Purina and other pet food from the grocery store is made from the waste of the human food industry, the not-fit-for-human-consumption ingredients.
Please read this article from the Animal Protection Institute:
API: What's Really in Pet Food -- An API Report
This is pinned in the Health & Diet section. It's a great guide to finding a high quality pet food.
http://www.yorkietalk.com/forums/yor...e-samples.html