diet vs fasting

diet and fasting both are nouns.

Word	Noun	Adjective	Verb	Adverb
diet	Yes	No	Yes	No
fasting	Yes	No	No	No

As nouns, fasting is a hypernym of diet; that is, fasting is a word with a broader meaning than diet:

diet: the act of restricting your food intake (or your intake of particular foods)
fasting: abstaining from food

Other hypernyms of diet include fast.

diet (noun)	fasting (noun)
the act of restricting your food intake (or your intake of particular foods)	abstaining from food
a prescribed selection of foods
the usual food and drink consumed by an organism (person or animal)
a legislative assembly in certain countries (e.g., Japan)

diet (verb)	fasting (verb)
eat sparingly, for health reasons or to lose weight
follow a regimen or a diet, as for health reasons