sustenance vs fast food

sustenance and fast food both are nouns.

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

As nouns, fast food is a hyponym of sustenance; that is, fast food is a word with a more specific, narrower meaning than sustenance:

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

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