The Collaborative International Dictionary of English v.0.48:

Dietetics \Di`e*tet"ics\, n.
   That part of the medical or hygienic art which relates to
   diet or food; rules for diet.
   [1913 Webster]

         To suppose that the whole of dietetics lies in
         determining whether or not bread is more nutritive than
         potatoes.                                --H. Spencer.
   [1913 Webster]

WordNet (r) 3.0 (2006):

dietetics
    n 1: the scientific study of food preparation and intake

Moby Thesaurus II by Grady Ward, 1.0:

27 Moby Thesaurus words for "dietetics":
   acid-ash diet, allergy diet, bland diet, calorie chart,
   calorie counter, diabetic diet, diet, diet book, dietary, dieting,
   dietotherapy, enzymology, high-protein diet, high-vitamin diet,
   low-carbohydrate diet, low-salt diet, macrobiotic diet, pap,
   regime, regimen, salt-free diet, soft diet, spoon victuals,
   ulcer diet, vegetarianism, vitamin-deficiency diet, vitaminology