Eat a lot and don't put on weight?
In the digestion system, mainly the pancreas is affected by Cystic Fibrosis. This means that the fat molecules can not be split and therefore do not get absorbed by the body. The missing natural enzyme can be taken by artifical pills with every meal. Nevertheless most CF patients seem thin and undernourished. It's hard for them to keep their weight not even thinking of putting on weight. What seems to be a dream for most of the healthy persons is a real challenge for every CF patient. Often they have to take supplements such as high calory drinks.
Beside the missing fat enzyme there's also the issue with insulin which is also produce in the pancreas. It is needed to absorb sugar. If there is too less insulin in the body, the sugar level in the blood will rise and this can cause depositions in the venes and arteries. Therefore many CF patients are also diabetics and have to inject insulin with every meal. There's a serious conflict. While regular diabetic are put on a special diet because most of them are overweight, CF patients absolutely need the calories and carbohydrates. This is why there's no way you can put a CF patient on a diet. The only reasonable approach is to measure the sugar level before every meal and inject the right amount of insuln.
Another problem with the insulin is cause by Prednison. Many CF patients have to take it in order to reduce the inflammation of the lungs and bronchia. But Prednison "kills" the insulin which is produced by the pancreas and so it occurs that a CF patient's natural insulin level would be sufficient but it is badly affected by Prednison. Apart from that, Prednison is also affecting the bones which can cause an early osteoporosis (lower densitiy of the bones).
This is the usual curse of effects and by-effects.