I definitely think that the major changes come from a switch to an "unregulated" food consumption habit, to any sort of deliberate (and often better-quality) diet. I gained some weight in late high school, and a lot of weight in early college, because I basically ate whatever without thinking much about it (and in my freshman year of college I had access to an unlimited meal plan. I think I ate 4,000 calories a day easily). After freshman year, I spent that summer dieting (mostly focusing on eating salads, and otherwise eating less, and only homemade food). I got very into cooking, and just generally ate a lot of mostly healthy, and almost always homemade food. I now eat vegetarian/vegan and overall I think the main change comes from the fact that my diet is consciously chosen (or comes from habits I developed while focusing on eating healthily).