nutritionist (n.)

1550s, "act or process by which organisms absorb their proper food into their systems and build it into living tissue," from Old French nutrition (14c.) and directly from Latin nutritionem (nominative nutritio) "a nourishing," noun of action from past-participle stem of nutrire "to nourish, suckle, feed," from PIE *nu-tri-, suffixed form (with feminine agent suffix) of *(s)nau- "to swim, flow, let flow," hence "to suckle," extended form of root *sna- "to swim." Meaning "that which nourishes, nutriment" is from c. 1600. Related: Nutritional.

word-forming element meaning "one who does or makes," also used to indicate adherence to a certain doctrine or custom, from French -iste and directly from Latin -ista (source also of Spanish, Portuguese, Italian -ista), from Greek agent-noun ending -istes, which is from -is-, ending of the stem of verbs in -izein, + agential suffix -tes.

Variant -ister (as in chorister, barrister) is from Old French -istre, on false analogy of ministre. Variant -ista is from Spanish, popularized in American English 1970s by names of Latin-American revolutionary movements.