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]