Obesity, the most common food-related problem, is easy to correct. First, talk to your vet about the weight-loss program that's right for your cat. In many cases, this will involve switching to a high-fiber, low-fat diet and scheduled feedings. Introduce any new diet slowly suddenly placing an overweight cat on a strict diet can cause fat to accumulate in the liver, leading to liver failure. Along with a change of food, it's also a good idea to encourage a sedentary cat to become more active through play.
The vet should see a scrawny cat that doesn't gain weight. If there is no health problem, ask for a good food to bulk up your bag of bones. It's not a big deal if your cat skips a meal every now and then, but a finicky cat that repeatedly turns up his nose at his food is another story. If he's holding out because a new food is being introduced, make the change even more gradual. Some cats simply don't like a variety of foods and may not eat again until they get their favorite dish. It will become more dangerous to hold out, in the hope that your cat will come around to the new diet, than to feed the same food daily. Don't let your cat go without food any longer than 24 hours before you take him to the vet; disinterest in food may be the first sign of a serious illness.