A polyphenol family, and one molecule that took the spotlight
The catechins are flavan-3-ols — a subgroup of the broader polyphenol class — and they sit at roughly 25-35% of the dry weight of an unoxidised green tea leaf, depending on cultivar, elevation, and pluck. Four show up in meaningful quantities: epicatechin (EC), epigallocatechin (EGC), epicatechin gallate (ECG), and epigallocatechin gallate (EGCG). The last of these, EGCG, is both the most abundant and the most studied, which is why so much of the green tea health literature is really EGCG literature in disguise.
The research arc is relatively recent. Catechins were structurally characterised across the early twentieth century, but the modern wave begins in 1987, when Fujiki and colleagues at Japan’s National Cancer Center Research Institute identified EGCG as the active anti-tumor-promoting component in green tea extract. The decades since have produced thousands of papers on antioxidant capacity, cardiovascular markers, hepatic enzymes, neuroprotective signalling — and a parallel, smaller body of work on what actually reaches human blood after a cup of tea. That second question is the one most worth pinning down, and it is where our companion piece, How much EGCG is actually in a real brew, lives: a working brew of Lóng Jǐng (龙井) from a Hangzhou garden such as Meijiawu is not a standardised 300 mg capsule, and the difference matters for any reading of the trial literature.
Pharmacologically, catechins are fragile travellers. They are unstable above roughly 80 °C in alkaline conditions, they oxidise on exposure to air, and EGCG specifically has a measured oral bioavailability in humans of only a few percent — the rest is metabolised by the gut microbiome into smaller phenolic acids or cleared rapidly. This is why a leaf that tests at 80 mg EGCG per gram dry does not deliver 80 mg of circulating EGCG. Brewing temperature, steep time, water mineral profile, and the drinker’s own microbiome each shift the final number considerably, often by more than half.
Regionally, catechin profiles track cultivar and processing. Zhejiang green teas — Lóng Jǐng, Ān Jí Bái Chá (安吉白茶) — tend to be catechin-forward and lower in theanine balance; the Bái Háo Yín Zhēn (白毫银针) white teas from Fuding lean differently, with slower oxidation creating a milder catechin signature. For the producer side of this story, our colleagues at puerh.app cover how microbial fermentation in shú chá (熟茶) breaks catechins down into theabrownins entirely, which is why aged pu-erh shows almost none of the EGCG-driven activity that green tea trials report.
The honest summary, after thirty-five years of work: catechins are biologically active, EGCG is real, and the cup is not the capsule. Readers wanting deeper grounding in the chemistry side can also visit tea.school, which carries the technical breakdown of polyphenol oxidation across the six tea classes. Nothing on this site is medical advice — see our disclaimer — but the literature is genuinely interesting once the marketing layer is peeled off.