No, this isn't true. There's no conservation of matter anywhere. If you take 2 hydrogen atoms and weight them, then bond them together in a hydrogen molecule and weight that, the molecule weights less than twice one hydrogen atom.
But you still have the two hydrogen atoms, before and after. They are matter. Tiny (really tiny!) differences in weight don't change that.
I guess when you said "conservation of matter" I interpreted that as meaning "conservation of mass", seeing as we're talking about weight loss.