WordNet records no synonym and no antonym for digestion. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
the process of decomposing organic matter (as in sewage) by bacteria or by chemical action or heat
the organic process by which food is converted into substances that can be absorbed into the body
Synonyms, antonyms and the broader and narrower relations for digestion come from Princeton University’s WordNet. Full sources and licences.
learning and coming to understand ideas and information
“his appetite for facts was better than his digestion”