I'm the opposite; I have had my permanent ileostomy for 16 years and have absolutely no trouble keeping the weight on and then some. I can eat anything and everything, and it certainly shows!
I was severely underweight before the surgery due to Crohn's, so my surgeon's advice in the hospital was to "eat everything that isn't nailed down"! - and I did and still do to a certain extent. For me, being a bit of a heifer (not obese, just pleasantly plump😉) is healthier than being a stick insect. Put it this way: I NEVER have to see a doctor (I haven't been for about 8 years, and that was just a checkup, not for anything wrong with me) and feel well and have plenty of energy. "Fat and fit," I believe it's called.
Directly after surgery, it did take about 3 months before I started stacking it on. Go for the full-fat version of everything (it tastes better and is actually better for you in the long run), plenty of meat, poultry, eggs, and dairy. Don't eat too much sugar and enjoy the return of appetite as you "live to eat" (like me)! 😅😋😊