Epidemiological and laboratory studies have linked green tea to reduced risks of breast, lung, prostate and ovarian cancer. As a result, green tea sales have been steadily increasing in Europe and the US.
Breast cancer is the most common form of cancer for European women. Over one million new cases are diagnosed every year worldwide, with the highest rates in the Netherlands and the US. China has the lowest rates.
The anti-cancer effects of tea 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.
The four primary polyphenols found in fresh tealeaves are epigallocatechin gallate (EGCG), epigallocatechin, epicatechin gallate, and epicatechin.
The meat-analysis, published on-lined in the journal Carcinogenesis (doi: 10.1093/carcin/bgi276), draws on three cohort and one population-based study for green tea, while five cohort and eight case-control studies were analysed for a link between black tea and breast cancer.
"Overall summary odds ration [a measure of risk] showed an approximately 20 per cent statistically significant reduction in risk of breast cancer associated with high intake of green tea," reported lead author Can-Lan Sun from the University of Minnesota.
No such protective effect was found for black tea. Indeed, the results were contradictory: the cohort studies showed a small increased risk of breast cancer, while the case-control studies showed a small decreased risk.
"We speculate that the overall cohort findings are compatible with the notion of black tea intake having a late-stage, promotional effect on breast cancer," said the researchers.
The protection from breast cancer by tea is related specifically to the catechin content, say the researchers. EGCG and epigallocatechin have been shown to be promote apoptosis (programmed cell death).
"Hence, if the beneficial effect of tea on breast cancer risk comes maily from the tea catechines, an explanation for the relative lack of risk reduction associated with black tea drinking can be due to the much lower level of catechins in black tea compared to green tea (up to 10-fold difference in catechin contents)," wrote Sun.
It seems unlikely that this meta-analysis will have any effect on the FDA ruling. The researchers themselves admit: "the number of published studies on this topic is too small for the results to be conclusive."
The scientists make two calls to help simplify the situation. Firstly, long term exposure to a wider range of green tea, and long term follow-up are needed to clarify if black tea protects or promotes breast cancer development at different stages.
This view was supported by Dr Julie Sharp, science information manager at Cancer Research UK, who said that more studies are needed before the effects can be known for sure.
Sharp told NutraIngredients.com however that the growing body of science does point towards a benefit: "Green tea is reported to have many health benefits and a number of studies have suggested it could even help to prevent certain cancers."
The global tea market is worth about €790 (£540, $941) million. Green tea accounts for about 20 per cent of total global production, while black tea (green tea that has been oxidized by fermentation) accounts for about 78 per cent.