How EGCG moves through the body
Epigallocatechin gallate (EGCG) is the most abundant catechin in green tea. After drinking, plasma levels typically peak in roughly one to two hours and decline with a half-life on the order of a few hours, so it does not accumulate strongly from a single serving. Absorption is modest and variable, and a substantial fraction is metabolised or not absorbed, which is one reason effect sizes in studies tend to be small.
The proposed metabolic mechanism involves EGCG inhibiting catechol-O-methyltransferase (COMT), the enzyme that breaks down norepinephrine. By slowing that breakdown, norepinephrine signalling that promotes fat breakdown and thermogenesis may be modestly prolonged, an effect often studied alongside caffeine, which acts through a complementary pathway.
How large the fat-oxidation effect really is
Reported increases in 24-hour energy expenditure from green tea catechins are small, often in the low single-digit percent range, and findings are inconsistent across studies. Any effect on body weight in trials has generally been minor and is unlikely to substitute for diet and activity.
This is general educational information, not medical or supplementation advice. Concentrated green tea extracts in high doses have been linked in rare cases to liver injury; consult a qualified professional before using them.