They're cousins of alkaloids and humanoids, too.
Or maybe they aren't.
They'll stop us from going all rusty, though: they're antioxidants, and help keep our capillaries and veins elastic.
So eat 100 g (4 ounces) of one or the other high-flavonoid berries a day - that'll not only help your capillaries, it might also help keep your aching joints less achy, and will give you lots of vitamins and trace minerals as well - at least if you chose wild berries over commercially grown ones.
We're used to thinking of flavonoids as strongly-colored things, as in dark purple, dark red, dark yellow, and so on.
Hawthorn flowering twigs and berries aren't all that bright, though, and they work admirably, partly because they're loaded with flavonoids.
And one of the strongest known flavonoids, quercetin, is found in oak bark.
But then, tannins are just concentrated flavonoids ... dunno just how much flavonoid action there actually is in tannins, though. Perhaps not all that much, seeing that their astringent action is so strong, eh?
Other entries: Constituents - Hawthorn berries