Pošto ste pitanje već zatvorili, samo želim da vam skrenem pažnju da bi prvi deo rečenice trebalo da glasi: „Prvi i SAMO dijetetski proizvod...“ (pretpostavljam da se želi naglasiti da to nije lek, tj. farmaceutski preparat), pošto je „only“ ovde u funkciji priloga, a ne prideva (jer je „nutritional“ pridev, a ne imenica), pa se ne može prevesti sa „jedini“:


Ignjatić Z. et al, ESSE rečnik:
only (adv) - samo
only (adj) - jedini, sam



Oxford OALD Dictionary:

only

- adjective [onlybeforenoun]
1 used to say that no other or others of the same group exist or are there:
She's their only daughter. * We were the only people there. * His only answer was a grunt. * Only five people turned up.
2 used to say that sb/sth is the best and you would not choose any other:
She's the only person for the job.

- adverb
1 nobody or nothing except:
There are only a limited number of tickets available. * The bar is for members only. * You only have to look at her to see she doesn't eat enough. * Only five people turned up.
2 in no other situation, place, etc:
I agreed, but only because I was frightened. * Children are admitted only if accompanied by an adult.

--------------------------------------------------
Note added at 7 hrs (2006-06-09 21:01:44 GMT) Post-grading
--------------------------------------------------

DOPUNA:
Vi svakako bolje znate šta odgovara suštini celine teksta. Bitno je da ste na osnovu šireg konteksta sigurni da se želi reći da je to JEDINI takav dijetetski proizvod (znači, ne postoji nijedan drugi takav), a ne da se želi naglasiti da je to SAMO dijetetski (tj. da to nije lek - farmaceutski preparat).
Inače, samo na osnovu date rečenice, “only” bih svakako prevela sa “samo” - mislim da gornja argumentacija nije sporna.
Srećno!