Green tea has a reputation as a fat-burning, cancer-fighting beverage, in part because of its role in traditional — and healthful — Asian diets, but also because of its catechins, a group of compounds largely found in tea, red wine, cocoa and berries. There are thousands of studies on tea and health. Some are high-quality and based on strong evidence, but many are not.
Catechins are types of polyphenols, which in turn are types of phytochemicals — plant compounds that have benefits for human health. One of the richest and best studied sources of catechins is Camellia sinensis, the plant that green, black, white and oolong teas come from. Catechins are particularly high in green tea, because the leaves are dried and steamed instead of fermented.
Camellia sinensis has historically been used for medicinal purposes. Catechins have strong antioxidant activity, especially epigallocatechin gallate (EGCG), which accounts for about two-thirds of the catechins in green tea. EGCG is the catechin that gets the most attention, but studies that have demonstrated a cancer-preventive effect of green tea catechins have clearly shown that all the catechins are needed to provide those benefits. Some of green tea’s effects also come from its other polyphenols and compounds, including theanine, an amino acid that research suggests may promote a relaxed but alert mental state, among other health benefits. So despite the hype around EGCG, it’s not all about the EGCG. Just another example of the synergistic effects of natural substances as they appear in their intact forms, rather than as isolated compounds.
Overall, the strongest evidence for the benefits of tea catechins comes from laboratory and animal studies, with human studies being less conclusive. Unfortunately, results from laboratory studies have been unreasonably extrapolated to human health, leading to weak health claims that spread widely through traditional and social media.
Humans are complex, and many factors — genetic, dietary, lifestyle — can affect whether any food or beverage has health benefits beyond simply nourishment. This is why some epidemiological (observational) studies find that green tea may protect against cancer, but others don’t. Human intervention studies — where people are randomly assigned to ingest green tea or catechins, or not — usually last for a few years at most, which isn’t exactly a long term. These studies also miss the early part of participants’ life histories — including prior tea consumption, or lack of it.
Researchers have observed that three to four 8-ounce cups of green tea per day, which supplies about 600-900 milligrams of catechins, may help prevent Type 2 diabetes, but lower levels of intake would still contribute to someone’s total intake of polyphenols. If you’re thinking, “Maybe taking a catechin supplement would be better,” think twice. Although green tea is considered safe, with a long history of use, taking dietary supplements that provide high doses of green tea extract has caused liver toxicity in some people, especially individuals who have preexisting conditions or who take medications that already affect the liver.
Researchers tell me that the heady optimism in the early days of green tea research has mellowed. Yes, green tea has some beneficial health effects, but it’s not a miracle beverage. It’s just one of the many things we can eat or drink that may benefit health, and it’s arguably healthier than many other beverages we might drink. So drink up if you enjoy it, skip it if you don’t.