I'm glad to hear you're enjoying life post-UC. When it comes to gaining or losing weight with an ileostomy, my experience has been that it depends on where your ileostomy is located in your small intestine. The farther down it is, the more nutrients you will absorb and vice versa. I've also learned that everyone's body reacts differently to having an ileostomy.
When I was sick with Crohn's, I was very thin, so I gained weight when I got healthy after my ileostomy, which is located at the end of my small intestine. Six weeks ago, I had to have a small bowel resection done due to a blockage caused by scar tissue, and the surgeon gave me a second temporary ileostomy much higher up in the small intestine. Because of where it's located, I've lost 35 pounds since surgery.
The surgeon told me to drink Ensure every day. I've been doing that, but it goes right through me just like everything else, so I don't know if it's helping. Have you told your surgeon about your weight loss? I would let them know, so you can find out what their suggestions might be. Is your output watery? If so, make sure you drink plenty of fluids, so you don't get dehydrated.
In the meantime, go ahead and eat high calorie foods and dessert whenever you want. Lately, I've been able to maintain my weight that way. Good luck!
Dx'd Crohn's in '99 at 28. Proctocolectomy and ileostomy in '06.
Disease-free, medicine-free, and very thankful to be healthy again.