A diet consisted of one's nutritional intake.
A change in diet could be prescribed by a physician to compensate for one's changing nutritional needs. In 2152, Jonathan Archer asked Doctor Phlox whether Porthos would require any special diet after receiving a chameleon's pituitary gland. (ENT: "A Night in Sickbay") In 2266, Dr. Leonard McCoy changed James T. Kirk's diet card to dietary salad following a two pound increase in the captain's weight. (TOS: "The Corbomite Maneuver")
In 2372, Harry Kim compared the fluffy content of "A Briefing with Neelix" to a steady diet of dessert, saying one would eventually become tired of it and want meat and potatoes. (VOY: "Investigations")
Different races had varying dietary requirements; some were biologically based, while others were cultural. In 2151, Dr. Phlox sent the Xyrillians Trip Tucker's dietary requirements so that he would not need to return to Enterprise NX-01 to eat.(ENT: "Unexpected") The Anticans preferred live animals freshly killed rather than preserved meat. In 2364, Natasha Yar discussed these requirements with William T. Riker when Antican delegates aboard the USS Enterprise-D declined their offer to preserve the livestock as meat. (TNG: "Lonely Among Us") The Kadi's dietary protocols prohibited highly flavored foods, something which Neelix reminded Ambassador Tomin of when he requested to try hasperat. (VOY: "Someone to Watch Over Me") In 2374, Seven of Nine was given a list of dietary requirements by The Doctor ("two hundred fifty grams glycoproteins consisting of fifty three percent polypeptides and twenty six percent fibrous glycogen") and provided it to Neelix, but the Talaxian suggested a plate of his chadre'kab would be an acceptable substitute. (VOY: "The Raven")