Food
The first thing we learn in It Happened One Night is that Ellie's "on hunger strike," and from there on out, the subject of food keeps coming up throughout the film. What's with all the donuts, chocolates, and carrots, not to mention the constant references to hunger?
Well, for one thing, Ellie comes from high society, which in this movie is kind of an empty place. Sure, it's fun and flashy, but it doesn't have much substance. In a way, Ellie is "starving" for something real—and she finds it in Peter. It's no accident that her turning point comes when she finally decides to eat raw carrots. They're not fancy, but they're actually nutritious, and they come from the earth. In a way, Ellie is returning to her human roots (pun totally intended).
On another note, appetite for food is often associated, in literature and film, with the another kind of appetite—you know, the kind of appetite that has to do with good old-fashioned S-E-X. And It Happened One Night can also be seen as a story about Ellie's quest to find—and ultimately to fulfill—her desire. She's never force-fed (even though her father threatens to force-feed her); in fact, Ellie's given space to find what it is she craves—and though that may not be carrots, exactly, it turns out to have a lot in common with the workaday carrot.