People have been drinking tea for thousands of years, but in the last couple of decades a number of potential health benefits have been attributed to this ancient beverage. Black tea and green tea are made from the same plant, but a higher level of the original substances endure in the less-processed green form.
Green tea contains high levels of substances called catechin polyphenols, known to possess strong antioxidant, anticarcinogenic, antitumorigenic, and even antibiotic properties.1,2 Based on these findings, as well as observational studies,3-8 green tea has become popular as a daily drink for preventing cancer and heart disease. However, some observational trials failed to find indications of benefit with green tea.9,10,11 Furthermore, only double-blind, placebo-controlled studies can prove a treatment effective, and there is little direct evidence of this type regarding green tea and cancer or heart disease prevention.52 (For more information on why double-blind studies are so important, see Why Does this Database Rely on Double-blind Studies?)
Researchers have studied the potential benefits of green tea in people with high cholesterol. The overall evidence has been positive. One study found that green tea produced short-term improvements in cholesterol profile, but the benefits disappeared after 4 weeks.26 Another study evaluated a form of green tea enriched with the substance theaflavin.21 In this large, 3-month trial, use of the tea product resulted in significant, ongoing reductions in LDL ("bad") cholesterol as compared to placebo. In a somewhat flawed double-blind study, a green tea extract enhanced with catechins also showed promise for reducing LDL levels.42 Lastly, a 2011 review of 20 randomized trials involving 1,415 people found evidence that green tea catechins (145-3,000 mg/day for 3-24 weeks) reduced total and LDL cholesterol.56
Preliminary studies suggest that certain green tea polyphenols may help prevent skin cancer if they are applied directly to the skin.12 In addition, there is some evidence that green tea constituents might help protect the skin from sun damage.13,14,15 Unlike normal sunscreen preparations, green tea does not physically block ultraviolet light. Rather, it seems to protect cells from some of the damage caused by ultraviolet light. Because it works by such a different mechanism of action, green tea might offer synergistic benefits if combined with standard sunscreens. However, in an 8-week double-blind, placebo-controlled study of 40 women who already had symptoms of aging skin, combined use of oral green tea and a topical green tea cream failed to prove more effective than placebo.27 Some possible benefits were seen in microscopic evaluation of skin condition.
Topical green tea extracts have also shown a bit of promise for the treatment of cervical dysplasia,22 while oral green tea extracts might reduce risk of prostate cancer, according to a small pilot study.28 Combining the results of 13 observational studies, researchers found conflicting evidence for green tea's effect on the risk of stomach cancer.53 In a Japanese pilot study, green tea extract supplements lowered the risk of recurrent colorectal polyps.54 In a review of nine observational studies involving over 5,600 cases of breast cancer, researchers failed to find reliable evidence for a reduction in the incidence of breast cancer. However, they did find weak evidence for a decrease in breast cancer recurrence among women who consumed more than 3 cups of green tea daily. .55
For more information on green tea's capacity to fight cancer see the Cancer Prevention article.
On a completely different note, one interesting study tested the effectiveness of gargling with green tea catechins as a means of preventing influenza.39 In this double-blind, placebo-controlled study, 124 residents of a Japanese nursing home gargled with green tea catechins or placebo for three months. All participants received standard influenza vaccine. The results showed that residents who gargled with the tea extract were less likely to develop influenza than those using the placebo. In addition, another double-blind study found preliminary evidence that oral consumption of a green tea extract might help prevent colds and flus.44
Oral use of green tea extracts has shown a bit of promise for treating borderline diabetes.41 However, one double-blind study failed to find that a combined extract of black and green tea was helpful for controlling blood sugar levels in people with type 2 diabetes.43 And a subsequent study failed to find any benefit for green tea extract in controlling blood sugar levels in obese people with diabetes.57
Green tea is sometimes recommended for weight loss on the basis of rather theoretical evidence that it speeds up metabolism.23 However, there is little direct scientific backing for this use. If green tea increases metabolism at all, the effect is extremely small.29-30 One study conducted in Thailand reported weight-loss benefits with green tea,45 as did a second study of oolong tea enriched with green tea extracts.24 However, a Dutch study failed to find green tea helpful for preventing weight regain after weight loss.46 In another study, use of green tea failed to produce significant weight loss in overweight women with polycystic ovary syndrome.31 Green tea extract enriched with catechins has done somewhat better, enhancing weight loss in one substantial, but flawed trial.42 However, a study in overweight Japanese children did not support the effectiveness of green tea catechins for weight reduction.47 Similar results were obtained in another placebo-controlled trial involving 78 overweight women after 12 weeks of treatment.49
One preliminary study, available only in abstract form, found some evidence that green tea cream may be helpful for rosacea.25 The results of another study weakly hint that green tea extracts taken orally might reduce symptoms of benign prostatic hyperplasia.32
One study found that inhaled tea catechins could reduce levels of resistant staph carried in the sputum of disabled seniors.33Note: Do not attempt to inhale green tea products.
Studies weakly suggest that 3 cups of green tea daily might provide protection against cancer. However, because not everyone wants to take the time to drink green tea, manufacturers have offered extracts that can be taken in pill form. A typical dosage is 100 mg to 150 mg 3 times daily of a green tea extract standardized to contain 80% total polyphenols and 50% epigallocatechin gallate. Whether these extracts offer any benefit remains unknown. Furthermore, there are growing concerns about liver toxicity with use of green tea extracts. (See Safety Issues.)
Warning: In an analysis performed in 2006 by the respected testing organization ConsumerLabs.com, some tested green tea products were found to be contaminated with lead.34
As a widely consumed beverage, green tea is generally regarded as safe. It does contain caffeine, at perhaps a slightly lower level than black tea, and can therefore cause insomnia, nervousness, and the other well-known symptoms of excess caffeine intake.
Green tea extracts, however, may not be safe. There are a growing number of case reports in which use of a concentrated green tea extract was associated with liver inflammation.35,40,50 In most cases, liver problems disappeared after the extract was discontinued. But, in two cases, permanent liver failure ensued requiring liver transplantation.36,40 While it is not absolutely certain that the green tea extract caused the liver problems, nor how it might do so, these reports do raise significant concerns about use of green tea extracts, especially by those with liver disease or prone to it.
Green tea should not be given to infants and young children. There are theoretical concerns that high dosages of EGCG might be unsafe for pregnant women.37
Dried green tea leaf contains significant levels of vitamin K on a per-weight basis. On this basis, it has been stated that people using blood thinners in the warfarin (Coumadin) family should avoid green tea, because vitamin K antagonizes the effect of those drugs. However green tea taken as a beverage provides such small amounts of the vitamin that the risk seems minimal for normal consumption. There is one case report of problems that developed in a person on warfarin who consumed as much as a gallon of green tea daily.38
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Last reviewed December 2015 by EBSCO CAM Review Board Last Updated: 12/15/2015