Crusaders Against Rancidity

Debbie Eldredge, D.V.M.

Antioxidants are preservatives  added to food to help protect the  fats, oils, and fat soluble compounds such as vitamins from undergoing decomposition. Unsaturated fats can mix with oxygen, “oxidation,” and become rancid.1 Once foods have started to decompose, the process speeds up with the good fats being influenced by the rancid fats.

Fats go rancid if they are exposed to heat (even room temperature) and to air (which contains oxygen).2  All pet foods have some unsaturated fats in them, and therefore, they all require some sort of antioxidant preservative.

A rancid fat is more than just foul smelling. Besides developing  a bad odor, foods lose their flavor and texture. More importantly, there can be harmful effects both from toxins formed in the “bad” fat and from the degeneration of essential vitamins, biotin and fatty acids. “Yellow Fat Disease” or steatitis was a common disease of cats back when they were fed a great deal of tuna with nothing to preserve the oils. The cats showed signs of extensive Vitamin E deficiency.3

Antioxidants come in both synthetic and natural varieties. Vitamin E, which requires some protection itself, is one of the best-known natural antioxidants. The form of Vitamin E most commonly used is alpha-tocopherol. Vitamin E helps to prevent oxidation by donating electrons to block the addition of oxygen to the unsaturated fats.

Vitamin C in the form of ascorbyl palmitate, a slightly modified form of the natural vitamin, is also an approved anti-oxidant. This compound acts by donating a hydrogen atom to the fats it protects. Ascorbyl palmitate is considered more effective than BHA or BHT in its antioxidant duties.1

The two vitamins are both considered to be natural antioxidants. They can be used in unrestricted amounts because of that designation. They are not as stable as the synthetic antioxidants and some of their usefulness may be destroyed in the processing of a food. Foods whose fat is totally preserved with natural antioxidants will have a shorter shelf life than most foods preserved with synthetic antioxidants.

The primary synthetic antioxidants are BHT (butylated hydroxytoluene), BHA (butylated hydroxyanisole) and ethoxyquin. These compounds have been in use since the 1950’s, and are on the list of Generally Regarded As Safe (GRAS) chemicals. Toxicity studies, particularly using BHT, have raised questions about the cancer-causing potential, but other studies have shown that the antioxidants help prevent cancer. Work is being done with antioxidants to try to counter the effects of aging, many of which are attributable to “free oxygen radicals.”

The synthetic antioxidants have a restricted use level which is 150 parts per million for ethoxyquin, and 200 parts per million for BHA and BHT. Ethoxyquin also tends to stand up to the rigors of food processing very well. Because of its efficiency and stability, ethoxyquin greatly increases the shelf life of foods to which it is added.1

It is quite clear that unless we cook fresh meals for our pets every day, we need to have some type of antioxidant added to their foods. Synthetic or natural, which is better? Only time and research will tell.


1. Hilton, John, PhD. “Antioxidants: function, types and necessity of inclusion in pet foods.  Canadian Veterinary Journal, Vol. 30, Aug., 1989, pg. 682.
2. Kallfelz, F., DVM. “Probing preservatives.” Cat Fancy, Nov., 1989.
3. Corbin, J., PhD. “Ethoxyquin answers for pet food clients.” Pet Store Marketing (PSM). Oct., 1989, pg. 26.

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