Webster’s Revised Unabridged Dictionary · 1913
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.
The breaking down of already existing chemical compounds into simpler ones, sometimes called disassimilation. Martin.