It’s a question you see a lot: How can someone who is thin be a good ally to fat people?
There are a few basic things you can do, some easier than others.
Start With Yourself
Stop engaging in diet talk, body hatred, food moralizing, and so on. Stop telling fat jokes and equating eating a lot or being lazy with being fat. Even if there are no fat people around. Even if you only mean it in a self-directed way.
Every time you say something negative about bodies, including your own, you are enforcing the hierarchy that makes thin more socially acceptable than fat. It doesn’t matter if no fat people are around when you say it. Thin people will hear this and legitimize their own prejudices.
Also, its just a rude and nasty way to be and you don’t need that kind of negativity in your life.
This sort of talk includes moralization of food. Referring to things as “good” or “bad” (or calling yourself one of those things because of something you ate), talking about how you need to “work off” something you ate, or discussing whether someone needs to or should eat something all enforce the same fatphobia. People do all of these things because of a fear of becoming fat. They may claim its because of health but if that were the case there would be no talk of working off food, there would be talk of vitamin content or eating for joy (because mental health is health, too!).
Stop using the term “flattering” when talking about clothes. That word just means making someones body appear to conform to ideals (meaning, making it look smaller). If someone is wearing something that isn’t “flattering,” that’s OK! They probably want to be wearing it, and it probably makes them happy, which is what matters. Or maybe it’s all they can afford, so laughing at them makes you twice the jerk.
Saying “get fat” when you mean eat lots of delicious foods is harmful as well. It reinforces stereotypes. People of all sizes have lazy days or go to events where they eat a ton of rich dessert. One indulgent meal will not make a thin person fat.
Address Other Thin People
Speak up when you hear others doing these things. Tell people you don’t want to hear about diets and that what someone else eats is their own business. Tell people that it’s not cool to make fun of someone’s clothes, even if they are wearing something that’s not normally shown on their body type. Point out that eating a lot doesn’t equal getting fat, and that it’s not OK to make that false equivalence. Don’t let people get away with casually reinforcing stereotypes.
Say something when you see a fat person being bullied. Confront people who are making fun of someone and tell them its not cool. Obviously, be mindful of safety, but if something feels unsafe to you imagine what it must feel like for the victim. If you can’t directly intervene, go up to the victim and ask if they’re OK. This guide to intervening when you see Islamophobia can be applied here, though in some cases with fat-shaming, speaking up works well.
(If the fat person gets mad and brush you off, or even if they are outright rude to you, don’t retort. They’re not actually angry with you; they’re mad at the situation and probably extremely embarrassed.)
If you decide it’s safe, you can shame the bully as strongly as possible, but do so without relying on stereotypes. Things like “that’s not cool” or “their body/health/outfit/meal/etc is none of your business” or simply “leave them alone” are fine. If in public, don’t be afraid to be loud (unless it would make things worse for the victim).
Bullies don’t react well to being embarrassed or to having the tables turned on them. They’re cowards. Most of the time they’ll back off. When I loudly shamed a douche on the subway who told me to go running every morning, it didn’t take much to get him to slink away. Once attention was turning to him, and people were giving HIM the stink eye, he lost his power.
Speaking up online can help a lot as well. Comment sections are rife with anti-fatness. Your voice can be a big help and take the emotional burden off of fat folks. Plus, sadly, many people are more likely to listen to a thin person than a fat one.
This means if you are privileged, it’s your duty to speak up. Thin people need to call out fat-shaming, similar to how white people should call out racism, straight people should call out homophobia, men should call out sexism, and so on. It will mean so much to the victim, and it makes the world just a tiny bit more pleasant.