The Collaborative International Dictionary of English v.0.48:

Dietetic \Di`e*tet"ic\, Dietetical \Di`e*tet"ic*al\, a. [Gr. ?:
   cf. F. di['e]t['e]tique. See Diet.]
   Of or performance to diet, or to the rules for regulating the
   kind and quantity of food to be eaten.
   [1913 Webster]

WordNet (r) 3.0 (2006):

dietetical
    adj 1: of or relating to the diet; "dietary restrictions" [syn:
           dietary, dietetic, dietetical]