%disassimilation%

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. [ 1913 Webster ]