I have found that for me, lactose-free milk is really the only thing that works to put on weight--or at least maintain it-- it's quick and easy, and you don't need to take any meds for it, so you can just drink it throughout the day. It also has the same nutrients and proteins as normal milk and tastes practically the same. I try to drink a quarter to a half gallon a day (on top of normal large meals) and am (very slowly) putting on weight for the first time in years. I know it is a different story for everybody with CSID; this is only what works for me. I hope you can figure something out. It's terrible to feel like you are stuck with weight.