obesity diet vs diet

obesity diet and diet both are nouns.

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

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

obesity diet (noun)	diet (noun)
a diet designed to help you lose weight (especially fat)	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)

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