First, a little background on cat's nutritional needs. Unlike dogs that are omnivores, cats are strict carnivores. In the wild, they would typically eat small mammals like rabbits and mice, so their protein and fat requirements are high while their need for carbohydrates is low. When you break down a typical mouse they are around 55% protein, 38% fat, 2% carbohydrates, and the remainder a mixture of minerals and vitamins (with a moisture level of around 65%). Looking at many commercial diets you may see that they can have carbohydrate loads upwards of 20-30% (which is typically very high for cats) and the dry diets have moisture levels of around 10% (which is very low for cats).
So then, what do I suggest that a cat eat. First, feeding an all wet food diet or mostly wet food diet is helpful. This helps a lot with hydration, which is important (especially in male cats). Many cats can get urinary issues when they are chronically dehydrated and keeping them on a wet diet helps prevent this. Also, a diet low in carbohydrates (hopefully less than 15%) with a protein level higher than 30% would be ideal. You can check out this table to see where your pet food stands. Making sure that the protein sources are from a meat source and not plant source is important as well.
The other issue is feeding the correct amount of food for your cat. Most cats only need between 150-250 calories per day of food, which typically comes to around one 6 oz can of food or 1/3-1/2 cup dry food per day. More active cats will need more food while less active ones need less. A typical cat is ideally around 12 pounds.
Another good resource for feeding your cat can be found here.
