Too much tea is may be carcinogenic, especially to the unborns of pregnant mothers.
Rather they generally act as genotoxins, so the health benefits probably act via hormesis, priming your system to properly deal with cancer forming toxins
Hormesis may explain this. This makes sense when you consider 95% of polyphenols are immediately flushed by the liver immediately, like a standard toxin. A regular moderate consumption is like your bodies fire drill, in case of emergency. Or this is my working theory anyway.
Long story short, in moderate amounts it may be healthful to ingest polyphenols, but in large doses or amounts it may be harmful.What level is harmful is hard to say exactly, because genotoxicity, like with ciggerettes usually takes decades to show given our DNA's defense systems, and is thus pretty nebulous to predict.
Wikipedia takes a more middle road line, basically saying that generally theres no strong proof these "anti-oxidant" substances are anti-oxidant in vivo, or that they mitigate cancer, and they may be harmful in high doses according to some studies, although it claims mostly in vulnerable populations(they also suggest someting like hormesis, saying that maybe increased uric acid as a defense to fructose is often responsible for anti-cancer effects of fruits).
But as I said earlier, the effect of genotoxins like polyphenols or ciggerettes is a bit nebulous due to the fact our DNA has redundancy and bodily defenses. Its not like any genotoxic substance apart from high doses of super cancer promoter safrole is going to make you sick semi-immediately.
To be honest the exact mechanics of cancer formation and protection escapes me, but I do know that polyphenols both decrease and increase cancer, in a dose dependant manner, and depending on which polyphenol as well.
Its inaccurate to say they are just "anti-oxidants", in veiw of this, and what
What data there is, suggests that there probably is an upper limit of excess consumption that is harmful, at least when your older, or infant. Certainly it seems like a downright stupid idea to take refined and extracted "anti-oxidants" like reversetrol or grape seed extract.
But then the level of excess consumption of tea that is harmful is probably well above a moderate one or two cups a day, and in a non-pregnant adult, would take many many years to potentially manifest.
I find that tea is a significant anti-nutrient quite interesting too.