fasting vs diet

fasting and diet both are nouns.

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

As nouns, diet is a hyponym of fasting; that is, diet is a word with a more specific, narrower meaning than fasting:

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

Other hyponyms of fasting include dieting, hunger strike, Ramadan.

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

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