About 7 years ago, I learned that most cat food contains ingredients that are bad for cats. I've since picked up some additional knowledge that I want to share. I'm no expert, but it is almost impossible to find good information as the only "healthy cat" information out there seems aimed at rich people, so this is what I've pieced together. In general, cats are healthiest on an all-wet-food diet but if you're poor (like me) that ain't happenin. Here's what to look for when you're figuring out what to get:
1) meat as the first ingredient and preferably the first 2-3 ingredients (if it has 'meal' after it, it does not count as meat: that's including ground bone & skin).
2) food with the highest protein & fat percentages (as long as your cat is pretty active).
Avoid (if you can't avoid it entirely, choose the option that has them listed later on in the ingredients (they're ordered by proportion):
1) avoid anything that has meat by-products listed as an ingredient. Not only are these gross, but they can make your cat food go rancid and then your cat either won't eat it or will get sick.
2) avoid corn and grains. Corn and grains will make your cat eat more because they are not getting enough nutrition; eventually your cat may get diabetes because of this.
3) avoid vegetables in the first few ingredients, especially especially high-carbohydrate ones. Some high-fiber veggies like peas can actually be good, especially on an all-dry diet, but it should be a fairly low percent.
It is not necessarily the more expensive the better: Iams is just horrible and costs more than Maxximum, for instance. Also, when you buy the food with less filler, cats eat less, so the cost isn't as big of a difference as you might think.
* When cats are kittens, feed them regular, recommended amounts, and don't give extra. Once they are very used to this, you can just fill the bowl and leave it and (usually) they will only eat what they need. As long as they are getting enough exercise they should be fine.
* Cats need daily exercise; if you play with them with toys while they are kittens they will continue to play when they are older, but if they don't get this early training they may not be active enough to be fully healthy as an adult.
* When cats are over 7 years old, they'll probably need wet food as well as dry or else they might end up with urinary tract blockages, because they don't have much of a thirst drive.
* Tuna fish is bad for cats due to high mercury content and can cause nerve problems as cats age.
I order my cat food via Amazon because it's significantly cheaper that way. For wet food, I get Natural Balance Platefulls and mix it with Rachael Ray Nutrish Natural (I don't recommend the Rachael Ray, but if I just give Kanika the healthy stuff they won't eat it more than once a week, and that's not enough to keep UTIs away) and for dry food I get Blue Buffalo Sensitive Stomach and mix it with Merrick Purrfect Bistro Healthy Kitten (because of good ingredients, relatively low price (it's usually about $15 for 7lbs), and good protein/fat/etc ratio). I give wet food every other day (because I can't afford every day) and leave dry food out at all times. If money wasn't an issue, I'd probably make other choices (no 'chicken meal' and no grains at all), but these are the ones I feel like I can manage.