FOOD AND RELATED MATTERS.
Food need never be eaten. Well, maybe your characters do eat from time to time, but you certainly don't need to mention it. That would just be boring. Even if you go to great lengths to show that they are poor, they should never be hungry, or worry about where the next meal's coming from, or have to eat anything that's not completely sanitary and safe and appetizing. You should not say anything about food unless it's a feast, in which case you should describe every inch of the table setting at every course.
Your Tomboy Princess hero(ine) should receive constant instruction in etiquette, hate it, and disregard all of it. However, this doesn't make her eat like a slob. No one eats like a slob, unless they're one of your Tragically Misunderstood Villain's idiot sycophants. Manners should never be noted at all unless they are "impeccable." The Tragically Misunderstood Villain should have dinner with the heroine at least once, and should be quite charming. With impeccable manners.
Feasts will never be seen as opulent or wasteful, unless the host is morbidly obese and therefore evil. Your hero(ine) can pack it away and never gain an ounce, but the evil people are all ridiculously fat because of their "self-indulgent lifestyle" which is really exactly the same as the hero(ine)'s. But they're evil. So they get fat. The end.
The characters who are fat but not evil should be either a) jovial, continuously making jokes about their LARD, b) a somewhat annoying sidekick who dies, c) not really fat - she just angsts because she thinks she's fat, and her Twoo Wuv cures her of that misperception right off!
Food that was built on the suffering of others tastes bad.
Veganism is perfectly practical in a post-apocalyptic society where everyone lives hand-to-mouth. It will never be inconvenient, and everyone will see the vegan character as noble and outstanding as opposed to frivolous and idiotic.
Every culture, in any time period, ever, has Hot Pockets, Arby's restaurants, buttered cinnamon toast, and... mutton stew. Failure to include at least one of these shall result in shunning.
Living for long periods on nothing but bread and cheese will not have any adverse effect. Likewise, prison fare will be described as paltry and revolting, but it will not weaken your imprisoned protagonists significantly. They can still make a daring escape after three weeks of refusing to eat the swill given them by the (malicious, vicious-minded, yet fundamentally stupid) guards, because they are Just That Awesome. However, if your Young King with Magic and Destiny and s**t ever holds any prisoners, he will give them the best food ever and treat them with utmost courtesy, and they will be ungrateful because they are evil.
