Tea is more and more consumed as a health promoting beverage. The health benefits of tea are mainly attributed to its polyphenols such as catechins, theaflavins and thearubigins. Catchins are flavanol monomers. The most important catechins are epicatechin, epigallocatechin, epicatechin gallate and epigallocatechin gallate . Fresh tea leaves contain naturally polyphenol oxidase enzymes. When the tea leaves are crushed these polyphenol oxidase enzymes are released and start to polymerize the phenols into flavanol dimers and the polymers theaflavins and thearubigins. Black tea leaves have undergone this oxidation process (also called fermentation although no real fermentation with bacteria is involved) and contain higher levels of theaflavins and thearubigins and lower levels of catechins. Green tea is first steamed to destroy the polyphenol oxydases. As a result green tea contains still high levels of catechins and lower levels of theaflavins and thearubigins. Green and black tea also contain the flavonols kaempferol, quercetin and myricitin.
The polyphenols act in three ways.
- Polyphenols act as direct antioxidants by neutralizing free radicals.
- They chelate transitional metals, such as iron and copper. Tea polyphenols can reduce LDL oxidation induced by cupper. It is unknown if tea polyphenols also chelate metal ions in vivo because metal ions are bound to proteins.
- Polyphenols regulate the activity and production of proteins and enzymes.
Polyphenols inhibit the redox-sensitive transcription factors, nuclear factor-B and activator protein-1. They also inhibit pro-oxidant enzymes (such as inducible nitric oxide synthase) and induce the production of phase II (such as glutathione S-transferases) and antioxidant enzymes (such superoxide dismutases). Green tea polyphenols can stimulate the production of phase II detoxification enzymes, which are responsible for the excretion of carcinogens. Glutathione S-transferases is such enzyme which catalyzes the reaction of glutathione with electrophiles, which can damage DNA and proteins. When macrophages are stimulated by bacterial endotoxins or other inflammatory cytokines they stimulate the production of inducible nitric oxide synthase. This enzyme produce nitric oxide which can damage DNA and proteins. In-vitro tests have shown that tea polyphenols can reduce the production of inducible nitric oxide synthase.
Many of the beneficial properties of tea polyphenols have been demonstrated in in-vitro studies. But because tea catechins are quickly metabolized it is important to determine the effect of oral administration of tea catechins in in-vivo studies. The plasma levels of tea catechins are normally 100 to 1000 times lower than those of other antioxidants, such as gluthatione or ascorbic acid, even when used at high intake of tea catechins. Animal studies allow testing the antioxidant properties of tea catechins when they are exposed to oxidative stress. For example, it is possible to determine the effects of tea catechins in animal models were DNA oxidation is induced with carcinogens.
The purpose of this study was to make an overview studies investigating the antioxidant activity of tea polyphenols in animals. In-vivo tests have demonstrated that intake of tea resulted in inhibition of oxidation of lipoproteins but there is some data that suggests that tea catechins can inhibit atherogenesis. Studies have shown that the intake of tea prevented decreases in tissue glutathione levels after the animals were exposed to oxidative stress, for example after treatment with carbon tetrachloride, intake of ethanol or infection with Mycobacterium tuberculosis. Studies with humans have not shown that tea consumption increases antioxidant concentrations or antioxidant enzyme activities. One study illustrated that mice fed with green tea extract in their drinking water had less accumulation cholesterol in their aorta. Results are consistent that the intake of tea polyphenols reduces oxidation of DNA induced by carcinogens.
The study concluded that in-vivo tests provide some evidence that tea polyphenols act as antioxidants. More studies are required to determine the in-vivo activity of tea polyphenols by using special biomarkers of the oxidative damage to DNA, lipoproteins and proteins.
Source: Balz Frei and Jane V. Higdon . Antioxidant Activity of Tea Polyphenols In Vivo: Evidence from Animal Studies . Journal of Nutrition. October 2003, 133:3275S-3284S