I don't know, Gem. I can only tell you what I know about
how insulin and the lack of it can affect us. When the body is working properly, insulin helps the glucose running around in our blood stream to get into the cells so it can be used for energy. However, if the glucose can't get into the cells, then the body is usually tired because it can't get energy and it starts using fat and muscle (the latter in extreme cases) to survive, and this does cause weight loss but not in a good way because essentially, the body is devouring muscle to survive. In the meantime, the blood sugar levels are extremely high and this can become critical. That can only go on for so long before serious health problems arise. All this can happen if there is no insulin available.
So, when the pancreas cannot produce insulin, or if it can't produce enough, then insulin needs to be part of a person's therapy on a regular and daily basis.
Now, how diet affects all this is that carbs need insulin to be escorted into the cells for energy and if there are too many carbs, then they are stored for future need (and this can be 'fat'). The more carbs eaten, the more insulin needed but if insulin in lacking and carbs are eaten, then the blood sugar levels sky rocket.
This is why a low-carb regime is best for any diabetic because it will help keep blood sugar levels closer to normal with less medication and even less insulin.
But - here is where I do not know
if having CF complicates this. I would think that these general facts about
blood sugar are true for everybody. I do think it's a good idea for you to look into this further because high blood sugar is not a good condition for anyone.
Best of luck. (Yeah, I'm fine, just older.