upkeep vs sustainment

upkeep and sustainment both are nouns.

Word	Noun	Adjective	Verb	Adverb
upkeep	Yes	No	No	No
sustainment	Yes	No	No	No

As nouns, upkeep and sustainment are synonyms defined as:

upkeep and sustainment: the act of sustaining life by food or providing a means of subsistence

Other synonyms of upkeep include maintenance, sustenance, sustentation.

upkeep (noun)	sustainment (noun)
activity involved in maintaining something in good working order	the act of sustaining life by food or providing a means of subsistence
the act of sustaining life by food or providing a means of subsistence