Mike Lyle writes

I think there are nicer things available at this time of year than
inferior rhubarb. Oxalic acid in the diet is on the whole not good
news, since the body's way of getting rid of it is to combine with
one's blood calcium. The older we get, the more important it is to keep
up the calcium. Another nifty trick it plays is that the excreted
compound, calcium oxalate, isn't water-soluble, so the crystals can
collect in your waterworks

And also, apparently, in your joints

-- this is worse for male animals, which
have a longer and narrower tube, and presumably also in men.

That said, I find it hard to believe that an occasional crumble made
with greenish rhubarb would make any difference. If in doubt, a calcium
tablet or two should prevent the oxalic getting into the blood: it'll
combine in the gut and, because the oxalate is insoluble, pass out
through the bowel instead of going into the bloodstream.

I was brought up to believe that one should always eat custard or some
similar milky substance with rhubarb, so that the oxalic acid would
combine with the calcium in that rather than the calcium in you..
Whether this is any more effective than eating spinach for iron, I
haven't a clue.

--
Kay