The Collaborative International Dictionary of English v.0.48:

Diet \Di"et\, v. t. [imp. & p. p. Dieted; p. pr. & vb. n.
   Dieting.]
   1. To cause to take food; to feed. [R.] --Shak.
      [1913 Webster]

   2. To cause to eat and drink sparingly, or by prescribed
      rules; to regulate medicinally the food of.
      [1913 Webster]

            She diets him with fasting every day. --Spenser.
      [1913 Webster]

WordNet (r) 3.0 (2006):

dieting
    n 1: the act of restricting your food intake (or your intake of
         particular foods) [syn: diet, dieting]

Moby Thesaurus II by Grady Ward, 1.0:

68 Moby Thesaurus words for "dieting":
   acid-ash diet, allergy diet, appetite, bland diet, calorie chart,
   calorie counter, cannibalism, carnivorism, carnivority,
   carnivorousness, chewing, consumption, cropping, deglutition,
   devouring, devourment, diabetic diet, diet, diet book, dietary,
   dietetics, dining, eating, epulation, feasting, feeding, gluttony,
   gobbling, grazing, herbivorism, herbivority, herbivorousness,
   high-protein diet, high-vitamin diet, hunger, ingestion, licking,
   low-carbohydrate diet, low-salt diet, macrobiotic diet,
   manducation, mastication, messing, munching, nibbling, nutrition,
   omnivorism, omnivorousness, omophagy, pantophagy, pap, pasture,
   pasturing, pecking, regalement, regime, regimen, relishing,
   rumination, salt-free diet, savoring, soft diet, spoon victuals,
   tasting, ulcer diet, vegetarianism, vitamin-deficiency diet,
   wolfing