Green tea, native to China and India, has been consumed and hailed for its health benefits for centuries globally, but has only recently gained popularity in the US. Tea is considered the most consumed beverage in the world behind water, however 78% of the tea consumed worldwide is black and only about 20% is green. All types of tea except herbal tea are brewed from the dried leaves of the Camellia sinensis bush. The level of oxidation of the leaves determines the type of tea. Green tea is made from un-oxidized leaves and is one of the less processed types of tea (with white tea the least) and therefore contains one of the most antioxidants and beneficial polyphenols. Green tea was used in traditional Chinese and Indian medicine to control bleeding and heal wounds, aid digestion, improve heart and mental health and regulate body temperature. Recent studies have shown green tea can potentially have positive effects on everything from weight loss to liver disorders to type 2 diabetes. This MNT Knowledge Center feature is part of a collection of articles on the health benefits of popular foods.
Unsweetened brewed green tea is a zero calorie beverage. The caffeine contained in a cup of tea can vary according to length of infusing time and the amount of tea infused. In general, green tea contains a relatively small amount of caffeine (approximately 20-45 milligrams per 8 oz cup), compared with black tea which contains about 50 milligrams and coffee with 95 milligrams per cup. Green tea is considered one of the world's healthiest drinks and contains one of the highest amount of antioxidants of any tea. The natural chemicals called polyphenols in tea are what are thought to provide its anti-inflammatory and anti-carcinogenic effects. Epigallocatechin-3-gallate (EGCG) is the most studied and bioactive polyphenol in tea and has been shown to be the most effective at eliminating free radicals. Green tea is approximately 20% to 45% polyphenols by weight, of which 60% to 80% are catechins such as EGCG.
According to the National Cancer Institute, the polyphenols in tea have been shown to decrease tumor growth in laboratory and animal studies and may protect against damage caused by ultraviolet UVB radiation. In countries where green tea consumption is high cancer rates tend to be lower, but it is impossible to know for sure whether it is the green tea that prevents cancer in these specific populations or other lifestyle factors. One large-scale clinical study compared green tea drinkers with non-drinkers and found that those who drank the most tea were less likely to develop pancreatic cancer, particularly women, who were 50% less likely to develop the disease. Studies have also shown the positive impacts of green tea on breast, bladder, ovarian, colorectal, esophageal, lung, prostate, skin and stomach cancer. Researchers believe that it is the high level of polyphenols in tea that help kill cancerous cells and stop them from growing, however the exact mechanisms by which tea interacts with cancerous cells is unknown. Other studies have shown a lack of preventative effects of tea on cancer. The amount of tea required for cancer-preventive effects has also varied widely in studies - from 2- 10 cups per day. In 2005, the FDA stated that "there is no credible evidence to support qualified health claims for green tea consumption and a reduced risk of gastric, lung, colon/rectal, esophageal, pancreatic, ovarian, and combined cancers.
A 2006 study published in the Journal of the American Medical Association concluded that green tea consumption is associated with reduced mortality due to all causes, including cardiovascular disease. The study followed over 40,000 Japanese participants between the ages of 40 and 79 for 11 years, starting in 1994. The participants who drank at least 5 cups of green tea per day had a significantly lower risk of dying (especially from cardiovascular disease) than those who drank less than one cup of tea per day. Another study found that consuming 10 cups of green tea per day can lower total cholesterol, however, consuming 4 cups or less had no effect on cholesterol levels.
Green tea may promote a small, non-significant weight loss in overweight and obese adults; however, since the weight lost in the studies were so minimal, it is unlikely that green tea is clinically important for weight loss. Other studies have found that green tea is helpful in preventing dental cavities, stress, chronic fatigue, treating skin conditions and improving arthritis by reducing inflammation.
Green tea is available bottled and sweetened with sugar or an artificial sweetener, in single tea bags, loose-leaf, and in instant-powder. Green tea supplements are sold in capsule form or liquid extracts. According to 2010 research presented at the American Chemical Society, bottled teas are not equivalent to brewed teas as some 16-oz bottled teas can contain fewer polyphenols than one cup of brewed tea. Green tea extract ointments have been approved by the FDA to topically treat genital warts.
There are little to no known side effects or contraindications to drinking green tea for adults. Those with severe caffeine sensitivities could experience insomnia, anxiety, irritability, nausea or upset stomach. Those taking anticoagulant drugs such as Coumadin/warfarin should drink green tea with caution due to its vitamin K content.
If taken with stimulant drugs, green tea could possibly increase blood pressure and heart rate. Green tea supplements however, contain high levels of active substances that can trigger side effects and interact with other herbs, supplements, or medications.4 Green tea supplements are unregulated by the FDA and may also contain other substances unsafe for health or with unproven health benefits. Always check with a physician before starting any herb or supplement regimen. In particular, pregnant or breastfeeding women, those with heart problems or high blood pressure, kidney or liver problems, stomach ulcers, or anxiety disorders should not take green tea supplements or extracts.