dietetics vs life science

dietetics and life science both are nouns.

Word	Noun	Adjective	Verb	Adverb
dietetics	Yes	No	No	No
life science	Yes	No	No	No

As nouns, life science is a hypernym of dietetics; that is, life science is a word with a broader meaning than dietetics:

dietetics: the scientific study of food preparation and intake
life science: any of the branches of natural science dealing with the structure and behavior of living organisms

Other hypernyms of dietetics include bioscience.

dietetics (noun)	life science (noun)
the scientific study of food preparation and intake	any of the branches of natural science dealing with the structure and behavior of living organisms