Green tea is considered an ultra-healthy beverage … drink it regularly and you’ll enjoy a long healthy life! But where is the scientific evidence to support this marvellous reputation?
There are three basic varieties of tea … green, black and oolong.
All are made from the leaves of the Camellia sinensis plant and the only difference between them is the processes used to make them.
Green tea is made by steaming the leaves quickly. To make black tea, the leaves are exposed to the air or fermented, which darkens the leaves and gives them flavour, while for oolong the tea leaves are only partially fermented.
The flavours of teas vary depending on where the plants are grown. Different growing conditions, horticulture methods, production processing, and harvesting times also affect flavours.
Teas from different areas can be combined to make teas with a particular flavour, called blended teas. English Breakfast Tea, for example, is a blend of Chinese, Ceylonese and Indian teas.
Herbal teas (such as chamomile and mint) are not really teas at all, as they are not made from the Camellia sinensis plant. Drinking herbal teas is not considered as healthful as drinking green, black or oolong teas.
Green tea and health
Green tea has long been associated with a long and healthy life in many Eastern cultures. Nowadays, extracts from the tea are used in beverages, health foods, and dietary supplements.
But does green tea actually contribute to health?
Free radicals are molecules that are damaged. These damaged molecules can, in turn, damage cells which may become cancerous. Antioxidants may reduce or prevent some of this damage.
Catechins are a type of antioxidant found in tea leaves. They are part of a family of molecules called flavonoids which have anti-oxidative and anti-carcinogenic functions.
It’s the flavonoids that give green tea its reputation as a healthful drink.
The average quantity of flavonoids in a cup of green tea is higher than the quantity found in the same volume of other healthy drinks, such as fresh fruit juices, wine or vegetable juices.
However, the quantity of flavonoids can vary widely between different teas and tea products.
Tea making and drinking
You get more antioxidants from freshly brewed tea, compared with other forms of the drink such as instant tea and decaffeinated tea.
However, to maximise the anti-oxidants in green tea, you need to steep the tea for at least three minutes; five minutes is ideal.
Most people in most countries drink their tea hot. America, of course, is the exception … about 85% of the tea drunk in the US is iced tea.
The problem is that iced tea often contains relatively small amounts of catechins compared with hot tea. This is due to the way iced tea is made.
Iced tea is usually made by boiling water to which tea is added. Once the tea has been stewed for about five minutes, the liquid is cooled by adding water to double its volume, after which it is refrigerated.
Adding water dilutes the concentration of catechins. To make sure that your iced tea contains the same amounts of antioxidants as your hot tea, allow for the dilution by adding 50% more tea than usual to the boiling water.
Research and health effects
Green tea contains a variety of enzymes, amino acids, carbohydrates, lipids, sterols, polyphenols, carotenoids, tocopherols, vitamins, caffeine and related compounds, phytochemicals and dietary minerals.
Over the last few decades it has been the subject of many scientific studies to determine the extent of its reputed health benefits.
There is some evidence suggesting that regular green tea drinkers may have a lower risk of developing certain types of cancer and heart disease. But nothing much has been proved conclusively through rigorously-conducted clinical trials.
Indeed, most of the claims made for the health benefits of green tea are based on analyses of its chemical composition, some in vitro experiments, and animal studies, rather than studies made with humans.
Cancer: a systematic review conducted in 2012 stated that the evidence that green tea can prevent cancer ‘is inadequate and inconclusive’.
The report did state however that there is some evidence that green tea can cause a reduction in certain types of cancer (ie, breast, prostate, and ovarian cancers).
However, there’s no hard evidence that drinking tea can prevent cancer in general and more research is needed.
Heart disease: some studies show that drinking green tea may curb several risk factors for heart disease, such as weight, blood pressure, and cholesterol.
However, America’s FDA (Food & Drug Administration) has refused to allow labels on packets of green tea to claim that the tea contributes to a healthy heart, on the grounds that this claim is not supported by credible scientific evidence.
Cholesterol: green tea may lower low-density lipoprotein and total cholesterol levels in the blood, according to several short studies. But it is not known whether the effects of green tea resulted in fewer deaths.
In addition, the evidence does not support the claim that green tea reduces the risk of coronary artery disease.
Diabetes: there is some evidence that green tea may help control blood glucose levels. However, this has not been widely tested in people and more research is needed.
As you can see, the healthful benefits of drinking green tea have not been proved in human trials to any degree of confidence, though some recent studies in Japan have found that the consumption of green tea does result in a decreased risk of many cancers, cardio-vascular disease, and dementia including Alzheimer’s.
Unnatural concentrations: one study found that the particular catechins found in green tea, if taken at extremely high doses, may damage DNA.
But to do so, consumption would have to be many hundreds of times greater than the amounts that could be obtained from drinking vast quantities of tea, which, for me, makes the finding pretty irrelevant.
Similar results from consuming unnatural concentrations of other antioxidants, such as vitamins E and C, have been obtained in human trials.
Adverse effects of green tea
My research has failed to uncover any adverse effects of drinking regular amounts of green tea. By regular amounts I mean a few cups a day.
However green tea does contain caffeine. As people with irregular heartbeats or anxiety disorders need to be cautious about the amount of caffeine they ingest, they should only drink moderate amounts of green tea, taking their other sources of caffeine into account.
The presence of caffeine also means, according to the American Dietetic Association, that pregnant or breast-feeding women should not drink more than one or two cups of green tea a day.
There is also some evidence that green tea may interfere with the actions of certain anti-cancer drugs, such as Bortezomib (Velcade) and other boronic acid-based proteasome inhibitors. If you are taking these drugs, you should consult your medical advisor about drinking green tea.
Conclusion
There seems to be absolutely no harm in drinking reasonable amounts of green tea … in fact, doing so may be very beneficial to your health, whether or not you are diabetic.
Green tea is also delicious and I find it helps me to relax. I drink at least one cup last thing every night and always sleep like a log afterwards.
