[Dynseli]

That's a good attitude to have. I looked at this page PubMed Health: Cystic Fibrosis, because I'm unfamiliar with CF. It does mention how CF makes it harder to gain weight. I'm curious, how physically active are you? Walking around, playing sports or exercising for many hours and over-training also causes high calorie demands like that. Rest, and eating, even a snack, immediately when you get hungry helps. Good luck with your condition.