What are the health benefits of green tea, and is it safe?

Answer:

What is green tea?

Green tea is made from the plant Camellia sinensis. It contains polyphenolic compounds called catechins that, in test tube studies, show antioxidant, anticarcinogenic, antitumorigenic, and anti-microbial properties. The main catechin found in green tea is epigallocatechin gallate (EGCG). Green tea also naturally contains caffeine.

What is the best form of green tea?

Green tea comes in many forms — leaves for brewing, matcha powders, bottled beverages, and supplements — so, it can be hard to know which product to choose. When comparing green tea products, it is helpful to look for ones that state the amount of EGCG or catechins. Catechins, including EGCG, are antioxidants and may help explain the link between green tea intake and any potential health benefit. Levels of EGCG and other catechins can vary greatly among products.

ConsumerLab has tested many green tea products to determine the amounts of catechins and EGCG they contain (as well as caffeine and potential contaminants such as lead). Test Results by Product can be found in our Green Tea Review. Matcha powders and brewed green tea can provide about 40 mg to 100 mg of EGCG per serving. While some matcha products claim to provide more than 130 times the EGCG that you get from brewed green tea, this is not correct. Get the details about why in the Brewed Green Tea vs. Matcha section of our Green Tea Review. Some bottled green teas provide similar amounts of active ingredients per serving as brewed green tea and matcha powders, but others provide very little. In some cases, you would have to drink an unrealistic amount in order to get the amount of EGCG found in brewable teas or supplements. To learn about which bottled green teas might be the best option, see our test results. Green tea supplements typically provide the largest amounts of these EGCG and other catechins — often hundreds of milligrams per serving. To learn which specific product we think is best for each of the common forms of green tea, see our Top Picks for brewed, matcha, bottled, and supplement green tea.

How does green tea compare to other teas?

Green tea, black tea, and white tea come from the same plant, so people may assume the teas are the same. This is not true. Green tea is made by lightly steaming freshly cut leaves, while black tea is fermented. Compared to green tea, black tea has lower amounts of catechins and more caffeine. The leaves used to prepare white tea are picked at a less mature stage compared to green tea. While green tea and white tea tend to provide comparable amounts of catechins, white tea tends to have less caffeine and is less likely to be contaminated with lead. Bear in mind, however, that there can be considerable variation from product to product.

How should brewed green tea be prepared?

To brew, steep green tea leaves (loose or in a tea ball or tea bag) in recently boiled water for only 3 to 5 minutes. This duration of brewing allows a significant amount of EGCG to go into the water while avoiding bitter-tasting tea. A microwave oven can also be used, but this is not very worthwhile. Get the details about why in the How to Use section of our Green Tea Review. After brewing, let tea cool to a comfortable temperature before drinking. Drinking hot tea has been associated with an increased risk of both esophageal and gastric cancer.

When selecting water for brewing green tea, it seems that the safest bet is to brew with water that is low in minerals ("soft" water) to maximize EGCG content. But this can affect flavor, making tea bitterer.

Does adding other ingredients to green tea, such as milk, lemon, or honey, increase its benefits?

Many people add other ingredients to brewed green tea, either to alter the flavor or to boost health benefits. While some of these ingredients may improve the potential benefits of green tea, others might be detrimental.

Some people add milk to green tea. This is not recommended for most people. Adding milk to green tea will significantly reduce the bioavailability of green tea catechins such as EGCG. These catechins are thought to be responsible for many of the health benefits of green tea. However, if you are prone to kidney stones, there may be a benefit to adding milk to your tea. For details, see the How to Take section of our Green Tea Review.

Other people add lemon. Lemon seems to stabilize and increase the absorption of catechins in green tea, which might be beneficial. One study in pigs showed that giving green tea extract with lemon juice increased the peak blood level of EGCG by about 1.7-fold compared to giving the same dose of green tea extract without lemon (Fang, Antioxidants 2019). While interesting, it is still too soon to know if adding lemon to green tea has notable benefits in people.

Some people add honey to green tea. While adding honey to green tea does not seem to have any health benefit, it might make the flavor more palatable. Just keep in mind that honey has about 65 calories per tablespoon, so it should be used in moderation.

Should green tea be taken with food?

Green tea extract should be taken with food to reduce the risk of liver injury. Green tea beverages, which provide lower amounts of catechins per serving, do not need to be taken with food. In fact, drinking tea, including green tea, may inhibit the absorption of iron from food. To avoid this, delay tea for at least one hour after an iron-containing meal. For more information, see the How to Use section in our Green Tea Review.

Can green tea still be used after the "best by" date?

Most brewable green tea has a "best by" date on the package, but the product can still be safely used past this date provided it was properly stored. However, the flavor and amount of EGCG in the product might not be optimal. For more information, see the Storage and Shelf Life section in our Green Tea Review.

What are the health benefits of green tea?

Weight loss - Some studies have found green tea to modestly aid in weight loss and protect against weight gain, although not all studies have found a benefit. Some researchers have suggested that green tea catechins may help with weight management by inhibiting starch digestion and absorption (by inhibiting digestive enzymes). However, other researchers have proposed that weight loss associated with green tea could be largely attributed to its caffeine content. Caffeine alone can reduce appetite and is a key component other herbal weight loss supplements, and studies using decaffeinated green tea extract have generally not shown a significant benefit. For more details, read the Weight Loss section of our Green Tea Review.

Flu prevention - In laboratory and animal studies, green tea catechins have been shown to have anti-viral effects. Some preliminary evidence has shown that drinking green tea may decrease the risk of flu infection in people. Some, but not all, studies have also found that gargling with green tea helps prevent infection with the flu. The evidence for green tea supplements is less clear. For more details, read the Flu section of our Green Tea Review.

Heart health - Regularly drinking green tea — typically 5 cups per day or more — has been associated with a lower risk of coronary artery disease. Drinking at least 3 cups of green tea per day has also been associated with a lower risk of stroke, although this benefit may be significant in only men. Drinking lower amounts (more than 3 cups per week) has also been linked with a lower risk of heart-related events. Clinical research has shown that drinking 4 cups of decaffeinated green tea per day lowers blood pressure and reverses left ventricular hypertrophy in people with high blood pressure. Drinking green tea or taking green tea extract can also lower total cholesterol and low-density lipoprotein (LDL, "bad" cholesterol). For more details, read the Cardiovascular Disease section of our Green Tea Review.

Cancer prevention - Test tube and animal studies hint that green tea constituents might help prevent various cancers. The majority of studies examined the effects of drinking brewed tea, rather than the effects of taking green tea supplements. Studies in people have not always found green tea to reduce the risk of cancer, but those that have typically involved a larger number of cups per day (at least 2), and, regarding gastric cancer specifically, it may be preferable to drink green tea that is not very hot. For more details, read the Cancer Prevention section of our Green Tea Review.

Fibroids - Laboratory research has suggested a role of green tea extract in shrinking uterine fibroids, which are non-cancerous tumors of the uterine wall affecting up to 70% of reproductive-age women and which may cause pelvic pain. Taking green tea extract for 4 months has been shown to reduce fibroid size and symptoms such as pelvic pain and anemia compared to placebo in women with uterine fibroids. For more details, read the Fibroids section of our Green Tea Review.

Diabetes - Drinking at least 3 cups of green tea per day has been associated with a 16% reduced risk of developing type 2 diabetes. Green tea may also have benefit in people who already have diabetes. Green tea polyphenols, particularly catechins, may decrease fasting and postprandial ("after a meal") blood sugar levels, and timing of intake may matter. For more details, read the Blood Sugar Control/Diabetes section of our Green Tea Review.

Memory and cognition - Higher consumption of green tea has been associated with lower prevalence of cognitive impairment in older adults. Even "regular" green tea consumption — at least 1 cup per week — has been associated with reduced risk of cognitive decline among older people with normal cognitive function. However, a study in Japan suggests that green tea may not help to improve cognition in people with more severe forms of cognitive impairment.

Green tea may also benefit some aspects of memory and cognition in younger healthy adults, but results have been inconsistent. One small study among healthy young men and women found that 4 grams of matcha tea powder consumed as brewed tea (2 cups) or as a matcha tea bar increased certain aspects of cognition (attention and response time) one hour after consumption compared to placebo. Some excitement was also generated by a Swiss study in 2014 reported by some news sources as showing that green tea helped improve working memory (i.e., short term storage and retrieval of information). Unfortunately, these accounts were exaggerated and misleading. Although there was a strong trend toward improved task performance with the extract, the memory improvement was not statistically significant. In addition, the dose of green tea extract was enormous. Such as dose of green tea extract would not be recommended and, interestingly, the study failed to mention whether adverse effects occurred. For more details, read the Memory and Cognition section of our Green Tea Review.

Men's health - Some forms of green tea have shown benefit for men's health. For instance, taking green tea extract may help lower the risk of prostate cancer and reduce serum PSA levels, which tend to be elevated in men with prostate cancer. However, drinking green tea does not seem to be beneficial. For more details, read the Cancer Prevention section of our Green Tea Review. Green tea intake has also been associated with a reduced risk of stroke in men. Men who drink green tea daily have been shown to have an 11% lower risk of stroke (ischemic and hemorrhagic) compared to those who don't drink green tea. Risk of stroke was also decreased in men with duration (years) of tea consumption.

Digestive health - Polyphenols in green tea may have a prebiotic effect, stimulating the growth of beneficial bacteria in the gut and inhibiting the growth of pathogenic bacteria, such as Clostridium species, including C. difficile. Some research has shown that drinking about 4 cups of green tea daily can increase the proportion of Bifidobacterium species, which are thought to be beneficial. However, a study of green tea extract given to people as capsules (providing at least 560 mg of EGCG daily) did not show any effect on bacterial species in the gut.

How much green tea should be taken?

As discussed above, studies have shown that drinking about 3-5 cups of green tea daily or taking green tea extract providing about 200 mg to 300 mg of EGCG can have health benefits. Green tea extract that contains catechins at doses at or above 800 mg per day should be avoided.

Does green tea cause any side effects?

Green tea contains a significant amount of caffeine. Consequently it can cause caffeine-related side effects. Even products listed as "decaffeinated" may contain up to 2% caffeine, and "caffeine free" products can contain small amounts.

Women who are attempting to conceive or are in their first trimester of pregnancy should avoid large amounts of green tea. Women who are nursing should also avoid large amounts of green tea in order to limit caffeine exposure to infants.

Green tea also contains small amounts of fluoride. Excess fluoride can cause teeth and bones to become brittle (a condition called "fluorosis"). This is unlikely to occur with consumption of just a few cups of tea daily but has occurred in people habitually consuming very large amounts of tea. To avoid this risk, healthy adults should consider limiting intake of green tea to 5 cups daily. Although green tea extract supplements are concentrated forms of green tea, there are no published reports of fluorosis from green tea supplements.

There are reports of several cases of liver toxicity beginning from five days to four months after beginning use of green tea extract. Liver function returned to normal in most cases after discontinuation. It may be the rapid ingestion, particularly on an empty stomach, of high amounts of catechins that explains the toxicity reported with green tea supplements. In light of potential liver toxicity, the United States Pharmacopeia (USP) added a cautionary labeling requirement in March 2019 for products that claim compliance with USP quality standards for green tea extract that reads as follows: "Do not take on an empty stomach. Take with food. Do not use if you have a liver problem and discontinue use and consult a healthcare practitioner if you develop symptoms of liver trouble, such as abdominal pain, dark urine, or jaundice (yellowing of the skin or eyes)." Green tea drinks (such as brewed tea), in contrast, are often taken throughout the day, provide lower amounts of catechins per serving, and are often consumed with food — slowing absorption.

Green tea contains oxalate, high levels of which can contribute to the development of kidney stones in some people. However, for a number of reasons, this is not much of a concern with green tea (which may even help). Get the details in the Concerns and Cautions section of our Green Tea Review.

Some concerns have been raised about the possibility of radiation contamination of green tea from Japan. Since the Fukushima Daiichi nuclear power plant accident in 2011, radiation levels in foods and beverages in Japan have been closely monitored, and there is currently no indication of excessive levels of radiation in green tea. See the ConsumerTips section of our Green Tea Review for more details.

Should green tea be avoided when taking certain prescription drugs?

Due to its caffeine content, green tea can interfere with drugs that are MAO inhibitors. People taking MAO inhibitors should use decaffeinated green tea products to minimize caffeine intake.

Cancer patients taking drugs called proteasome inhibitors should be aware that EGCG and other polyphenols in green tea may block the action of these drugs and negate their therapeutic efficacy. Green tea should not be used when taking boronic acid-based proteasome inhibitors such as bortezomib (BZM), MG-262, and PS-IX.

Green tea has been shown to reduce the absorption of the beta-blocker nadolol (Corgard). The effect of nadolol on systolic blood pressure was also significantly reduced. For more details, see the Concerns and Cautions section of our Green Tea Review.

Modest daily intake of green tea is not problematic for most people taking statins, but be aware that certain individuals may be more sensitive than others and certain statins may be more affected, as described in the Concerns and Cautions section of our Green Tea Review. If you are on a statin, it may be best to limit green tea intake one cup per day, take your statin at a different time of day than green tea, and eliminate green tea altogether if you experience statin-related muscle pain.

High doses of green tea extract and green tea catechins have been found to impair thyroid function by decreasing levels of thyroid hormones T3 and T4, increasing levels of TSH (thyroid stimulating hormone). To be safe, it may be best to consume green tea at least one hour apart from thyroid medication such as levothyroxine (Synthroid). For more information about other supplement ingredients that may interaction with levothyroxine, see our answer to Levothyroxine (Synthroid) Supplement Interactions.

While green tea contains vitamin K and catechins — both of which have the potential to counteract the effects of some blood thinning medications — drinking a moderate amount of regular brewed green tea would not be expected to cause a problem. However, be aware that matcha green tea could potentially interfere with certain blood-thinning medications. For more details about this interactions of green tea, see the Concerns and Cautions section of our Green Tea Review.

The bottom line on green tea

Health benefits of green tea are generally associated with its catechins, most notably EGCG. Benefits include a modest reduction in LDL cholesterol, reduced growth of uterine fibroids and associations with lower risks of cardiovascular disease, certain cancers and type 2 diabetes. These benefits are generally associated with consumption of 3 or more cups per day. Studies with green tea supplements typically provide 200 mg to 300 mg per day. The evidence regarding weight loss and memory benefits is mixed and may relate to the caffeine in green tea. Green tea can interfere with a range of drugs. Liver toxicity is a concern with high doses of EGCG from green tea supplements. Avoid excessive green tea when pregnant. Excessive tea consumption can make bones and teeth brittle. Don't drink very hot tea due to an association with gastric cancer.