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Stay away from food containing "byproduct meal" or "meat and bone meal." These rendered products are the most inexpensive sources of animal protein. The contents and quality of these meals can vary tremendously from batch to batch and are not a reliable source of nutrition for your animal.
Avoid foods that rely on byproducts as the sole source of animal protein. Byproducts consist of organs and other parts, either not desired or condemned for human consumption. An occasional byproduct based food may be okay, since in the wild, carnivores do consume the whole prey including the organs, but these foods are not acceptable as a steady diet.
Look for a named meat or meal ("lamb" or "chicken meal," for example, instead of the generic term "meat") as the first ingredient.
In general, select brands promoted to be "natural." While they are not perfect, they may be better than most. Several brands are now preserved with vitamins C and E instead of chemical preservatives (such as BHA, BHT, ethoxyquin and propyl gallate). While synthetic preservatives may still be present, the amounts will be less.