I had a foster, Tara, that I pulled from the pound and was malnourished. The vet suggested feeding like 4 (small) meals a day until she got to a healthy weight/condition. She was skin and bones but had a potbelly, we thought for sure she had worms but she did not
we used canned food, and then mixed it with kibble.
After the first week I put her on 'normal' food (fosters here always ate TOTW) and she did fine! It wasn't an overnight thing, but she did get a lot better fairly quick and when she got to a good condition I cut back to the normal 2 meals a day and she kept it.
One thing I wanted to say is with Tara, she was a stray, so we suspected she ate a lot of garbage and scraps. She had a stomach of STEAL I kid you not. But about 2-3 weeks in she went through a terrible 'detox' - her coat blew, her skin coat and eyes were nasty, as were her stools. I took her to the vet twice that week and they ran tests, but she was fine, just detoxing. After that her coat and everthing looked a lot better