Pancreatic sufficient, but losing weight

Quick recap: My DS, 12 years old, was diagnosed with CRMS because of one mutation & one variant (DF508 & 5T/12TG) and borderline sweat tests (40/41). He has chronic constipation and wasn't gaining weight steadily on a growth curve so his Dr prescribed enzymes as a "trial" to see if they would help. He did begin to gain weight but I couldn't help but wonder if he was just catching up with a normal growth spurt or if the enzymes were actually doing something. The Dr that started him on enzymes left her practice and the new Dr agreed that we could do a little testing to see if he actually needed them or not. We did the one-time fecal elastase test and it came back normal at 380 (normal being >200). The Dr said he was PS so we stopped taking the enzymes. That was one week ago and he hasn't had any new GI symptoms like pain, diarrhea, or greasy stool (he's never had any of those), he is still constipated and taking Miralax daily, but now I have noticed a weight loss. We had finally gotten him up to 82 pounds (4'11" tall), and now after a week he is at 79 pounds, so maybe the enzymes were helping him after all. Here is my question: If he truly is PS, why would taking enzyme supplements help him to gain weight? In theory, his pancreas is producing all the enzymes he needs, so how would introducing more help anything? I'm so confused.
Source: Cystic Fibrosis Families Forum - Category: Respiratory Medicine Authors: Tags: Families Source Type: forums