Antioxidant Properties of Tea : Drink More Tea for better Health  

                                                                                     

                                                                  

Introduction

Evidence is increasingly available from research labs highlighting the role antioxidants play in protecting human beings against  heart disease, stroke and cancers. It has been

proposed that the mechanisms leading to these diseases are promoted by free

radicals and that antioxidants  oppose the action of these molecules. In addition

to the well known antioxidants such as Vitamins C and E, there is growing research

demonstrating the potentially beneficial effects of plant-derived antioxidants like

polyphenols which are  found in fruits, vegetables, nuts, cereals and  in drinks such as tea.

Free Radicals  

Free radicals are unstable molecules that include the hydrogen atom, nitric oxide

(NO) and molecular oxygen (O2). These molecules naturally occur in human body as a result of chemical reactions during normal cellular processes. They can also be formed in response to excess pollution, too much UV sunlight and exposure to cigarette

smoke. In an attempt to stabilise, they attack other molecules in the body potentially

leading to cell damage and triggering the formation of other free radicals resulting

in a chain reaction. Some scientists believe that this type of free radical action has

been implicated in certain chronic and ageing diseases such as cancer, heart

disease, stroke, rheumatoid arthritis, cataracts and Alzheimer’s disease.

Role of Antioxidants

Antioxidants are compounds that help to inhibit the many oxidation reactions

caused by free radicals thereby preventing or delaying damage to the cells and

tissues. Their mechanisms of action include:-

·  Scavenging reactive oxygen and nitrogen free radical species.

·  Decreasing the localised oxygen concentration thereby reducing molecular

   oxygen’s oxidation potential.

·  Metabolising lipid peroxides to non-radical products.

·  Chelating metal ions to prevent the generation of free radicals.

The free radicals cause the following  damages :

·  Oxidising Low Density Lipoprotein (LDL) cholesterol, which may increase

    the risk of atherosclerosis.

·  Promoting platelet adhesion, which can lead to thrombosis thereby

    increasing the risk of heart disease or stroke.

·  Damaging the cell’s DNA, which may lead to cancer.

·  Blocking the normal endothelial cell function and vasodilatation in response

   to nitric oxide, a potential mechanism for heart disease and cancer.

·  Triggering inflammation.

·  Impairing immune function.

Some antioxidants are synthesised within the cells themselves (endogenous) and

others need to be provided externally in the diet. Table 1 gives examples of antioxidants with established   activity in the body :

Table 1

Endogenous Antioxidants	Antioxidants provided in the diet
Polyamines Melatonin Oestrogen Superoxide dismutase Glutathione peroxidase Catalase Lipoic Acid Caeruloplasmin Albumin Lactoferrin Transferrin	Vitamin E Vitamin C Carotenoids Polyphenols Copper

Sources of dietary antioxidants

Traditionally dietary antioxidants were thought of as Vitamin E and C and the

carotenoid - carotene. In recent years there has been particular interest in the

antioxidant activity and health benefits of other phytochemicals. Table 2 lists two

examples of phytochemicals and their food sources :

Table 2

Phytochemical	Categories	Sub-category	Food Sources
Carotenoids	Carotene	alpha--carotene	Carrots, pumpkins, avocados
		beta- carotene	Carrots, red peppers, apricots, spinach
	Lycopene		Tomatoes, pink grapefruit, watermelons
	Lutein		Spinach, kale, brussel sprouts
Polyphenols	Flavonoids	Anthocyanins	Berries, red wine, black grapes
		Flavones	Celery, parsley, olives
		Flavonols; Quercetin, Rutin	Tea, apples, onions, wine, garlic
		Flavonols; Catechins	Tea, wine, pears, apples, chocolate
		Flavanones	Citrus fruit
		Isoflavones	Legumes
	Phenolic Acids	Hydroxybenzoic Acid; Gallic Acid, Ellagic Acid, Salicylic Acid	Berries, Tea, Grapes, Walnuts
	Other Phenolic compounds	Capsaicin	Chillies, Peppers
		Tannins	Tea, red wine, grapes

Tea has one of the highest total flavonoid contents of all plants being about  15% of the leaf by dry weight.

Tea Flavonoids

The types and amounts of flavonoids present in tea will differ depending on the

variety of leaf, growing environment, processing, manufacturing, particle size of

ground tea leaves and infusion preparation.  Typically 93% of total tea phenolic

compounds are flavonoids  Green teas contain more of the simple flavonoids

called catechins, while the oxidation that the leaves undergo to make black tea

converts these simple flavonoids to the more complex varieties called theaflavins (TF)

and thearubigins(TR).  

Tea flavonoids are water-soluble and one study  has shown that a cup of   tea

that has been allowed to brew for 40-60 seconds will typically deliver approximately

140 mg of flavonoids whilst other analysis  gives a figure of

125 mg / 235 ml brew. The longer the tea is left to brew, the higher the

concentration of flavonoids.

Tea flavonoids demonstrate antioxidant activity   and while not a replacement for

fruit and vegetables, the antioxidant activity of tea has been compared to that of

fruit and vegetables in a number of studies. One study concluded that at the typical

  daily consumption of 3 cups a day,  tea has approximately the same antioxidant

power as eating six apples. Another study found that one or two cups of tea has

the same ‘radical scavenging capacity as five portions of fruit and vegetables or

400mg vitamin C equivalents.

Health benefits of tea flavonoids

For many years it has been known that the plant polyphenols are antioxidant in

vitro, in fact many common flavonoids are several times more potent than Vitamin C

or E. This growing interest in the antioxidant activity of phenolic compounds has

led to increased research into their potential health benefits ,as already mentioned above.

The antioxidant activity of tea flavonoids may account for the results of a number of

epidemiological studies suggesting that they may have a protective role in

conditions  as noted below :

-Cardiovascular disease.

-Cancer.

-In vitro studies have demonstrated that the initiation stage of cancer can be

prevented by the action of tea flavonoids.

-Tea polyphenols have been shown to inhibit DNA synthesis of leukaemia

cells and lung carcinoma cells.

-Animal studies have shown that tea and its flavonoids protect against

many types of cancer e.g. skin tumors in mice, lung cancer in mice.

and digestive cancer in mice and rats.

-Antibacterial effects.

-Tea extracts exhibit inhibitory effects against Salmonella typhi.

Campilobacter jejuni, Campilobacter coli, Helicobacter pylori, Shigella,

Clostridium, Pseudomonas, Candida and others.

-Dental Caries

-Green tea and various catechins have exhibited inhibitory effects on the

growth of cariogenic bacteria by preventing the adherence and growth of

bacteria at the tooth surface.

Absorption of tea flavonoids 

Until recently the majority of  research demonstrating the antioxidant activity of

tea flavonoids was either using animal models or laboratory cellular studies.

Emerging evidence is concluding that the body does in fact absorb some of these

Antioxidants, e.g., when green tea extract is consumed by healthy human

volunteers, various catechins are found in the plasma in a dose-dependent

concentration varying between 0.2-2.0% of the ingested amount, with a maximum

concentration being achieved 1.4 to 2.4 hours after consumption. Some studies

have shown that plasma antioxidant activity peaks 30-60 minutes after tea

consumption and returns close to baseline by 90 minutes. Further research is

currently being undertaken on the metabolism, distribution and excretion of tea

flavonoids and its metabolites.

The addition of milk to tea, as enjoyed by the majority of people, does

not appear to affect the bioavailabilty of the tea flavonoids.

Conclusion

It is well known that fruit and vegetables are good sources of antioxidants. However,

what is less well known is the amount of antioxidants present in tea. The major

group of antioxidants in tea are flavonoids that appear to be readily digested, absorbed and metabolised by the body. There is a wealth of evidence demonstrating that tea and

flavonoids exhibit beneficial effects in animal and in vitro studies and provide a

promising area of research.

So as well as eating more fruit and vegetables, antioxidant intake can be topped up

by drinking more tea, helping to promote overall health and well-being.

Acknowledgement  : http://www.teahealth4.com and the references cited therein.