Disassimilation \Dis`as*sim`i*la"tion\, n. (Physics) The decomposition of complex substances, within the organism, into simpler ones suitable only for excretion, with evolution of energy, -- a normal nutritional process the reverse of assimilation; downward metabolism; -- now more commonly called {catabolism}. [1913 Webster +PJC]
The breaking down of already existing chemical compounds into simpler ones, sometimes called disassimilation. --Martin.