How do I avoid gaining weight in hospital

How do I get away with not gaining weight because they’re scared if I start eating normally I’ll have refeeding syndrome. I don’t know when I’m gunna be admitted sometime this/next week when they have a bed. I don’t want to gain weight, but the ED clinic said they can’t help me if I don’t because they’ve deemed me medically unstable bc symptoms and behaviours. It’ll be for 2-3 weeks. I don’t know what to do