fasting vs dieting

fasting and dieting both are nouns.

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

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

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

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

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