Tea polyphenols: bioactive compounds from leaves of tea plant

Tea polyphenols: from the leaf to the cup

Tea Polyphenols

The leaves of the tea plant, Camellia sinensis, and tea are rich in bioactive compounds.
More than 4000 molecules have been found in the beverage.
Approximately one third of these compounds are polyphenols, the most important molecules in determining nutritional value and health benefits of the beverage.
Tea polyphenols are mostly flavonoids, such as catechins in green tea (e.g. EGCG), and thearubigins and theaflavins in black tea.
Other bioactive compounds present in tea are:

  • alkaloids, such as caffeine, theophylline and theobromine;
  • amino acids, and among them, theanine (r-glutamylethylamide), that is also a brain neurotransmitter and one of the most important amino acids in green tea;
  • proteins;
  • carbohydrates;
  • chlorophyll;
  • volatile organic molecules, that is, compounds that easily produce vapors and contribute to the odor of the beverage;
  • fluoride, aluminum and trace elements.

These molecules provide the nutritional value of the tea, affecting human health in many ways. Their composition is highly influenced by processing of tea leaves.

Biological activities of polyphenols

Polyphenols, both in vivo and in vitro, have a broad spectrum of biological activities such as:

  • antioxidant properties;
  • reduction of various types of tumors;
  • inhibition of inflammation;
  • protective effects against hyperlipidemia and diabetes.

Therefore, they have a protective role against the development of many diseases.
Thanks to the abundance of tea polyphenols, there has been a growing interest in recent years about the possible preventive effects of beverage against several diseases, particularly cardiovascular disease, for example in the development and progression of atherosclerosis.

Mechanisms of action of tea polyphenols

Currently, there is limited information on how tea polyphenols exert their effects at cellular level.
It seems, at least in vitro, that catechins in green tea, and theaflavins and thearubigins in black tea are the bioactive compounds responsible for the physiological effects and health benefits of tea.
And among the observed mechanisms by which tea polyphenols act at the cellular level, in addition to the antioxidant effect, it has been observed, as a consequence of polyphenol binding to specific receptors on the cell membrane, changes in the activity of various protein kinases, and growth and transcriptional factors.
Moreover, it seems that these molecules, or at least EGCG, may enter the cell and directly interact with their intracellular specific targets.

References

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

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

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

Lenore 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

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.

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

Isoflavones: definition, structure and soy

CONTENTS

What are isoflavones?

Isoflavones are colorless polyphenols belonging to the class of flavonoids.
Unlike the majority of the other flavonoids, they have a restricted taxonomic distribution, being present almost exclusively in the Leguminosae or Fabaceae plant family, mainly in soy.
Since legumes, soy in primis, are a major part of the diet in many cultures, these flavonoids may have a great impact on human health.
They are also present in beans and broad beans, but in much lower concentrations than those found in soy and soy products.
Also red clover or meadow clover (Trifolium pratense), another member of Leguminosae family, is a good source.
Currently, they are not found in fruits and vegetables.

Together with phenolic acids, such as caffeic acid and gallic acid, and quercetin glycosides, they are the most well-absorbed polyphenols, followed by flavanones and catechins (but not gallocatechins).

In plants, some isoflavones have antimicrobial activity and are synthesized in response to attacks by bacteria or fungi; thus they act as phytoalexins.

Chemical structure of isoflavones

While most flavonoids have B ring attached to position 2 of C ring, isoflavones have B ring attached to position 3 of C ring.

Isoflavones
Fig. 1 – Isoflavone Skeleton

Even if they are not steroids, they have structural similarities to estrogens, particularly estradiol. This confers them pseudohormonal properties, such as the ability to bind estrogen receptors; therefore, they are classified as phytoestrogens or plant estrogens. The benefits often ascribed to soy and soy products (e.g. tofu) are believed to result from the ability of isoflavones to act as estrogen mimics .
It should be underlined that the binding to estrogen receptors seems to lose strength with time, therefore their potential efficacy should not be overestimated.
In foods, they are present in four forms:

  • aglycone;
  • 7-O-glucoside;
  • 6′-O-acetyl-7-O-glucoside;
  • 6′-O-malonyl-7-O-glucoside.

Soy isoflavones: genistein, daidzein and glycitein

Soy and soy products, such as soy milk, tofu, tempeh and miso, are the main source of isoflavones in the human diet.
The isoflavone content of soy and soy products varies greatly as a function of growing conditions, geographic zone, and processing; for example, in soy it ranges between 580 and 3800mg/kg fresh weight, while in soy milk it range between 30 and 175 mg/L. The most abundant isoflavones in soy and soy products are genistein, daidzein and glycitein, generally present in a concentration ratio of 1:1:0,2.; biochanin A and formononetin are other isoflavones present in less concentrations.

Isoflavones
Fig. 2 – Isoflavones

The 6′-O-malonyl derivatives have a bitter, unpleasant, and astringent taste; therefore they give a bad flavor to the food in which they are contained. However, being sensitive to temperature, they are often hydrolyzed to glycosides during processing, such as the production of soy milk.
The fermentation processes needed for the preparation of certain foods, such as tempeh and miso, determines in turn the hydrolysis of glycosides to aglycones, i.e. the sugar-free molecule.
Isoflavone glycosides present in soy and soy products can also be deglycosylated by β-glucosidases in the small intestine.
The aglycones are very resistant to heat.
Although many compounds present in the diet are converted by intestinal bacteria to less active molecules, other compounds are converted to molecules with increased biological activity. This is the case of isoflavones, but also of prenylflavonoids from hops (Humulus lupulus), and lignans, that are other phytoestrogens.

Phytoestrogens and menopause

Vasomotor symptoms, such as night sweats and hot flashes, and bone loss are very common in perimenopause, also called menopausal transition, and menopause. Hormone replacement therapy (HRT) has proved to be a highly effective treatment for the prevention of menopausal bone loss and vasomotor symptoms.
The use of alternative therapies based on phytoestrogens is increased as a result of the publication of the “Women’s Health Initiative” study, that suggests that hormone replacement therapy could lead to more risks than benefits, in particular an increased risk of developing some chronic diseases.
Soy isoflavones are among the most used phytoestrogens by menopausal women, often taken in the form of isoflavone fortified foods or isoflavone supplements.
However, many studies have highlighted the lack of efficacy of soy isoflavones, and red clover isoflavones, even in large doses, in the prevention of vasomotor symptoms (hot flushes and night sweats) and bone loss during menopause.

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

Lethaby A., Marjoribanks J., Kronenberg F., Roberts H., Eden J., Brown J. Phytoestrogens for menopausal vasomotor symptom. Cochrane Database of Systematic Reviews 2013, Issue 12. Art. No.: CD001395. doi:10.1002/14651858.CD001395.pub4

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


Green tea benefits for health

Benefits of green tea: science and myths

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.

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