dietician n.

c. 1200, "regular food," from Old French diete (13c.) "diet, pittance, fare," from Medieval Latin dieta "parliamentary assembly," also "a day's work; daily food allowance, food," from Latin diaeta "prescribed way of life," from Greek diaita, originally "way of life, regimen, dwelling," related to diaitasthai "lead one's life," and from diaitan, originally "separate, select" (food and drink), frequentative of *diainysthai "take apart," from dia "apart" (see dia-) + ainysthai "take," from PIE root *ai- (1) "to give, allot."

From late 14c. as "customary way of eating," also "food considered in relation to its quantity and effects," and "a course of food regulated by a physician or by medical rules," often a restriction of food or certain foods; hence to put (someone) on a diet (mid-15c.).  The adjective in the sense of "slimming, having reduced calories" (Diet Coke, etc.) is attested by 1963, originally in American English. 

c. 1200, fisicien, fisitien, later phisicien, "healer, one who practices the art of healing disease and of preserving health, doctor of medicine" (as distinguished from a surgeon), from Old French fisiciien "physician, doctor, sage" (12c., Modern French physicien means "physicist"), from fisique "art of healing," from Latin physica "natural science" (see physic). The restored classical ph- spelling is attested in English from late 14c. (see ph). Related: Physiciancy; physicianly; physicianship.

see dietician.