Tag Archives: polyphenols

Green tea benefits for health

Green Tea Benefits
Fig. 1 – Green Tea Benefits

Tea drinking, particularly green tea, has always been associated, at least in East Asia cultures (mainly in China and Japan) with health benefits. Only recently, the scientific community has begun to study the health benefits of tea consumption, recognizing its preventive value in many diseases.

Green tea benefits in preventing cancer

Several epidemiological and laboratory studies have shown encouraging results with respect to possible preventive role of tea, particularly green tea and its catechins, a subgroup of flavonoids (the most abundant polyphenols in human diet) against the development of some cancers like:

  • oral and digestive tract cancers;
  • lung cancer among those who have never smoked, not among smokers.

Tea polyphenols, the most active of which is epigallocatechin-3-gallate (EGCG), seem to act not only as antioxidants, but also as molecules that, directly, may influence gene expression and diverse metabolic pathways.

Green tea and cardiovascular disease

Cardiovascular disease is the main cause of deaths worldwide, particularly in low- and middle-income countries, with an estimate of about 17 million deaths in 2008 that will increase up to 23.3 million by 2030.
Daily tea consumption, especially green tea, seems to be associated with a reduced risk of developing cardiovascular disease, such as hypertension and stroke.
Among the proposed mechanisms, the improved bioactivity of the endothelium-derived vasodilator nitric oxide (NO), due to the action of tea polyphenols that enhance nitric oxide synthesis, and/or decrease superoxide-mediated nitric oxide breakdown seem to be important.

Green tea and antioxidant properties

Tea polyphenols may act, in vitro, as free radical scavengers.
Since radical damage plays a pivotal role in the development of many diseases such as atherosclerosis, rheumatoid arthritis, cancer, or in ischemia-reoxygenation injury, tea polyphenols, particularly green tea catechins, may have a preventive role.

Green tea benefits in weight loss and weight maintenance

Green tea, but also oolong tea, that is, catechins and caffeine rich teas, has a potential thermogenic effect. This has made them a potential tool for:

  • weight loss, by increasing energy expenditure and fat oxidation;
  • weight maintenance, ensuring a high energy expenditure during the maintenance of weight loss.

Indeed, it has been shown that green tea and green tea extracts are not an aid in weight loss and weight maintenance, since:

  • they are not able to induce a significant weight loss in overweight and obese adults;
  • they are not helpful in the maintenance of weight loss.

Green tea and preventing dental decay

Animal and in vitro studies have shown that tea, and in particular its polyphenols, seems to possess:

  • antibacterial properties against pathogenic action of cariogenic bacteria, as Streptococcus mutans, particularly green tea EGCG;
  • inhibitory action on salivary and bacterial amylase (it seems that black tea thearubigins and theaflavins are more effective than green tea catechins);
  • it is able to inhibit acid production in the oral cavity./li>

All these properties make green tea and black tea, beverages with potential anticariogenic activity.

References

Arab L., Khan F., and Lam H. Tea consumption and cardiovascular disease risk. Am J Clin Nutr 2013;98:1651S-1659S doi:10.3945/ajcn.113.059345

Dwyer J.T. and Peterson J. Tea and flavonoids: where we are, where to go next. Am J Clin Nutr 2013;98:1611S-1618S doi:10.3945/ajcn.113.059584

Goenka P., Sarawgi A., Karun V., Nigam A.G., Dutta S., Marwah N. Camellia sinensis (Tea): implications and role in preventing dental decay. Phcog Rev 2013;7:152-6 doi:10.4103/0973-7847.120515

Grassi D., Desideri G., Di Giosia P., De Feo M., Fellini E., Cheli P., Ferri L., and Ferri C. Tea, flavonoids, and cardiovascular health: endothelial protection. Am J Clin Nutr 2013;98:1660S-1666S doi:10.3945/ajcn.113.058313

Hursel R. and Westerterp-Plantenga M.S. Catechin- and caffeine-rich teas for control of body weight in humans. Am J Clin Nutr 2013;98:1682S-1693S doi:10.3945/ajcn.113.058396

Hursel R., Viechtbauer W. and Westerterp-Plantenga M.S. The effects of green tea on weight loss and weight maintenance: a meta-analysis. Int J Obesity 2009;33:956-961 doi:10.1038/ijo.2009.135

Jurgens T.M., Whelan A.M., Killian L., Doucette S., Kirk S., Foy E. Green tea for weight loss and weight maintenance in overweight or obese adults. Editorial group: Cochrane Metabolic and Endocrine Disorders Group. 2012:12 Art. No.: CD008650 doi:10.1002/14651858.CD008650.pub2

Lagari V.S., Levis S. Phytoestrogens for menopausal bone loss and climacteric symptoms. J Steroid Biochem Mol Biol 2014;139:294-301 doi:10.1016/j.jsbmb.2012.12.002

Lambert J.D. Does tea prevent cancer? Evidence from laboratory and human intervention studies. Am J Clin Nutr 2013;98:1667S-1675S doi:10.3945/ajcn.113.059352

Lethaby A., Marjoribanks J., Kronenberg F., Roberts H., Eden J., Brown J. Phytoestrogens for menopausal vasomotor symptoms. Cochrane Database Syst Rev 2013:10;12 Art. No.: CD001395 doi:10.1002/14651858.CD001395.pub4

Levis S., Strickman-Stein N., Ganjei-Azar P., Xu P., Doerge D.R., Krischer J. Soy isoflavones in the prevention of menopausal bone loss and menopausal symptoms: a randomized, double-blind trial. Arch Intern Med 2011:8;171(15):1363-9 doi:10.1001/archinternmed.2011.330

Lorenz M. Cellular targets for the beneficial actions of tea polyphenols. Am J Clin Nutr 2013;98:1642S-1650S doi:10.3945/ajcn.113.058230

Sharma V.K., Bhattacharya A., Kumar A. and Sharma H.K. Health benefits of tea consumption. Trop J Pharm Res 2007;6(3):785-792.

Yang Y-C., Lu F-H., Wu J-S., Wu C-H., Chang C-J. The protective effect of habitual tea consumption on hypertension. Arch Intern Med 2004;164:1534-1540 doi:10.1001/archinte.164.14.1534

Yuan J-M. Cancer prevention by green tea: evidence from epidemiologic studies. Am J Clin Nutr 2013;98:1676S-1681S doi:10.3945/ajcn.113.058271


Flavonols: definition, structure, food sources

Flavonols are polyphenols belonging to the class of flavonoids.
They are colorless molecules that accumulate mainly in the outer and aerial tissues, therefore skin and leaves, of fruit and vegetables, since their biosynthesis is stimulated by light. They are virtually absent in the flesh.

They are the most common flavonoids in fruit and vegetables, where they are generally present in relatively low concentrations.
Due to their widespread in nature and human diet, they should be taken into consideration when the positive effect on health associated with fruit and vegetable consumption is examined. Their effect is probably related to their ability to:

  • act as antioxidants;
  • act as anti-inflammatory agents;
  • act as anticancer factors;
  • regulate different cellular signaling pathways; an example is the action of quercetin, the most widespread flavonols, on the oxidative stress-induced MAPK activities.

CONTENTS

Chemical structure of flavonols

Chemically, these molecules differ from many other flavonoids since they have a double bond between positions 2 and 3 and an oxygen (a ketone group) in position 4 of the C ring, like flavones from which, however, they differ in the presence of a hydroxyl group at the position 3. Therefore, flavonol skeleton is a 3-hydroxyflavone.

Basic Skeleton of Flavanols
Fig. 1 – 3-Hydroxyflavone

The 3-hydroxyl group can link a sugar, that is, it can be glycosylated.
Like many other flavonoids, most of them is found in fruit and vegetables, and in plant-derived foods, in glycosylated form. The sugar associated with flavonols is often glucose or rhamnose, but other sugars may also be involved, such as:

  • galactose;
  • arabinose;
  • xylose;
  • glucuronic acid.

Flavonol are mainly represented by glycosides of:

  • quercetin;
  • kaempferol;
  • myricetin;
  • isorhamnetin.
Flavonols
Fig. 2 – Flavonols

The most ubiquitous compounds are glycosylated derivatives of quercetin and kaempferol; in nature, these two molecules have respectively about 280 and 350 different glycosidic combinations.
Finally, it should be underlined that sugar moiety influences flavonol bioavailability.

Foods rich in flavonols

The major sources in human diet are:

  • fruit;
  • vegetables;
  • beverages such as red wine and tea.

In human diet, the richest source are capers, which contain up to 490 mg/100 g fresh weight (FW), but they are also abundant in onions, leeks, broccoli, curly kale, berries (e.g. blueberries), grapes and some herbs and spices, for example dill weed (Anethum graveolens). In these sources, their content ranges between 10 and 100 mg/100 g FW.
Even cocoa, tea, both green and black ones, and red wine are good sources of flavonols. In wine, together with other polyphenols such as catechins, proanthocyanidins and low molecular weight polyphenols, they contribute to the astringency of the beverage.

The main flavonols in foods

The main flavonols in foods, listed in decreasing order of abundance, are quercetin, kaempferol, myricetin and ishoramnetin.

Quercetin

The richest sources of quercetin are capers, followed by onions, asparagus, lettuce and berries; in many other fruit and vegetables, it is present in smaller amounts, between 0.1 and 5 mg/100 g FW.
This flavonol is also present in cocoa and it could be one of its main protective agents against LDL oxidation.
Together with isoflavones, quercetin glycosides are the most well-absorbed polyphenols, followed by flavanones and catechins (on the contrary, gallic acid derivatives of catechins are among the least well absorbed polyphenols, together with anthocyanins and proanthocyanidins).

Kaempferol

Typical dietary sources of kaempferol include vegetables, such as spinach, kale and endive, with concentrations between 0.1 and 27 mg/100 g FW, and some spices such as chives, fennel and tarragon, with concentrations between 6.5 and 19 mg/100 g FW.
Fruit is a poor source of the molecule, with content down to 0.1 mg/100 g FW.

Myricetin

Myricetin is the third most abundant flavonol. It is found in some spices, such as oregano, parsley, and fennel, with concentrations between 2 and 20 mg/100 g FW, but also in tea, 0.5-1.6 mg/100 ml, and red wine, 0-9.7 mg/100 ml.
In fruit, it is only found in high concentrations in berries, while in most other fruit and vegetables it is present in a content of less than 0.2 mg/100 g FW.

Isorhamnetin

A fourth flavonol, less abundant than the previous ones, is isorhamnetin. It is only present in some foods such as some spices: chives, 5.0-8.5 mg/100 g FW, fennel, 9.3 mg/100 g FW, tarragon, 5 mg/100 g FW.
In fruit and vegetables it is only present in almonds, with a concentration between 1.2 and 10.3 mg/100 g FW, pears and onions.

References

de la Rosa L.A., Alvarez-Parrilla E., Gonzàlez-Aguilar G.A. Fruit and vegetable phytochemicals: chemistry, nutritional value, and stability. 1th Edition. Wiley J. & Sons, Inc., Publication, 2010

Han X., Shen T. and Lou H. Dietary polyphenols and their biological significance. Int J Mol Sci 2007;9:950-988. doi:10.3390/i8090950

Manach C., Scalbert A., Morand C., Rémésy C., and Jime´nez L. Polyphenols: food sources and bioavailability. Am J Clin Nutr 2004;79(5):727-47 doi:10.1093/ajcn/79.5.727

Tsao R. Chemistry and biochemistry of dietary polyphenols. Nutrients 2010;2:1231-46. doi:10.3390/nu2121231


Anthocyanins: definition, structure and pH

Anthocyanins are a subgroup of flavonoids, therefore they are polyphenols, which give plants their distinctive colors.
They are water soluble pigments and are present in the vacuolar sap of the epidermal tissues of flowers and fruit.
They are responsible for the colors of the most of the petals, fruits and vegetables, and of some varieties of cereals such as black rice. In fact, they impart red, pink and purple to blue colors to berries, red apples, red grapes, cherries, and of many other fruits, red lettuce, red cabbage, onions or eggplant, but also red wine.
Together with carotenoids, they are responsible for autumn leaf color.
Finally, anthocyanins contribute to attract animals when a fruit is ready to eat or a flower is ready for pollination.

They are bioactive compounds found in plant foods that have a double interest for man:

  • the first one, a technological interest, due to their effects on the organoleptic characteristics of food products;
  • the other due to their healthy properties, being implicated in the protection against cardiovascular risk.
    In fact:

in vitro, they have an antioxidant activity, due to their ability to delocalize electrons and form resonance structures, and a protective role against oxidation of low density lipoproteins (LDL);

like other polyphenols, such as catechins, proanthocyanidins and other uncolored flavonoids, they can regulate different signaling pathways involved in cell growth, differentiation and survival.

CONTENTS

Chemical structure of anthocyanins

The basic chemical structure is flavylium cation (2-phenylbenzopyrilium), which links hydroxyl (-OH) and/or methoxyl (-OCH3) groups, and one or more sugars.
The sugar-free molecule is called anthocyanidins.

Basic Skeleton of Anthocyanins
Fig. 1 – 2-Phenylbenzopyrilium

Depending on the number and position of hydroxyl and methoxyl groups, various anthocyanidins have been described, and of these, six are commonly found in vegetables and fruits:

  • pelargonidin
  • cyaniding
  • delphinidin
  • petunidin
  • peonidin
  • malvidin
Antocyanins
Fig. 2 – Antocyanins

Anthocyanins, as most of the other flavonoids, are present in plants and plant foods in the form of glycosides, that is, linked to one or more sugar units.
The most common sugars present in these natural pigments are:

The sugars are linked mainly to the C3 position as 3-monoglycosides, to the C3 and C5 positions as diglycosides (with the possible forms: 3-diglycosides, 3,5-diglycosides, and 3-diglycoside-5-monoglycosides). Glycosylations have been also found at C7, C3′ and C5′ positions.
The structure of these molecules is further complicated by the bond to the sugar unit of different acyl substituents such as:

  • aliphatic acids, such as acetic, malic, succinic and malonic acid;
  • cinnamic acids (aromatic substituents), such as sinapic, ferulic and p-coumaric acid;
  • finally, there are pigments with both aromatic and aliphatic substituents.

Furthermore, some anthocyanins have several acylated sugars in the molecule; these anthocyanins are sometimes called polyglycosides.

Depending on the type of hydroxylation, methoxylation and glycosylation patterns, and the different substituents linked to the sugar units, more than 500 different anthocyanins have been identified that are based on 31 anthocyanidins. Among these 31 monomers:

  • 30% are from cyanidin;
  • 22% are from delphinidin;
  • 18% are from pelargonidin.

Methylated derivatives of cyanidin, delphinidin and pelargonidin, namely peonidin, malvidin, and petunidin, all together represent 20% of the anthocyanins.
Therefore, up to 90% of the most frequently encountered anthocyanins are related to delphinidin, pelargonidin, cyanidin, and their methylated derivatives.

Role of pH

The color of these molecules is influenced by the pH of the vacuole where they are stored, ranging in color from:

  • red, under very acidic conditions;
  • to purple-blue, in intermediate pH conditions;
  • until yellow-green, in alkaline conditions.

In addition to the pH, the color of these flavonoids can be affected by the degree of hydroxylation or methylation pattern of the A and B rings, and by glycosylation pattern.
Finally, the color of certain plant pigments result from complexes between anthocyanins, flavones and metal ions.
It should be noted that anthocyanins are often used as pH indicators thanks to the differences in chemical structure that occur in response to changes in pH.

References

de la Rosa L.A., Alvarez-Parrilla E., Gonzàlez-Aguilar G.A. Fruit and vegetable phytochemicals: chemistry, nutritional value, and stability. 1th Edition. Wiley J. & Sons, Inc., Publication, 2010

de Pascual-Teresa S., Moreno D.A. and García-Viguera C. Flavanols and anthocyanins in cardiovascular health: a review of current evidence. Int J Mol Sci 2010;11:1679-1703. doi:10.3390/ijms11041679

Escribano-Bailòn M.T., Santos-Buelga C., Rivas-Gonzalo J.C. Anthocyanins in cereals. J Chromatogr A 2004:1054;129-141. doi:10.1016/j.chroma.2004.08.152

Han X., Shen T. and Lou H. Dietary polyphenols and their biological significance. Int J Mol Sci 2007;9:950-988. doi:10.3390/i8090950

Manach C., Scalbert A., Morand C., Rémésy C., and Jime´nez L. Polyphenols: food sources and bioavailability. Am J Clin Nutr 2004;79(5):727-47 doi:10.1093/ajcn/79.5.727

Ottaviani J.I., Kwik-Uribe C., Keen C.L., and Schroeter H. Intake of dietary procyanidins does not contribute to the pool of circulating flavanols in humans. Am J Clin Nutr 2012;95:851-8. doi:10.3945/​ajcn.111.028340

Tsao R. Chemistry and biochemistry of dietary polyphenols. Nutrients 2010;2:1231-1246. doi:10.3390/nu2121231


Proanthocyanidins: definition, structure and absorption

Proanthocyanidins or condensed tannins, also called pycnogenols and leukocyanidins, are polyphenolic compounds (in particular they are a flavonoid subgroup) widely distributed in the plant kingdom, second only to lignin as the most abundant phenol in nature.
They are present in high concentrations in various parts of the plants such as flowers, fruits, berries, seeds (e.g. in grape seeds), and bark (e.g. pine bark).

Together with anthocyanins and their oxidation products, and catechins, they are the most abundant flavonoids in human diet and it has been suggested that they constitute a significant fraction of the polyphenols ingested in the Western diet.
Therefore, condensed tannins should be taken into consideration when the epidemiological association between the intake of polyphenols, especially flavonoids, and chronic diseases are examined.

CONTENTS

Chemical structure of proanthocyanidins

Condensed tannins have a complex chemical structure being oligomers (dimers to pentamers) or polymers (six or more units, up to 60) of catechins or flavanols, which are joined by carbon-carbon bonds.

Proanthocyanidins
Fig. 1 – Procyanidin Skeleton

They may consist exclusively of:

  • (epi)catechin, and they are named procyanidins;
  • (epi)afzelechin, and they are named propelargonidins;
  • (epi)gallocatechin, and they are named prodelphinidins.

Propelargonidins and prodelphinidins are less common in nature and in foods than procyanidins.

Depending on the bonds between monomers, proanthocyanidins have a:

  • B-type structure, if the polymerization occurs via carbon-carbon bond between the position 8 of the terminal unit and the 4 of the extender (or C4-C6);
  • A-type structure, less frequent, if monomers are doubly linked via an ether bond C2-O-C7 or C2-O-C5 plus a B-type bond.

Procyanidins

The most common dimers are B-type procyanidins, B1 to B8, formed by catechin or epicatechin; in B1, B2, B3 and B-4 dimers, the two flavanol units are joined by a C4-C8 bond; in B5, B6, B7 and B8 dimers the two units are joined by C4-C6 bond.

Proanthocyanidins
Fig. 2 – Procyanidins B1-B4

Procyanidin C1 is a B-type trimer.

Procyanidin A-2 is an example of A-type procyanidin.

Intestinal absorption of proanthocyanidins

Condensed tannins are poorly absorbed from the intestine; together with anthocyanins and gallic acid ester derivatives of tea catechins, they are the least well-absorbed polyphenols.
It seems that low molecular weight oligomers (2-3 monomers) may be absorbed as such while polymers are not.
In the systemic circulation, dimers reach concentrations of two orders of magnitude lower than those of catechins.
It seems that condensed tannins with a degree of polymerization greater than three transit into the stomach and small intestine without significant modifications, and then, into the large intestine, they are catabolized by colonic microflora, with production of phenylpropionic, phenilvaleric and phenylacetic acids. These degradation products have been suggested to be the major metabolites of proanthocyanidins in healthy humans.

Procyanidins and catechins

It had been proposed that the catabolism of procyanidins in the gastrointestinal tract lead to the release of monomeric catechins, thus indirectly contributing to their systemic pool in humans. In recent years, it has been shown that this does not happen because procyanidins do not significantly contribute to:

  • the concentration of catechin metabolites in the systemic circulation;
  • the total catechin metabolites excreted in the urine;
  • finally, they do not significantly affect plasma metabolite profile derived from catechol-O-methyltransferase activity.

Therefore, analyzing the potential health benefits associated with the intake of foods containing these phytochemicals, catechins and procyanidins should be considered distinct classes of related compounds.

References

de la Rosa L.A., Alvarez-Parrilla E., Gonzàlez-Aguilar G.A. Fruit and vegetable phytochemicals: chemistry, nutritional value, and stability. 1th Edition. Wiley J. & Sons, Inc., Publication, 2010

Gu L., Kelm M.A., Hammerstone J.F., Beecher G., Holden J., Haytowitz D., Gebhardt S., and Prior R.L. Concentrations of proanthocyanidins in common foods and estimations of normal consumption. J Nutr 2004;134(3):613-617. doi:10.1093/jn/134.3.613

Han X., Shen T. and Lou H. Dietary polyphenols and their biological significance. Int J Mol Sci 2007;9:950-988. doi:10.3390/i8090950

Manach C., Scalbert A., Morand C., Rémésy C., and Jime´nez L. Polyphenols: food sources and bioavailability. Am J Clin Nutr 2004;79(5):727-47 doi:10.1093/ajcn/79.5.727

Nandakumar V., Singh T., and Katiyar S.K. Multi-targeted prevention and therapy of cancer by proanthocyanidins. Cancer Lett 2008;269(2):378-387. doi:10.1016/j.canlet.2008.03.049

Ottaviani J.I., Kwik-Uribe C., Keen C.L., and Schroeter H. Intake of dietary procyanidins does not contribute to the pool of circulating flavanols in humans. Am J Clin Nutr 2012;95:851-8. doi:10.3945/ajcn.111.028340

Santos-Buelga C. and Scalbert A. Proanthocyanidins and tannin-like compounds: nature, occurrence, dietary intake and effects on nutrition and health. J Sci Food Agr 2000;80(7):1094-1117. doi:10.3390/nu2121231

Wang Y.,Chung S., Song W.O., and Chun O.K. Estimation of daily proanthocyanidin intake and major food sources in the U.S. diet. J Nutr 2011;141(3):447-452. doi:10.3945/jn.110.133900


Catechins: definition, structure, green tea, black tea, cocoa

Catechins or flavanols, with flavonols such as quercetin, and flavones such as luteolin, are a subgroup of flavonoids among the most widespread in nature.
Flavanols and proanthocyanidins, together with anthocyanins and their oxidation products, are the most abundant flavonoids in human diet.

CONTENTS

Chemical structure of catechins

Chemically they differ from many other flavonoids as:

  • they lack the double bond between positions 2 and 3 of the C ring;
  • they not have a keto group at position 4;
  • they have a hydroxyl group in position 3, and for this reason they are also called flavan-3-ols.
Catechins
Fig. 1 – Basic Flavanol Skeleton

Another distinctive feature of flavan-3-ols is their ability to form oligomers (two to ten units) or polymers (eleven or more units, up to 60 units) called proanthocyanidins or condensed tannins.

Catechins in foods

Flavanols commonly found in plant-derived food products are catechin, epicatechin, gallocatechin, epigallocatechin, and their gallic acid ester derivatives: catechin gallate, gallocatechin gallate, epicatechin gallate, and epigallocatechin gallate or EGCG.

Catechins
Fig. 2 – Flavanols

Flavanols present with higher frequency are catechin and epicatechin, which are also among the most common known flavonoids, and almost as popular as the related flavonol quercetin.
Cocoa and green tea are by far the richest sources in flavanols. In these foods the main flavonoids are catechin and epicatechin (cocoa is also a good source of epigallocatechin), but also their gallic acid ester derivatives, the gallocatechins.
However, they are also present in many fruits, especially in the skins of apples, blueberries (Vaccinium myrtillus) and grapes, in vegetables, red wine and beer, and peanuts.
As in many cases flavanols are present in the seeds or peels of fruits and vegetables, their intake may be limited by the fact that these parts are discarded during processing or while eaten.
Furthermore, in contrast to other flavonoids, catechins are not glycosylated in foods.
Proanthocyanidins, that is polymeric flavan-3-ols, are also commonly found in plant-derived food products. Their presence has been reported in the skin of peanuts and almonds, as in the berries.

Green and black tea

Green tea is an excellent source of flavonoids. The main flavonoids present in the leaves of the tea (as in cocoa beans) are catechin and epicatechin, monomeric flavanols, together with their gallate derivatives such as EGCG.
Epigallocatechin gallate is the most abundant catechin in green tea and it seems to have an important role in determining green tea benefits, as the reduction of:

  • vascular inflammation;
  • blood pressure;
  • concentration of oxidized LDL.

Black tea (fermented tea) contains fewer monomeric flavanols, as they are oxidized during fermentation of the leaves to more complex polyphenols such as theaflavins (theaflavin digallate, theaflavin-3-gallate, and theaflavin-3′-gallate, all dimers) and thearubigins (polymers).
Theaflavins and thearubigins are present only in the tea; their concentrations in brewed tea are between 50- and 100-folds lesser than in tea leaves.

It should be noted that tea epicatechins are remarkably stable to heat in acidic environment: at pH 5, only about 15% is degraded after seven hours in boiling water (therefore, adding lemon juice to brewed tea does not cause any reduction in their content).

Cocoa and cocoa products

Cocoa has the highest content of polyphenols and flavanols per serving, a concentration greater than those found in green tea and red wine. Most of the flavonoids present in cocoa beans and derived products, such as black chocolate, are catechin and epicatechin, monomeric flavanols, but also epigallocatechin, and their derivatives such as the gallocatechins; among polymers, proanthocyanidins are also important.

Fruits, vegetables, and legumes

Catechin and epicatechin are the main flavanols in fruits. They are found in many fruits in different concentrations, respectively, between 5-3 and 0.5-6 mg/100 g fresh weight.
On the contrary, gallocatechin, epicatechin gallate, epigallocatechin, and epigallocatechin gallate are present in various fruits such as red grapes, berries, apples, peaches and plums, but in very low concentrations, less than 1mg/100 g fresh weight.
Except for lentils and broad beans, few legumes and vegetables contain catechins, and in very low concentrations, less than 1.5 mg/100 g fresh weight.

References

de la Rosa L.A., Alvarez-Parrilla E., Gonzàlez-Aguilar G.A. Fruit and vegetable phytochemicals: chemistry, nutritional value, and stability. 1th Edition. Wiley J. & Sons, Inc., Publication, 2010

de Pascual-Teresa S., Moreno D.A. and García-Viguera C. Flavanols and anthocyanins in cardiovascular health: a review of current evidence. Int J Mol Sci 2010;11:1679-1703. doi:10.3390/ijms11041679

Han X., Shen T. and Lou H. Dietary polyphenols and their biological significance. Int J Mol Sci 2007;9:950-988. doi:10.3390/i8090950

Manach C., Scalbert A., Morand C., Rémésy C., and Jime´nez L. Polyphenols: food sources and bioavailability. Am J Clin Nutr 2004;79(5):727-47 doi:10.1093/ajcn/79.5.727

Tsao R. Chemistry and biochemistry of dietary polyphenols. Nutrients 2010;2:1231-1246. doi:10.3390/nu2121231


Flavonoids: definition, structure and classification

Flavonoids are the most abundant polyphenols in human diet, representing about 2/3 of all those ones ingested. Like other phytochemicals, they are the products of secondary metabolism of plants and, currently, it is not possible to determine precisely their number, even if over 4000 have been identified.
In fruits and vegetables, they are usually found in the form of glycosides and sometimes as acylglycosides, while acylated, methylated and sulfate molecules are less frequent and in lower concentrations.
They are water-soluble and accumulate in cell vacuoles.

CONTENTS

Chemical structure of flavonoids

Their basic structure is a skeleton of diphenylpropane, namely, two benzene rings (ring A and B, see figure) linked by a three carbon chain that forms a closed pyran ring (heterocyclic ring containing oxygen, the C ring) with benzenic A ring. Therefore, their structure is also referred to as C6-C3-C6.

Flavonoids: the Skeleton
Fig. 1 – Skeleton of Diphenylpropane

In most cases, B ring is attached to position 2 of C ring, but it can also bind in position 3 or 4; this, together with the structural features of the ring B and the patterns of glycosylation and hydroxylation of the three rings, makes the flavonoids one of the larger and more diversified groups of phytochemicals, so not only of polyphenols, in nature.
Their biological activities, for example they are potent antioxidants, depend both on the structural characteristics and the pattern of glycosylation.

Classification

They can be subdivided into different subgroups depending on the carbon of the C ring on which B ring is attached, and the degree of unsaturation and oxidation of the C ring.
Flavonoids in which B ring is linked in position 3 of the ring C are called isoflavones; those in which B ring is linked in position 4, neoflavonoids, while those in which the B ring is linked in position 2 can be further subdivided into several subgroups on the basis of the structural features of the C ring. These subgroup are: flavones, flavonols, flavanones, flavanonols, flavanols or catechins and anthocyanins.
Finally, flavonoids with open C ring are called chalcones.

Flavonoids: the Subgroups
Fig. 2 – Flavonoid Subgroups
  • Flavones
    They have a double bond between positions 2 and 3 and a ketone in position 4 of the C ring. Most flavones of vegetables and fruits has a hydroxyl group in position 5 of the A ring, while the hydroxylation in other positions, for the most part in position 7 of the A ring or 3′ and 4′ of the B ring may vary according to the taxonomic classification of the particular vegetable or fruit.
    Glycosylation occurs primarily on position 5 and 7, methylation and acylation on the hydroxyl groups of the B ring.
    Some flavones, such as nobiletin and tangeretin, are polymethoxylated.
  • Flavonols
    Compared to flavones, they have a hydroxyl group in position 3 of the C ring, which may also be glycosylated. Again, like flavones, flavonols are very diverse in methylation and hydroxylation patterns as well, and, considering the different glycosylation patterns, they are perhaps the most common and largest subgroup of flavonoids in fruits and vegetables. For example, quercetin is present in many plant foods.
  • Flavanones
    Flavanones, also called dihydroflavones, have the C ring saturated; therefore, unlike flavones, the double bond between positions 2 and 3 is saturated and this is the only structural difference between the two subgroups of flavonoids.
    The flavanones can be multi-hydroxylated, and several hydroxyl groups can be glycosylated and/or methylated.
    Some have unique patterns of substitution, for example, furanoflavanones, prenylated flavanones, pyranoflavanones or benzylated flavanones, giving a great number of substituted derivatives.
    Over the past 15 years, the number of flavanones discovered is significantly increased.
  • Flavanonols
    Flavanonols, also called dihydroflavonols, are the 3-hydroxy derivatives of flavanones; they are an highly diversified and multisubstituted subgroup.
  • Isoflavones
    As anticipated, isoflavones are a subgroup of flavonoids in which the B ring is attached to position 3 of the C ring. They have structural similarities to estrogens, such as estradiol, and for this reason they are also called phytoestrogens.Neoflavonoids
    They have the B ring attached to position 4 of the C ring.
  • Flavanols or flavan-3-ols or catechins
    Flavanols are also referred to flavan-3-ols as the hydroxyl group is almost always bound to position 3 of C ring; they are called catechins as well.
    flavanols to have two chiral centers in the molecule, on positions 2 and 3, then four possible diastereoisomers. Epicatechin is the isomer with the cis configuration and catechin is the one with the trans configuration. Each of these configurations has two stereoisomers, namely, (+)-epicatechin and (-)-epicatechin, (+)-catechin and (-)-catechin.
    (+)-Catechin and (-)-epicatechin are the two isomers most often present in edible plants.
    Another important feature of flavanols, particularly of catechin and epicatechin, is the ability to form polymers, called proanthocyanidins or condensed tannins. The name “proanthocyanidins” is due to the fact that an acid-catalyzed cleavage produces anthocyanidins.
    Proanthocyanidins typically contain 2 to 60 monomers of flavanols.
    Monomeric and oligomeric flavanols (containing 2 to 7 monomers) are strong antioxidants.
  • Anthocyanidins
    Chemically, anthocyanidins are flavylium cations and are generally present as chloride salts.
    They are the only group of flavonoids that gives plants colors (all other flavonoids are colorless).
    Anthocyanins are glycosides of anthocyanidins. Sugar units are bound mostly to position 3 of the C ring and they are often conjugated with phenolic acids, such as ferulic acid.
    The color of the anthocyanins depends on the pH and also by methylation or acylation at the hydroxyl groups on the A and B rings.
  • Chalcones
    Chalcones and dihydrochalcones are flavonoids with open structure; they are classified as flavonoids because they have similar synthetic pathways.

References

de la Rosa L.A., Alvarez-Parrilla E., Gonzàlez-Aguilar G.A. Fruit and vegetable phytochemicals: chemistry, nutritional value, and stability. 1th Edition. Wiley J. & Sons, Inc., Publication, 2010

Han X., Shen T. and Lou H. Dietary polyphenols and their biological significance. Int J Mol Sci 2007;9:950-988. doi:10.3390/i8090950

Manach C., Scalbert A., Morand C., Rémésy C., and Jime´nez L. Polyphenols: food sources and bioavailability. Am J Clin Nutr 2004;79(5):727-47 doi:10.1093/ajcn/79.5.727

Tsao R. Chemistry and biochemistry of dietary polyphenols. Nutrients 2010;2:1231-1246. doi:10.3390/nu2121231


Polyphenols: definition, structure and classification

Polyphenols are one of the most important and certainly the most numerous among the groups of phytochemicals present in the plant kingdom.
Currently, over 8000 phenolic structures have been identified, of which more than 4000 belonging to the class of flavonoids, and several hundred occur in edible plants.
However, it is thought that the total content of polyphenols in plants is underestimated as many of the phenolic compounds present in fruits, vegetables and derivatives have not yet been identified, escaping the methods and techniques of analysis used, and the composition in polyphenols for most fruits and some varieties of cereals is not yet known.

They are present in many edible plants, both for men and animals, and it is thought to be their presence, along with that of other molecules such as carotenoids, vitamin C or vitamin E, the responsible for the healthy effects of fruits and vegetables.
In the human diet, they are the most abundant natural antioxidants, and the main sources are fruits, vegetables, whole grains, but also other types of foods and beverages derived from them, such as red wine, rich in resveratrol, the extra virgin olive oil, rich in hydroxytyrosol, chocolate or tea, in particularly green tea, rich in epigallocatechin gallate (EGCG).

CONTENTS

Chemical structure of polyphenols

The term polyphenols refers to a wide variety of molecules that can be divided into many subclasses, subdivisions that can be made on the basis of their origin, biological function, or chemical structure.
Chemically, they are compounds with structural phenolic features, which can be associated with different organic acids and carbohydrates.

Polyphenols: Phenolic Skeleton
Fig. 1 – Phenol

In plants, the most part of them are linked to sugars, and therefore they are in the form of glycosides. Carbohydrates and organic acids can be bound in different positions on polyphenol skeletons.
Among polyphenols, there are simple molecules, such as phenolic acids, or complex structures such as condensed tannins, that are highly polymerized molecules.

Classification

They can be classified into different classes, according to the number of phenolic rings in their structure, the structural elements that bind these rings each others, and the substituents linked to the rings.

Polyphenols: Flavonoid Skeleton
Fig. 2 – Flavonoid Skeleton

Therefore, two main groups can then be identified: the flavonoids group and the non-flavonoid group.
Flavonoids share a structure formed by two aromatic rings, indicated as A and B, linked together by three carbon atoms forming an oxygenated heterocycle, the C ring; they can be further subdivided into six main subclasses, as a function of the type of heterocycle (the C ring) that is involved:

Non-flavonoids can be subdivided into:

  • simple phenols
  • phenolic acids
  • benzoic aldehydes
  • hydrolyzable tannins
  • acetophenones and phenylacetic acids
  • hydroxycinnamic acids
  • coumarins
  • benzophenones
  • xanthones
  • stilbenes;
  • lignans
  • secoiridoids

Variability of polyphenol content of plant and plant products

Although several classes of phenolic molecules, such as quercetin (a flavonol), are present in most plant foods (tea, wine, cereals, legumes, fruits, fruit juices, etc.), other classes are found only in a particular type of food (e.g. flavanones in citrus, isoflavones in soya, phloridzin in apples, etc.).

Polyphenols: Quercetin
Fig. 3 – Quercetin

However, it is common that different types of polyphenols are in the same product; for example, apples contain flavanols, chlorogenic acid, hydroxycinnamic acids, glycosides of phloretin, glycosides of quercetin and anthocyanins.
The polyphenol composition may also be influenced by other parameters such as environmental factors, the degree of ripeness at harvest time, household or industrial processing, storage, and plant variety. From currently available data, it seems that the fruits with the highest content of polyphenols are strawberries, lychees and grapes, and the vegetables are artichokes, parsley and brussels sprouts. Melons and avocados have the lowest concentrations.

References

de la Rosa L.A., Alvarez-Parrilla E., Gonzàlez-Aguilar G.A. Fruit and vegetable phytochemicals: chemistry, nutritional value, and stability. 1th Edition. Wiley J. & Sons, Inc., Publication, 2010

Han X., Shen T. and Lou H. Dietary polyphenols and their biological significance. Int J Mol Sci 2007;9:950-988. doi:10.3390/i8090950

Manach C., Scalbert A., Morand C., Rémésy C., and Jime´nez L. Polyphenols: food sources and bioavailability. Am J Clin Nutr 2004;79(5):727-47 doi:10.1093/ajcn/79.5.727

Tsao R. Chemistry and biochemistry of dietary polyphenols. Nutrients 2010;2:1231-1246. doi:10.3390/nu2121231


Fruits and vegetables in season

Numerous studies showed that seasonality plays a key role in optimizing the antioxidant properties of fruits and vegetables. For example, a recent Chinese study have investigated the influence of growing season (summer vs winter) on the synthesis and accumulation of phenolic compounds and antioxidant properties in five grape cultivars. The study showed that both phenolic compounds and antioxidant properties in the skin and seed of winter berries were significantly higher than those of summer berries for all of the cultivars investigated. Finally, to choose seasonal vegetables and fruits also ensures considerable saving of money.

List of vegetables and fruits in season

Fruits and Vegetables: Fruits in Season
Fig. 1 – Fruits in Season
Fruits and Vegetables: Vegetables in Season
Fig. 2 – Vegetables in Season
References

Xu C., Zhang Y., Zhu L., Huang Y., and Jiang Lu J. Influence of growing season on phenolic compounds and antioxidant properties of grape berries from vines Grown in Subtropical Climate. J Agric Food Chem 2011:59(4);1078-1086. doi:10.1021/jf104157z