Tea and Antioxidant properties



Introduction

Increasing evidence is highlighting the role antioxidants may have in protecting against certain conditions such as heart disease, stroke, and cancers. It has been proposed that the mechanisms leading to the diseases may be promoted by free radicals and that antioxidants may 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 polyphenols, found in fruits, vegetables, nuts, cereal and drinks such as tea and red wine.

Free radicals explained

Free radicals are unstable molecules that include the hydrogen atom, nitric oxide (NO) and molecular oxygen (O2). These naturally occur in the body as a result of chemical reactions during normal cellular processes. They can also be formed in response to excessive pollution, too much UV sunlight and exposure to cigarette smoke. In an attempt to stabilize, they attack other molecules in the body potentially leading to cell damage and triggering the formation of another free radical 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.

Protective mechanisms 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 localized oxygen concentration thereby reducing molecular oxygen's oxidation potential
- Metabolizing lipid peroxides to non-radical products
- Chelating metal ions to prevent the generation of free radicals

In this way antioxidants limit the free radical damage from- - Oxidizing Low Density Lipoprotein (LDL) cholesterol, which may increase the risk of ahtersclerosis
- 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 vasodilatatioin in response to nitric oxide, a potential mechanism for heart disease and cancer
- Triggering inflammation
- Impairing immune function
Some antioxidants are synthesized within the cells themselves (endogenous) and others need to be provided in the diet.

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.

Tea has one of the Highest total flavonoid contents of all plants at 15% of the leaf by dry weight and is also the major source of flavonoids in the UK diet, providing approximately 80% of dietary flavonoids for the population as a whole.

Tea flavonoids

The types and amounts of flavonoids present in tea will differ dependent on the variety of leaf, growing environment , processing, manufacturing, particular size of ground tea leaves and infusion preparation typically 93% of the total tea phenolic compounds are flavonoids. Green teas contain more of the simple flavonoids called catechins, while the oxidization that the leaves undergo to make black tea converts these simple flavonoids to the more complex varieties called theaflavins and thearubigins.

Tea flavonoids are water-soluble and one study has shown that a cup of UK tea that has been allowed to brew for 40-60 seconds will typically deliver approximately 140mg of flavonoids whilst a second carried out by the UK Tea Trade Technical Committee using typical UK consumer brewing conditions and encompassing the range of blends and bag weights commonly on sale in the UK gives a figure of 125mg 235ml serving. 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 UK 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 e. g.

-Heart Disease and Stroke
-Several reports indicate that tea flavonoids inhibit the oxidation of LDL cholesterol in vitro
-A reduction in blood lipids has been demonstrated in animal studies
-Certain tea flavonoids exhibit anti-inflammatory actions in animals
-Athersclerosis is a disease with a strong inflammatory component
-Improvements in blood vessel function, specifically the vascular endothelium, has been seen in patients with established CHD
-Several in vitro studies and one human trial have found that platelet aggregation can be inhibited by various flavonoids.

The antioxidant activity of tea flavonoids may account for the results of a number of epidemiologocal studies suggesting that they may have a protective role in conditions such as 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 leukemia cells and lung carcinoma cells
-animal studies have shown that tea and its flavonoids protect against may 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 Campylobacter jejuni, Campylobacter coli, Helicobacter pylori, Shigella, Clostridium. Pseudomonas, Candida and others -Dental Caries
-Green tea and various catechins have exhibited inhibitory effects on the growth of cryogenic bacteria by preventing the adherence and growth of bacteria at the tooth surface.

Absorption of tea flavonoids

Until recently the majority of the 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 the UK population, does not appear to affect the bioavailability of the tea flavonoids.

In summary´┐Ż

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 digested, absorbed and metabolized 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 for the future human studies.

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.

References

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