 " Lisa the Vegetarian " was the first full @-@ length episode David X. Cohen wrote for The Simpsons . His most prominent work for the show to that point had been the " Nightmare Cafeteria " segment in the season six episode " Treehouse of Horror V " . The idea for " Lisa the Vegetarian " came to him while he was working on another Simpsons script . Cohen could not concentrate on his task because he was waiting for lunch , and on the back of the script he scribbled , " Lisa becomes a vegetarian ? " Cohen showed the note to Simpsons writer Brent Forrester , who liked the idea . Show runner David Mirkin then approved the story when Cohen pitched it to him . Mirkin had just become a vegetarian himself , and later noted that many of Lisa 's experiences in the episode were based on his own . 
