fast food vs sustenance

fast food and sustenance both are nouns.

Word	Noun	Adjective	Verb	Adverb
fast food	Yes	No	No	No
sustenance	Yes	No	No	No

As nouns, sustenance is a hypernym of fast food; that is, sustenance is a word with a broader meaning than fast food:

fast food: inexpensive food (hamburgers or chicken or milkshakes) prepared and served quickly
sustenance: a source of materials to nourish the body

Other hypernyms of fast food include aliment, alimentation, nourishment, nutriment, nutrition, victuals.

fast food (noun)	sustenance (noun)
inexpensive food (hamburgers or chicken or milkshakes) prepared and served quickly	the act of sustaining life by food or providing a means of subsistence
	a source of materials to nourish the body
	the financial means whereby one lives