If Y acid is rich in X compound, can we say

No. X compound, which is rich in Y acid.



No. It is X compound to which the adjective "rich" applies. If you wish to concentrate on X compound, you could say "X compound, which contains much Y acid", or if the focus is on Y acid, "Y acid, in which X compound is rich"

Thank you Contrex.
But you've given me a headache.

If I've used "in which x is rich" structure, I get a problem of expression (use two whichs in a row which have different meanings):

The existence of the hormone lowers the
metabolism of TAG in which Y acid is rich, the result of which is that TAG doubles in blood .

Of course, the second which above refers to The existence of the hormone lowers the metabolism of TAG in which Y acid is rich. I'm not sure whether the usage of two whichs would make native English speakers understand it at the first sight.

And even if they get it crystal clear, using two whichs in a row would be somewhat boring. But how to edit and improve it?