The Collaborative International Dictionary of English v.0.48:

Dietist \Di"et*ist\, Dietitian \Di`e*ti"tian\, n.
   One skilled in dietetics. [R.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

dietitian
    n 1: a specialist in the study of nutrition [syn: dietician,
         dietitian, nutritionist]