Question for thought:
Animals in the wild eat raw and do not need supplements. Why supplement our pets when feeding raw?
There are a couple of ways to feed "raw"":
1. Whole prey -- primarily birds & rodents -- contains hair, skin, feathers, bones & organs;
2. Body parts -- can be any type of protein a cat will eat -- doesn't necessarily include bones or organs, skin, feathers;
3. Raw ground (a) containing whole bodies (skin, feathers, hair, bones & organs), (b) containing some but not all of (a) or consisting only of meat.
Each part of an animal contains vitamins, minerals and amino acids necessary for balanced diet. The more we move away from the balance, the more we have to supplement to ensure health.
Whole prey -- the fresher the better and is a tidy package containing most nutrients required. I buy quail, chicks, guinea pigs, rats, mice in bulk and since taurine (an important amino acid that cats can't synthesize) can degrade when frozen, I supplement with taurine occasionally. This amino acid appears in dark muscle (such as thighs) and in organ meats such as the heart & brain.
Body parts -- When we feed parts, we have to be careful to include appropriate muscle to organ ratio to keep the diet balanced. You can't just throw a chicken leg on the floor every day and expect that it contains all nutrients required for a balanced diet. The farther you get from whole body, the more you have to supplement to insure balance. The way a food is processed can also degrade nutrients which will need to be readded into the diet.
Lastly, the quality of the food fed to the prey animal affects its nutritional value. We can't always find out how healthy the prey diet was, especially for supermarket bought meats, so that can be another reason for supplementing a raw diet.