Tea is a decoction of leaves from the plant Camellia sinensis. Steaming the leaves, as in green tea preparation, maintains much of the original flavonoid content of the plant, which includes high concentrations of catechins (flavan-3-ols). In contrast, black tea preparation involves fermentation of the leaves which converts the majority of catechins to related tannins. Oolong tea is partially fermented and therefore can be considered halfway between black and green tea in chemical composition. This is important nutritionally because catechins are bioavailable and possess antioxidant ability in vivo. The high content of antioxidant flavonoids in green tea may explain the anticancer effects reported in observational studies. In particular tea shows inverse association with cancers of the intestinal tract in humans. This may suggest that the antioxidants work within the gut tissue itself prior to absorption by inhibiting the oxidation reactions that may initiate cancers.
For example, in one study1, researchers investigated the association between regular consumption of tea and the risk of digestive tract cancers in Chinese women, using a prospective cohort study design. Subjects included did not drink alcohol or smoke tobacco, and reported drinking tea more than 3 times a week for more than 6 months. Most of the tea drinkers consumed green tea, which is more commonly consumed in China compared to black tea. The results showed that at the 11 year follow-up, there were 1255 cases of cancer amongst the 69310 participants. Those women who consumed tea regularly had a reduced risk of all digestive system cancers (hazard ratio 0.86) compared to women who never consumed tea. This risk decreased significantly for the women as the amount of tea and duration of consumption increased. The inverse association between digestive system cancers and tea consumption were strongest for stomach, oesophageal and colorectal cancers.
Women consuming over 150 grams of tea per month had a 21 % reduced risk of developing intestinal cancers. These results supports growing body of evidence that tea consumption, particularly green tea consumption, can protect from certain cancers of the gut. The most significant effects for tea are seen in studies that specifically investigate intestinal cancers. It is unlikely that one component of tea explains the beneficial effects on cancer, but rather that a number of components act synergistically following consumption. Because green tea seems to show the strongest protective effects, it is likely that the catechin flavonoids are responsible for cancer protection. In particular the flavan-3-ols (-)-eipigallocatechin-3-gallate (EGCG), (-)-epigallocatechin (EGC), (-)-epicatechin-3-gallate and (-)-epicatechin may inhibit oxidative damage to DNA through significant in vivo antioxidant effects. However, that this occurs in the gut may suggest that absorption is not required.
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