diet vs dietetical

diet is a noun but dietetical is not a noun.

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

diet (noun)	dietetical (noun)
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)

diet (adjective)	dietetical (adjective)
	of or relating to the diet

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