Green Tea

📜 Historical Use

The tea plant was first cultivated in China thousands of years ago, and NCCIH puts evidence of the use of tea for health purposes at about 3,000 years old. Green, black and oolong tea are all the same species, separated only at the processing stage: green tea leaves are steamed, pan fried and dried but not fermented, black tea is fermented, and oolong tea is partially fermented. LiverTox notes that green tea has long been believed to have health restoring properties and its ingredients to have antioxidant activity - a belief that long predates any attempt to test it. The modern commercial history is a different story from the cultural one. Green tea extract is now listed in more than 100 over-the-counter herbal preparations, is not approved for any specific medical indication, and is not regulated in regard to efficacy and safety by the FDA; it is promoted for weight loss, for reducing blood cholesterol, and for preventing chronic diseases such as heart disease and cancer, frequently in combination products containing other ingredients. That commercial history includes regulatory action. Exolise, a weight loss product, was withdrawn from Spain and France in 2003, and green tea is an ingredient in many over-the-counter weight loss agents - Hydroxycut, Dexatrim, Slimquick and Green Tea Fat Burner among them - which have been implicated in causing rare instances of clinically apparent acute liver injury.

📊 What the Research Shows

NCCIH's summary line is unusually blunt for a plant this heavily studied: 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. Taking the marketed uses one at a time. On cancer, many studies have looked at the relationship between green tea consumption and the risk of various types of cancer, and NCCIH reports the overall results of these studies have been inconsistent. On heart disease, green tea consumption has been linked to a lower risk of coronary heart disease in Asian populations but not in Western populations - a split that observational data cannot resolve into cause. On weight, the catechins and caffeine may have a modest effect on body weight, with effects varying by the composition of the product and the individual's level of physical activity; that is a long way from the metabolic claims printed on labels. Cholesterol is the one place where randomised evidence exists and is positive but small: in trials assigning people to either a green tea product or a placebo - mostly extract supplements rather than the beverage - green tea reduced total cholesterol and LDL cholesterol to a small extent, and did not affect HDL cholesterol or triglycerides at all. The clearest positive result is also the narrowest: the FDA-approved topical extract ointment for genital warts. Everything else on the list is inconsistent, small, or unresolved.

Brew 2-3 cups daily at 160-180°F for 2-3 min. No milk (blocks catechins).

Contains caffeine. Limit after 2PM.

⚠️ What happens if taken in excess:

The serious signal here belongs to the extract, not the cup, and the distinction carries the whole section. LiverTox assigns green tea extract a likelihood score of A - a well established cause of clinically apparent liver injury - and reports that more than 100 instances of clinically apparent liver injury attributed to green tea extract have been described in the literature, including instances of acute liver failure with either need for urgent liver transplantation or death. Injury typically arises within 1 to 6 months of starting the product, presents as an acute hepatitis-like syndrome with a markedly hepatocellular pattern of serum enzyme elevation, and most patients recover rapidly on stopping, with recovery expected in one to two months. Rechallenge reproduces the injury with a shorter latency: in one published case, four months to injury initially and four weeks on restarting. Susceptibility looks immunologic and idiosyncratic rather than dose-dependent - injury is closely associated with the HLA-B*35:01 allele, which NCCIH says between 5 and 15 percent of Americans carry. Drinking green tea, by contrast, has not been associated with liver injury or aminotransferase elevations; LiverTox notes cross-sectional studies suggesting regular use is associated with lower ALT and AST values, though a small number of cases have followed green tea infusions rather than capsules. NCCIH lists the milder extract side effects as nausea, constipation, abdominal discomfort and increased blood pressure; LiverTox adds headache, dizziness and nausea at high doses, and states that the safety and tolerability of long term use of green tea extracts has not been well defined.

Three named drug interactions are documented, and all three run the same direction - green tea lowering the drug's exposure. NCCIH reports that green tea at high doses has been shown to reduce blood levels and therefore the effectiveness of nadolol, a beta-blocker used for high blood pressure and heart problems; that green tea extract can reduce blood levels of the cholesterol-lowering drug atorvastatin; and that a study funded by NCCIH showed an interaction between green tea and raloxifene, used to treat osteoporosis - the published trial (Clarke JD et al., Clinical and Translational Science, 2023) found that co-consuming green tea with raloxifene decreases raloxifene systemic exposure in healthy adult participants. NCCIH adds that green tea may also interact with other medicines, and that anyone taking any type of medicine should talk with their health care provider before using green tea supplements or other herbal products. Green tea is also a source of caffeine, which matters in pregnancy: NCCIH states that during pregnancy caffeine intake should not exceed moderate levels and points to American College of Obstetricians and Gynecologists guidance on the point. For breastfeeding, LactMed reports that caffeine is excreted into breastmilk and that fussiness, jitteriness and poor sleep patterns have been reported in the infants of mothers with extremely high caffeine intakes, while breastfeeding infants usually do not have adverse effects at low-to-moderate maternal intake. The excretion of l-theanine into breastmilk has not been studied, and LactMed found no studies at all examining oral green tea extract during breastfeeding. Tea given directly to infants can interfere with iron absorption and cause anemia.

1. Green tea consumption and mortality due to cardiovascular disease, cancer, and all causes in Japan — Kuriyama S et al JAMA (2006)[doi ↗]
2. Tea — National Center for Complementary and Integrative Health NCCIH (NIH) (2020)[NIH ↗]
3. Green Tea Catechins: Their Use in Treating and Preventing Infectious Diseases — Reygaert WC Biomed Res Int (Hindawi) (2018)[doi ↗]