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Both black and green teas are high in flavonoids, a type of chemical found in all plants that helps to protect them from harm. Over 5,000 different flavonoids have been identified, but only about 20 are common in our diet. Green tea contains a type of flavonoid called "catechins," while black tea contains "theaflavins" and "thearubigens." Although both black and green tea comes from the same plant, Camellia sinensis, the oxidation of black tea during processing creates different flavonoids than those found in green tea, which does not go through the oxidation process.
Luckily, all these flavonoids appear to be beneficial to health. Flavonoids may act as antioxidants, helping to prevent damage to cells from free radicals. Free radicals are byproducts of normal metabolism, but can also be caused from exposure to pollution, smog, cigarette smoke, pesticides or drugs. Brewing tea for five minutes will extract about 80% of the antioxidant capacity of the flavonoids. Instant tea and bottled teas are much lower in antioxidant capacity than fresh-brewed tea from either loose leaves or tea bags.
Tea provides about 22% of the total intake of flavonoids in the U.S. diet, almost as much as fruits (28%) and vegetables (26%). It is easy to incorporate into any diet, has no calories when prepared without milk or sugar, and tastes great. Ideally, both black and green teas should be consumed on a regular basis to ensure intake of the full range of tea's different flavonoids. Less research has been done on white and oolong teas, but they too are high in flavonoids and have been shown to have similar health benefits.
There is not enough information to give a precise recommendation about how much tea to drink per day. Some studies have shown benefits at one cup per day, while other benefits don't occur until ten cups per day. It will take a lot more research to answer these questions with precision. But almost everyone can add a few cups of tea a day without concern. For those who need no caffeine, the good news is that decaffeinated tea is almost as high in flavonoids as regular tea.
Cardiovascular Benefits (www.teahealth.co.uk/th/facts/4.htm)
Consumption of both black and green teas is generally associated with a lower risk of heart disease in Europe, the US, and Japan. The mechanism for this protection is unknown. It has been speculated that the antioxidant activity of chemicals in tea called flavonoids may be at least partly responsible. Black and green teas each contain different flavonoids that appear to be beneficial to health (www.teahealth.co.uk/th/facts/3.htm). Antioxidants may help to decrease the risk of atherosclerosis by preventing lipid oxidation in the blood, making the lipids less likely to stick to artery walls. One recent study showed tea consumption appears to improve the function of the lining of the blood vessels. This means that arteries have a better capacity for expansion and contraction as needed, and will be less prone to developing plaques and lesions associated with heart disease. Vitamin C, another antioxidant, has similar effects. Abstract Link
While test tube studies consistently show these flavonoids to have a high antioxidant capacity, human studies do not always show the same activity in the body. But human studies are conducted in a very different manner from test tube studies, and it is difficult to compare results. Tea preparation methods and intake must be carefully measured, and factors such as lifestyle and socioeconomic status must be taken into consideration. It is also important to realize that human subjects rarely receive a dose of tea equivalent to that used in animal or test tube studies. Interestingly, there is evidence that tea flavonoids can accumulate in the body to levels comparable to those used in test tube studies. Abstract Link
You may have read recently that tea intake is associated with high levels of plasma homocysteine, a compound associated with increased risk of heart disease. The study that found these results had 20 subjects who were given the equivalent of 4 cups of strong black tea per day for seven days. Homocysteine levels increased an average of 11%. But the effect of caffeine was not evaluated, and other larger studies have shown that caffeine intake is related to homocysteine levels. When caffeine and/or coffee intake is corrected for, tea is actually associated with lower levels of homocysteine. This area obviously requires more study to tease out the complexities of these relationships.
Other interesting findings:
In laboratory tests, green and black teas lower serum cholesterol levels in animals.
Black and green teas inhibit platelet aggregation in dogs and help to prevent clot formation.
Incidence of heart attack deceases 11% with 3 cups of tea per day (abstract link:
Green and black teas slow the development and progression of atherosclerosis
Cancer prevention
In animal tests, both black and green tea and tea preparations have repeatedly been found to reduce tumor formation for cancers of the skin, lung, oral cavity, esophagus, stomach, small intestine, colon, liver, pancreas, and prostate. Although the antioxidant activity of tea is often reported as the mechanism behind these results, there is not strong evidence that this is the case. Tea flavonoids have been shown to have other properties besides their antioxidant activity that protect against cancer, such as
Inhibiting new blood vessel growth (tumors can't survive without a good blood supply)
Promoting death of damaged cells that may turn cancerous, without harming normal cells
Preventing cancer cells from multiplying
Interestingly, caffeine has also been shown to promote death of damaged cells, so care must be taken to separate the effects of tea from the effects of caffeine.
Human epidemiological studies, however, do not consistently show a reduced rates of cancer in populations that drink a lot of tea. Possible explanations for these mixed findings include
Differences in types of tea consumed and tea preparation among different populations
Lifestyle factors that are difficult to control for
Differences in reporting tea intake from culture to culture (for example, cups per day vs. dry weight of tea leaves used per week)
Intervention studies are scarce, but needed.
Skin Cancer
In contrast to the above, both human and animal studies consistently show that tea has a protective effect when applied topically or when consumed.
Application of green tea extract prior to exposure to UVA and UVB radiation reduces the damage to skin cells.
Consumption of black and oolong teas has been linked to reduced risk of skin cancer and improvement of skin lesions
Weight loss
Green tea appears to increase thermogenesis, or body heat production, which can cause weight loss. This effect used to be attributed to caffeine, but it seems that the effect is more likely due to the combination of caffeine and catechins in green tea.
Anti-Inflammatory/Anti-Arthritis
Recent animal studies suggest that green tea may have anti-inflammatory properties. Mice fed green tea experienced delayed onset and decreased severity of collagen-induced arthritis.
Results from a test tube study suggest green tea catechins decrease the movement of pro-inflammatory white blood cells.
Liver Protection
Green tea catechins have been shown to protect the liver against toxins such as alcohol and cigarette smoke.
A green tea extract has been used since 1976 to treat hepatitis in humans. Cyanidanol-3 (CatergenTM) activates T-cells, macrophages and natural killer cells, thus enhancing the body's immune response.
Dental Health (www.teahealth.co.uk/th/facts/9.htm)
Dental cavities are caused by a combination of oral bacteria, sugars, and acid. Tea drinking can help reduce cavities through several mechanisms:
The tea plant naturally contains fluoride in its leaves which makes tooth enamel stronger and more resistant to acids
Tea inhibits bacteria growth in the mouth
Tea helps to prevent bacteria from sticking to the tooth surface
Tea helps to decrease the acid production of oral bacteria
Tea decreases the activity of the oral enzyme amylase, which is responsible for breaking down starch into sugar in the mouth.
Caffeine (www.teahealth.co.uk/th/facts/7.htm)
Tea contains, roughly, 40 mg caffeine per 6 oz. cup, compared to brewed coffee at 100-150 mg.
It is generally agreed that, for most people, moderate caffeine consumption is harmless. Many people find that caffeine boosts mental clarity and alertness in a pleasing way. However, since caffeine is a cardiac stimulant, people with certain heart conditions should avoid caffeine, and excessive caffeine intake can cause restlessness, insomnia, and anxiety. Women should avoid caffeine throughout pregnancy and while nursing.
For more detail on caffeine content of various foods and beverages and general FAQs, see www.cs.unb.ca/~alopez-o/Coffee/caffaq.html.
(For a recent review of literature on tea and health, see Yang C. S. and Landau, J. M. Effects of tea consumption on nutrition and health. J. Nutr. 130:2409-2412, 2000)
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