The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]