"These findings extend related observations on the anticarcinogenic potential of tea ingredients and suggest that consumers may benefit more by drinking both green and black teas," wrote lead author Mendel Friedman from the USDA.
The health benefits of tea ranging from a lower risk of certain cancers to weight loss, and protection against Alzheimer's, have been linked to the polyphenol content of the tea. Green tea contains between 30 and 40 per cent of water-extractable polyphenols, while black tea (green tea that has been oxidized by fermentation) contains between 3 and 10 per cent. Oolong tea is semi-fermented tea and is somewhere between green and black tea.
The four primary polyphenols found in fresh tealeaves are epigallocatechin gallate (EGCG), epigallocatechin, epicatechin gallate, and epicatechin.
Friedman and his co-workers from Keimyung, Uiduk, and Yeungnam Universities in South Korea investigated the ability to induce cell death in human cancer and normal cells nine green tea catechins, three black tea theaflavins, and theanine extracted using either an as aqueous and or an ethanol(80 per cent)/water method.
"Because tumour promotion may be the only reversible event during cancer development, its suppression is regarded as an effective way to inhibit carcinogenesis," said Friedman.
The researchers report that the majority of the catechins, theaflavins, theanine, and all the general tea extracts decreased the numbers of human breast (MCF-7), colon (HT-29), hepatoma (liver) (HepG2), and prostate (PC-3) cell lines, with the ethanol/water extracts found to contain higher levels of flavonoids and to have a higher activity.
"The anticarcinogenic effects of tea compounds and of tea leaf extracts varied widely and were concentration dependent over the ranges from 50 to 400 microg/mL of tea compound and from 50 to 400 microg/g of tea solids," wrote the researchers.
However, the flavonoid levels of the teas were not found to correspond with the anti-cancer activities.
The researchers did not undertake a mechanistic study, but state that previous studies have suggested the flavonoids can induce programmed cell death (apoptosis), stop P450 enzymes that activate pro-carcinogens, stop the transmission of signals by tumour promoters, bind to damage DNA usually involved in cancer promotion, or inhibit the formation of new blood vessels in the tumour (angiogenesis).
Whether the tea compounds work independently, additively or synergistically merits further study, said the researchers.
"Because it may be too risky to translate results from cell assays to in vivo effects, the observed destruction of a broad range of cancer cells suggests the need for animal and human studies designed to ascertain whether the observed wide variation in potencies of tea compounds and teas can predict corresponding effects in vivo," they said.
Source: Journal of the Agricultural and Food Chemistry Published on-line ahead of print: ASAP Article doi: 10.1021/jf062276h S0021-8561(06)02276-X "Structure-Activity Relationships of Tea Compounds against Human Cancer Cells" Authors: M. Friedman, B.E. Mackey, H-J. Kim, I-S. Lee, K-R. Lee, S-U. Lee, E. Kozukue, and N. Kozukue