This explains a bit more, too.
The ingredients list on the label is an area of consumer preference and subjectivity. Pet owners who do or do not want to feed a pet a certain ingredient can look at the list of ingredients to make sure that particular substance is included or excluded.
Some people prefer to pass up animal by-products, which are proteins that have not been heat processed (unrendered) and may contain heads, feet, viscera and other animal parts not particularly appetizing. But protein quality of by-products sometimes is better than that from muscle meat, says Burkholder.
"Meal" is another ingredient that some people like to avoid. In processing meat meal or poultry by-product meal, by-products are rendered (heat processed), which removes the fat and water from the product. Meat or poultry by-product meal contains parts of animals not normally eaten by people.
Some consumers try to avoid pet foods with synthetic preservatives, such as butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), and ethoxyquin. Ethoxyquin, in particular, has been hotly debated. Current scientific data suggest that ethoxyquin is safe, but some pet owners avoid this additive because of a suspected link to liver damage and other health problems in dogs. CVM has asked pet food producers to voluntarily lower their maximum level of ethoxyquin in dog food while more studies are being conducted on this preservative, and the industry is cooperating.
Many products preserved with naturally occurring compounds, such as tocopherols (vitamin E) or vitamin C, are available. These products have a much shorter shelf life than those with synthetic preservatives, especially once a bag of food is opened.
Some animal nutritionists recommend switching among two or three different pet food products every few months. Burkholder says nutritional advice for people to eat a wide variety of foods also applies to pets. Doing so helps ensure that a deficiency doesn't develop for some as yet unknown nutrient required for good health. When changing pet foods, add the new food to the old gradually for a few days to avoid upsetting the pet's digestive system.
Like human foods, pet foods are regulated under the Federal Food, Drug, and Cosmetic Act, and must be pure and wholesome and contain no harmful substances. They also must be truthfully labeled. Foods for human or pet consumption do not require FDA approval before they are marketed, but they must be made with ingredients that are "generally recognized as safe" (GRAS) or ingredients that are approved food and color additives. If scientific data show that an ingredient or additive presents a health risk to animals, CVM can prohibit or modify its use in pet food.
Pet food ingredients must be listed on the label in descending order by weight. However, the weight includes the moisture in the ingredient, which makes it tricky to interpret. "A moist ingredient, such as chicken, which may be 70 percent water, may be listed ahead of a dry ingredient, such as soybean meal, which is only 10 percent water--yet the soy actually contributes more solids to the diet," says Susan Donoghue, V.M.D., owner of Nutrition Support Services, Inc., and past president of the American Academy of Veterinary Nutrition.
Similar materials listed as separate ingredients may outweigh other ingredients that precede them on the list of ingredients. For example, chicken may be listed as the first ingredient, then wheat flour, ground wheat, and wheat middlings. The consumer may believe that chicken is the predominant ingredient, but the three wheat products--when added together--may weigh more than the chicken.
http://www.fda.gov/fdac/features/2001/301_pet.html