In fact, these roles don't have to be mutually exclusive. Take, for example, milk, one of the most common transformation reagents, and certainly a popular food item generally in stories. It's probably my single favorite food item to write about, for a lot of reasons, both aesthetic and psychological.
Magical milk can be the cause of a transformation, like in one of my favorite Matt. L weight gain stories, The Simple Life or the interesting weight gain/lactation tale, Mama Latte. Milk consumption can also be seen a side effect of a transformation, to reflect changed appetite choices, like in my fanfic Super Size. But milk can also be both a cause and and effect. In my weight gain story Roll Model, the main character's consumption of milk is both a symptom of her transformation (it reflects changes in her behavior) and it causes further weight gain, which in turn causes more changes in her behavior.
Other common foods include candy, milkshakes, ice cream, cheese. Yes, diary tends to feature heavily, both in my stories and in the work of others. Also popular are energy drinks (like my age progression story Elixir), and junk food, like pizza. Even healthy food can be rendered transformative by surreptitiously altering its properties, like what was done in my age progression ebook Golden Girls Spa.
Another way food can act as a component for a transformationstory is as a setting. My Wilma series, for example, revolves around a chocolate factory. There are many, many, many, stories out there about a girl gaining after taking a new job at a donut shop, including my short weight gain ebook Donut Surprise. Working as a waitress is another common profession. Usually the formula is that after entering or working in a location abounding in fattening food, a thin woman loses her willpower and then gains way.
Erotic eating and weight gain often go together, for obvious reasons. Weight gain has featured a role in most of my stories, so it's not surprising that there's also a fair amount of focus on food. To keep food fresh (ha ha), I've tried to vary the kinds of food, the manner and location in which it is consumed, and the way it affects the story.