Source : Webster's Revised Unabridged Dictionary (1913)

Dietetics \Di`e*tet"ics\, n.
   That part of the medical or hygienic art which relates to
   diet or food; rules for diet.

         To suppose that the whole of dietetics lies in
         determining whether or not bread is more nutritive than
         potatoes.                                --H. Spencer.

Source : WordNet®

dietetics
     n : the scientific study of food preparation and intake