Tea 26 August 2009
There are many potentially healthy components in tea. However most attention is on the antioxidant polyphenols. Catechin polyphenols are potent antioxidants and anti-inflammatories.
All tea is made from the leaf of the plant Camellia sinensis. It is grown mostly in China, Japan and India. Harvesting time and processing are the chief determinants of its smell, taste and health properties. Black, green and oolong are the most common varieties. Tea is second only to water as the world’s most popular drink.
Green teas are not fermented. The leaves are sun-dried and pan roasted. This preserves useful antioxidants. Oolong teas are partially fermented. The leaves are bruised and oxidised to a brownish colour. Black teas are fully fermented by drying and oxidation. This enhances the flavour but the active polyphenols are oxidised and much of the health benefit is lost. Scented teas are made by adding natural oils or petals such as jasmine to any of the teas. Herbal teas do not have the same health properties as tea derived from Camellia sinensis.
Remember that the shelf life of a bag of quality, unopened green tea is about six months. Once it is opened the shelf life is around three months. Store the tea in an airtight container in a cool location (the refrigerator is appropriate).
Tea, and especially green tea is increasingly being shown to have powerful protective properties against both cancer and cardiovascular disease. Green tea is especially important for the prevention of breast, ovarian, skin and prostate cancer. An Australian study of Chinese hospital patients found that daily green tea consumption enhanced the survival of patients with ovarian cancer. Three years after diagnosis, 78% of women who drank at least 1 cup of green tea per day had survived versus 48% in the nondrinkers. A study reported in the December 2005 edition of the journal Archives of Internal Medicicine found that women who drank tea every day have a reduced incidence of ovarian cancer. The study followed over 61000 women aged 40-76 for 17 years. During that time, 301 women were diagnosed with ovarian cancer. Women who regularly drank 2 cups of tea per day had a 46% lower risk of developing ovarian cancer than women who didn’t drink tea.
A study in the December 2005 the edition of the Journal of Nutrition has found that mice drinking green tea have a reduced risk of skin cancer. Two groups of mice were exposed to ultraviolet B radiation three times a week for 24 weeks. Group 1 was given water and group 2 was given green tea to drink. The incidence of skin tumours was reduced by 35%, the number of skin cancers was reduced by 63% and tumour growth was decreased by 55% in the green tea group. Chemicals in green tea decrease the ability of tumours to form new blood vessels which is crucial for their growth. Green tea also increases the number of immune cells which fight cancer.
A study reported in the journal Cancer Research showed that green tea extract markedly reduced the progression of high grade prostate intraepithelial neoplasia (HGPIN) to prostate cancer. Sixty men with HGPIN either received a placebo or 3 200mg capsules of green tea extract. After one year, 1/30 of the green tea group had progressed to cancer while 9/30 in the placebo group had progressed. The green tea group also had a marked improvement in benign prostatic hypertrophy symptoms and improved quality of life scores. Research published in the December 2004 Cancer Research journal showed that mice fed green tea polyphenols for six months had substantially reduced blood levels of prostate cancer markers as well as markers for metastasis and new blood vessel formation (which is necessary for the rapid growth of cancers).
As well as the polyphenols, tea also contains a valuable amino acid called theanine. Theanine is found almost exclusively in tea. It appears to balance the stimulatory effect of caffeine by inducing relaxation. Studies have shown that theanine lowers blood pressure in people with hypertension (but not in people with normal blood pressure). It has also been shown to reduce oxidation of LDL cholesterol. A study of 6000 Japanese women has shown those who consume more than 5 cups of green tea a day (remember that’s the small Japanese cups, not our huge mugs) were significantly less likely than non-tea drinkers to suffer a stroke.
Click here to read the text of a television interview with Dr Greg Emerson on the health benefits of green tea.