diet vs dietary

diet and dietary both are nouns.

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

diet (noun)	dietary (noun)
the act of restricting your food intake (or your intake of particular foods)	a regulated daily food allowance
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 (adjective)	dietary (adjective)
	of or relating to the diet

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