, and blue pigments), the flavor (such as vanillin and eugenol) of foods is one of the primary polyphenols.

One of the main characteristics is the radical scavenging capacity, which is involved in its antioxidant properties and the ability to interact with proteins.

The high antioxidant capacity makes polyphenols a critical factor in plants’ chemical defense against pathogens and predators and plant-plant interference.

Main classes of polyphenolic compounds

phenolic acids

Phenolic acids consist of two subgroups: hydroxybenzoic and hydroxycinnamic acids.

Hydroxybenzoic acids include gallic, p-hydroxybenzoic, protocatechuic, vanillic, and syringic acids, which have a C6-C1 structure.

On the other hand, Hydroxycinnamic acids are aromatic compounds with a three-carbon side chain (C6-C3), with caffeic, ferulic, p-coumaric, and synaptic acids.

The primary sources of phenolic acids are cranberry, pear, cherry (sweet), apple, orange, grapefruit, cherry juice, apple juice, lemon, peach, potato, lettuce, spinach, coffee beans, tea, and cider.

Hydroxycinnamic acids are a significant class within phenolic compounds.


Flavonoids are the most abundant polyphenols in human diets, accounting for more than half of the eight thousand natural phenolic compounds found in blackberries, black currant, blueberries, grapes, strawberries, cherries, plums, pomegranate, and raspberry.

They are low molecular weight compounds consisting of fifteen carbon atoms in a C6-C3-C6 structure.

They are mainly divided into two classes: (a) anthocyanins (a glycosylated derivative of anthocyanidin, present in colorful flowers and fruits), anthoxanthins (a group of colorless compounds divided into several categories, including flavonoids, flavones, flavonols, isoflavones, and glycosides).


Tannins are relatively high molecular weight compounds

found in complexes with alkaloids, polysaccharides, and proteins, they are a group of water-soluble polyphenols.

These can be subdivided into hydrolyzable and condensed tannins.

Hydrolyzable tannins are gallic acid esters, while condensed tannins, also known as proanthocyanidins, are polyhydroxyflavan-3-of polymers.

A third subdivision, the phlorotannins consisting entirely of phloroglucinol, has been isolated from several genera of brown algae.

They are found in the seed and skin of grapes, apple juice, strawberries, raspberries, pomegranate, walnuts, peaches, blackberries, olives, and plums.

Also, chickpea, black-eyed peas, lentils, beans, red/white wine, cocoa, chocolate, tea, cider, coffee, and immature fruits are tannins.


Stilbenes are structurally characterized by the presence of a 1,2-diphenylethylene nucleus with hydroxyl groups substituted in aromatic rings.

They exist in the form of monomers or oligomers. The best-known compound is resveratrol (3,5,4′-trihydroxy-trans-stilbene).

The primary dietary sources of stilbenes include grapes, wine, soybeans, peanuts, and peanut products.

Polyphenols in health

The possible health benefits of phenolic compounds in the diet depend on their absorption and metabolism, which in turn are determined by their structure, including their conjugation with other phenolic compounds, degree of glycation and acylation, molecular size, and solubility.

These steps occur at various points during the passage through the small intestine wall into the circulatory system and subsequent transport to the liver in the port vein.

Polyphenol metabolites are rapidly cleared from plasma. Therefore daily consumption of plant products is essential to supply high concentrations of metabolites in the blood.

However, it should be noted that the most abundant polyphenols in the daily diet are not necessarily the ones with the best bioavailability.

For example, hydroxycinnamic acids are found in high concentrations in food, but esterification decreases their intestinal absorption.

The bioavailability of phenolic compounds can also be affected by differences in cell wall structures, location of glycosides in cells, and binding of phenolic compounds within the food matrix.

Now epidemiological insights emphasize that polyphenols show essential functions, such as pathogen inhibition, microorganism decomposition, and triglyceride antideposition.

Likewise, they reduce the incidence of non-communicable diseases such as cardiovascular diseases, diabetes, cancer, and heart attack, with anti-inflammatory and antiallergic effects through processes that involve reactive oxygen species.

These protective effects are attributed, in part, to phenolic secondary metabolites.

Initially, it was thought that the protective effect of phenolic compounds in the diet was due to their antioxidant properties, which are the decrease in the levels of free radicals present in the body.

However, there is now emerging evidence that dietary phenolic metabolites, which appear in the circulatory system in nmol / L at low concentrations of mmol / L, exert modulatory effects on cells.

These effects are carried out through the selective actions of different components of the intracellular signaling cascades for cellular functions such as growth, proliferation, and apoptosis.

Polyphenols are believed to show their antioxidant capacity, depending on the hydroxylation state of their aromatic rings, including:

  • Elimination of free radicals.
  • Chelation and stabilization of divalent cations.
  • Modulation of endogenous antioxidant enzymes.

Phenolic acids, hydrolyzable tannins, and flavonoids have anticancer and antimutagenic effects as they act as protective agents for DNA against free radicals and activate carcinogens.

They also inhibit enzymes involved in pro-carcinogenic activation and activate xenobiotic detoxifying enzymes.

In particular, flavonoids and L-ascorbic acid have a synergistic protective effect against oxidative DNA damage in lymphocytes.

A diet rich in vegetables reduces the risk of colon cancer.

Chlorogenic and caffeic acids are antioxidants in vitro and are potent inhibitors for forming mutagenic and carcinogenic N-nitroso compounds in vitro.

Flavonoids, catechins, and derivatives are considered therapeutic agents in brain aging studies and serve as possible neuroprotective agents in progressive neurodegenerative disorders such as Parkinson’s and Alzheimer’s diseases.

High intakes of flavonoids lead to a decrease in the oxidation of low-density lipoproteins.

Resveratrol, trans-3,5,4′-trihydroxystilbene, is the best known molecule for promoting health; it is present in grapes and red wine and has been studied for its effects on genes and the heart, breasts, the prostate, the uterus, and the immune system.

In addition, recent studies show that resveratrol maintains healthy nerves and essential brain functions, including cognitive processes.

Tannins, commonly referred to as tannic acid, have been responsible for decreased feed intake, growth rate, feed efficiency, metabolizable energy, and protein digestibility in experimental animals.

Therefore, foods rich in tannins, such as betel nuts and herbal teas, are considered low nutritional value.

However, many researchers indicated that the main effect of tannins was not due to their inhibition of food consumption or digestion but instead to a decrease in the efficiency in converting absorbed nutrients to new substances in the body.

Tannins’ anticancer and antimutagenic potentials may be related to their antioxidant property, essential for protecting oxidative cell damage, including lipid peroxidation.

Tannins have also been reported to exert other physiological effects, such as accelerating blood clotting, lowering blood pressure, lowering serum lipid levels, causing liver necrosis, and modulating immunosupposabilities.

Polyphenols (phenolic acid, stilbenes, tannins, isoflavones, and green tea catechins) have been reported to inhibit the production and growth of many fungi, yeasts, viruses, bacteria, such as Salmonella, Clostridium, Bacillus or Chlamydia pneumonia, Vibrio cholera, Enterotoxigenic, E.coli.

This antimicrobial property of phenolic compounds can be used in food processing to increase the shelf life of certain foods, such as catfish fillets, as they serve as a natural defense mechanism against microbial infections.

In recent years there has been an increase in the food industry related to newer developments on the role of phenolic components as antioxidants, antimutagens on free radicals, and the prevention of pathologies such as cancer and cardiovascular heart disease.

Some epidemiological studies have indicated a negative correlation between the consumption of polyphenols with the diet and the risk of infectious, degenerative, and chronic diseases.

There is limited information on possible adverse effects suggesting that some phenolic compounds, when ingested at high concentrations, may play a role in genotoxicity, thyroid toxicity, pharmaceutical interaction, and estrogenic activity.

However, most researchers stated that the curative effects of polyphenols depend on the amount of consumption and their bioavailability.