When you're feeding yourself with player weight gain enabled, it seems like some of the level up upgrades don't seem to function, notably the one that lets you have a third helping at a restaurant. I'm not sure if any of the others aren't functioning, like the calorie-boosting ones, because there's no information on what the calories of each meal are.
On that note, it'd be nice if the calories of each meal were known, since a lot of restaurants do provide nutritional information for their food in real life. Or at least add an upgrade you can purchase that lets you see it.