It's true that green tea can raise your metabolic rate, so you burn more calories, says David Nieman, DrPH, director of the human performance laboratory at Appalachian State University in Boone, N. Nieman, who has studied green tea extensively, says this effect is probably due to a combination of its caffeine and catechins—antioxidants that are plentiful in green tea and present in smaller amounts in some fruits, dark chocolate, and red wine.

But the effect is likely to lead to little, if any, change on the scale. And the evidence that supplements derived from green tea do much for weight loss is minimal. Some studies have found modest reductions in body weight associated with green tea supplementation, according to the National Institutes of Health Office of Dietary Supplements.

But other human trials have found no benefit. What's more, experts warn against taking the large amounts used in many studies—and recommended on many supplement labels. There have been reports of serious liver damage in people who used green tea supplements.

And one study published in in the journal Cancer Prevention Research found that postmenopausal women who'd taken green tea extract twice a day for a year as part of a breast cancer clinical trial had elevated liver enzymes—a sign of potential liver damage often seen in people who abuse alcohol.

Hoofnagle, MD, director of the Liver Disease Research Branch at the National Institutes of Health. The potential risk of liver damage is one reason green tea extract is included on CR's list of 15 supplements you should never take.

Still, when it comes to losing weight, working a few cups of brewed green tea into your diet isn't necessarily a bad idea. If you don't add sugar, it's calorie-free and can be a good replacement for sugary drinks.

Consumption of up to eight cups a day is believed to be safe, according to NIH, though people that are pregnant or breastfeeding should not have more than six cups daily to limit caffeine intake. Compounds called catechins are often credited for green tea's cancer-fighting potential.

These antioxidants have the ability to block the action of molecules called free radicals, which can cause changes in healthy cells that sometimes lead to cancer. But there's no solid proof yet that the catechins in green tea help to prevent cancer in humans, despite some promising studies in test tubes and animals.

In the Cochrane Library published a review of 51 studies that included a total of more than 1. Each study investigated whether consuming green tea reduced the risk of developing specific types of cancer, including those of the breast, colon , oral cavity, liver, pancreas, and prostate.

After scrutinizing the data, Boehm says, the evidence of a benefit wasn't strong enough to recommend using green tea to protect against cancer. In a update to that review , the authors concluded that the evidence for a beneficial effect against cancer remains limited, with inconsistent results between studies.

Some data suggests that green tea can reduce the risk of cardiovascular disease —a narrowing of the arteries that can lead to heart attack, chest pains, or stroke. For example, in a study of more than 40, adults, researchers in Japan found that those who consumed five or more cups of green tea per day were 26 percent less likely to die of cardiovascular disease over an year period than those who drank just one cup per day.

Similarly, a study of 76, people published in the Journal of Epidemiology and Community Health in found that women who drank one to six cups of green tea per day had a reduced risk of dying of cardiovascular disease over an average of 13 years compared with those who drank none.

How might green tea protect the heart? Experts say it may reduce high cholesterol levels and high blood pressure , which are important factors in cardiovascular disease. But while this is a potential benefit, it's not conclusive, and high doses of green tea can reduce the effectiveness of the drug nadolol, used to treat high blood pressure and heart problems, cautions NIH.

To reduce your risk of diseases such as cancer and heart disease as well as keep your weight in check , he says, you need to eat a healthy diet that includes plenty of fruit, vegetables, and whole grains.

You should also exercise and avoid smoking. The bottom line: From what we know now, drinking a few cups of green tea a day is a perfectly healthy habit as long as it's not loaded with sugar and may have a few small benefits. Although many studies have been done on green tea and its extracts, definite conclusions cannot yet be reached on whether green tea is helpful for most of the purposes for which it is used.

What Have We Learned? Green tea contains caffeine. Drinking caffeinated beverages throughout the day seems to prevent a decline in alertness. One study looked at the effect of taking only a main component of green tea—EGCG—on mental capabilities.

The FDA has approved a specific green tea extract ointment as a prescription drug for treating genital warts. Studies of green tea and cancer in people have had inconsistent results.

The National Cancer Institute does not recommend for or against using green tea to reduce the risk of any type of cancer. A small number of studies suggests that both green and black tea might have beneficial effects on some heart disease risk factors, including blood pressure and cholesterol.

The research has limitations though, including how the data was evaluated and differences in study populations, so no definite conclusions have been reached. The National Center for Complementary and Integrative Health NCCIH is funding research on green tea and its extracts, including studies on new forms of green tea extracts for preventing symptoms of inflammatory bowel disease and for lowering cholesterol.

What Do We Know About Safety? Green tea, when consumed as a beverage, is believed to be safe when used in amounts up to 8 cups per day. Keep in mind that only the amount of added caffeine must be stated on product labels and not the caffeine that naturally occurs in green tea.

Drinking green tea may be safe during pregnancy and while breastfeeding when consumed in amounts up to 6 cups per day no more than about mg of caffeine.

Drinking more than this amount during pregnancy may be unsafe and may increase the risk of negative effects. Green tea may also increase the risk of birth defects associated with folic acid deficiency. Caffeine passes into breast milk and can affect a breastfeeding infant.

Although uncommon, liver problems have been reported in a number of people who took green tea products, primarily green tea extracts in pill form. People with liver disease should consult a health care provider before taking products with green tea extract.

People taking green tea extracts, especially those with liver disease, should discontinue use and consult a health care provider if they develop symptoms of liver trouble such as abdominal pain, dark urine, or jaundice.

Green tea is an ingredient in many over-the-counter weight loss products, some of which have been identified as the likely cause of rare cases of liver injury. Green tea at high doses has been shown to reduce blood levels and therefore the effectiveness of the drug nadolol, a beta-blocker used for high blood pressure and heart problems.

It may also interact with other medicines. Keep in Mind. Take charge of your health—talk with your health care providers about any complementary health approaches you use. Together, you can make shared, well-informed decisions. For More Information. Using Dietary Supplements Wisely Know the Science: How Medications and Supplements Can Interact Know the Science: How To Make Sense of a Scientific Journal Article.

NCCIH Clearinghouse The NCCIH Clearinghouse provides information on NCCIH and complementary and integrative health approaches, including publications and searches of Federal databases of scientific and medical literature.

Toll-free in the U. gov Email: info nccih. PubMed® A service of the National Library of Medicine, PubMed® contains publication information and in most cases brief summaries of articles from scientific and medical journals.

Office of Dietary Supplements ODS , National Institutes of Health NIH ODS seeks to strengthen knowledge and understanding of dietary supplements by evaluating scientific information, supporting research, sharing research results, and educating the public.

gov Email: ods nih. Key References. Chung M, Zhao N, Wang D, et al. Dose-response relation between tea consumption and risk of cardiovascular disease and all-cause mortality: a systematic review and meta-analysis of population-based studies.

Advances in Nutrition. February 19, Filippini T, Malavolti M, Borrelli F, et al. Green tea Camellia sinensis for the prevention of cancer. Cochrane Database of Systematic Reviews.

com on March 21, Fujiki H, Watanabe T, Sueoka E, et al. Cancer prevention with green tea and its principal constituent, EGCG: from early investigations to current focus on human cancer stem cells.

Molecules and Cells. Green tea. Natural Medicines website. Accessed at naturalmedicines. Hartley L, Flowers N, Holmes J, et al. Green and black tea for the primary prevention of cardiovascular disease. com on March 23, Khan N, Mukhtar H. Tea polyphenols in promotion of human health.

LiverTox: clinical and research information on drug-induced liver injury. Bethesda, MD: National Institute of Diabetes and Digestive and Kidney Diseases; National Cancer Institute.