I agree with Amy22; I've done a bit of nutrition studies for vet, and its all really quite interesting, and damn pet food companies can be bloody sneaky!
There are so many ways to distinguish crappy supermarket brands, and the genuine premium brands (e.g. Hills Science Diet, Royal Canin, Advance- Personally, I do NOT consider brands like Iams to be a genuine 'premium' brand, for certain reasons; namely that they don't run feeding trials). Heres a bit of a tip for trying to find some genuine premium brands;
There's 'groups' I suppose you could call them that aid in a kind of classification of pet foods. The lowest would be the ones that don't reference AAFCO anywhere- these companies can virtually chuck whatever they want into the pet food. Then you get companies that will say "formulated to meet AAFCO requirements"- these foods have the theoretical quantities of the different nutrients animals require, but they are only CALCULATED and not actually tested. Say for example a cat needs "X" amount of calcium in its diet; a cheaper company can just use the cheapest substance/formulation that provides "X" amount of calcium, and then claim it meets AAFCO standards- which is true, it does, but not all of this calcium may actually be absorbable by your beloved pet. The best level/group is when they say something like "Nutritionally complete based on AAFCO feeding trial." These brands have conducted feeding trials to establish/prove that "X" amount of calcium is actually ABSORBED by the pet, if that makes sense. Royal Canin, Hills Science Diet and Advance have all conducted feeding trials, whilst brands like Whiskas, Iams etc have not. Different sized animals, different breeds, animals at different life stages (e.g. puppy/kitten, young adult, adult, elderly, lactating/pregnant etc) etc have different requirements for protein, energy, fats and other nutrients (calcium, magnesium etc)- there is an important balance between these nutrients and if the quantities are stuffed up, then your pet may get ill. Many cheaper brands don't factor for these, either
Personally, my cats are both on the Hills Science Diet- Oral formula
Anyway I apologize for taking things off track from the home made diets. I just thought I'd try to give a tip to those who are still using commercial foods and are trying to figure out which ones are good.
Ha don't get me started! Pet food was something that was just so intruiging to me, that and animal behaviour. I could lecture all day on pet food...
There are so many things that people would find suprising (for example that Advance is owned by Mars, the same people who make such dodge foods as Mars Bars, Pedigree and Dolmio sauces, and Hills is owned by Colgate/Palmolive).
And with the different requirements, there is such a huge range in premium foods that you just don't get with cheap stuff. Different ages, sizes, activity levels, diseases, even foods made just for specific breeds.
There are basically 3 levels of pet foods- your really crap stuff that is only ever sold in supermarkets, but never in a petshop or vet, then your moderately good stuff that is sold in the supermarket and in petshops, but not at the vet, then your really good stuff that is never sold at the supermarket but is in petshops and vets. That's a good way to know the quality you're really getting.
Another thing is that the top premium foods don't advertise on tv, etc. because they want to spend the money they earn on making the food even better. Although a few weeks ago I saw an ad for Hills in a magazine! :O *gasp!*I was suprised, nobody else cared though...