Green tea, derived from infusion of Camellia sinensis leaves, has more antioxidants and phytochemical components than black or oolong tea because it undergoes less processing than these other types. The effects of green tea on cancer prevention have been well-studied in laboratory research, but they remain under investigation in human clinical trials.
Mechanisms
Compounds called catechins, which are a type of phytochemical, are believed to be behind the potential cancer-protective effects of green tea. The major catechin in green tea is epigallocatechin gallate, or EGCG. Other catechins found in green tea include epicatechin, epigallocatechin and epicatechin gallate. These catechins act as antioxidants and also directly affect cellular processes. Caffeine in green tea may also have anti-cancer properties caused by its ability to modulate cellular enzymes involved in the development of cancer.
Specific Cancers
Green tea has been linked not only to the prevention of cancer in general, but more specifically to individual types of cancer. The consumption of green tea has been linked to a reduction in the risk of a range of female cancers, including breast cancer and ovarian cancer. Bladder, colorectal, mouth, esophageal, pancreatic, skin, stomach, prostate and lung cancer may also be prevented through green tea consumption.
Population Studies
The results of population studies involving the impact of green tea on cancer prevention have been mixed. While cancer rates tend to be lower in countries where green tea consumption is high, such as Japan, this effect may also be related to other dietary factors. Many studies have shown promise for the use of green tea in preventing cancer, but other studies have failed to find an association. A 2009 review of 51 different studies published in the journal "Cochrane Database Systems Review" found no conclusive evidence for the overall cancer-preventative activity of green tea. However, the authors emphasize that further studies and analysis should be done.
Laboratory Studies
In laboratory studies, catechins showed antioxidant activity, destroying free radicals before they could cause cellular damage. Catechins also reduce the size and occurrence of tumors in laboratory animals and cell culture. The catechins and caffeine in green tea have been studied independently as well as the overall effect of green tea or green tea extracts.
Recommendations
While the link between green tea and cancer remains under investigation, drinking this beverage seems to be generally safe. The amounts administered in laboratory studies are reasonable, equivalent to a few cups a day in most studies, so this level of consumption should not be problematic for most people. Three to five cups of green tea daily, providing 250mg of catechins, is a safe and potentially beneficial amount.
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