micki: You do not have to add all the fats and such to an extruded food, some companies choose not to, which is perferctly fine, and preferable to some people (such as myself) solid gold is a perfect example.
In comparion to extrusion vrs rendering, extrusion wins hands down, rendering is the cooking method that most companys (think of those found at popular pet stores and grocery stores) use.

Here's a quick little article for anyone whom is interested.

http://business.gorge.net/gorgevet/wysong.htm

Christians Mommy: The chicken Soup brand is one of my top 5 favorite foods, for dogs, cats and ferrets! I feed the cat formula here to my adult ferret in his mix, plus my cats love it as a treat.

Here's another interesting website about companion animal food, it's basically in a online course format, very neat
http://www.speedyvet.com/Learningcentre/index1.htm