Preservatives are necessary in commercial pet foods, primarily to prevent the fats from turning rancid and to inhibit mold growth. Synthetic preservatives have been used for many years, but concerns about the potential harm in feeding chemical preservatives with every meal have encouraged the pet food industry to turn to more natural preservatives. These are usually found in higher quality foods and are a better choice.
The most common synthetic preservatives are BHT, BHA and ethoxyquin. BHT and BHA are also used to stabilize rubber products, like tires, to keep them supple, and as herbicides. These are also effective preservatives and keep kibble and treats from spoiling almost indefinitely. Natural preservatives now found in better pet foods are antioxidants like vitamin E (usually listed as "mixed tocopherols"), vitamin C and rosemary extract. Natural preservatives are also very effective, but have a shorter shelf life; and once opened, the bag should be used within a month or kept in the freezer to preserve freshness.
BHT and BHA are known carcinogens. However, the FDA allows for low levels of these synthetic preservatives in both pet and human food and they are GRAS (generally known as safe) in small amounts. Ethoxyquin, while widely used, is still being studied. Some research has shown that it may compromise liver function in dogs, and in 1997 the FDA recommended that the allowed limit in pet foods be lowered. Some feel that the standards are still too lax and ethoxyquin should not be used at all. It is in very limited use in human foods. People do not eat food laced with ethoxyquin, BHT and BHA at every meal, unlike kibble-fed dogs and cats, and this has raised concerns.
Natural preservatives are becoming more common in pet foods, in response to concerns many owners have about the synthetic preservatives. Mixed tocopherols and antioxidants are very effective, but it's important to use the food by the due date printed on the packaging, and to use it within a month of opening or keep it in the freezer. The type of preservative used will be listed in the ingredients on the packaging.
Beware of food labeled "holistic" or "natural." These are marketing terms and have no legal definition. There are foods labeled holistic and natural, but they may contain many synthetic or highly processed ingredients like imported gluten, chemical food dyes and synthetic preservatives. It's important to read the ingredient list to understand exactly what you're giving your pet.
Few studies have conclusively proven that synthetic preservatives have caused significant health problems in pets. However, they have not been extensively studied and there is much speculation that they may be linked to a range of health issues, from minor skin problems to fatal cancers. It may well be that the nutritional advice given to people also applies to pets: Feed fresh foods and avoid a diet high in over-processed foods. Until more is known about the effects of chemical additives to food, choosing a more natural diet for a pet may be the wisest choice.