Med student here. My guess is the following: So with CF your pancreas can’t secrete many of the enzymes that are necessary to actually digest food. You can take medications to help with that digestion, but regardless, you aren’t actually getting all the calories in your food with CF because it isn’t making its way into your body (notably patients with CF have steatorrhea which is fat in their stool because they can’t absorb a lot of fat from their foods like a non-CF patient). The med helps the pancreas recover some of its ability to secrete digestive enzymes and so patients can now eat and get more from their food. The issue is that there needs to be a recalibration in terms of how much patients are eating. Previously a 2000 calorie diet may not have gotten them so far because they didn’t absorb much of it, but now they’re absorbing a lot more of it. Plus increased work of breathing with CF expends more calories compared to a non-CF patient (and patients on this drug).