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Ethoxyquin is a fat preservative, specifically an anti-oxidant, used in many different brands of commercial pet food. It is also used in some of our foods such as chili, paprika and in fats of some animals. The big difference is the amount of ethoxyquin allowed for pet foods and for human foods. While for human foods the maximum allowance is 0.5 ppm (parts per million), in pet foods the maximum allowance is 150 ppm, that is, the amount used in pet foods is 300 times greater than that used in human foods. Not to mention that our menu is varied while a pet usually eats only the commercial food and that with the exception of some giant breeds of dogs, we weigh a lot more than our pets.
Many studies have showed that ethoxyquin is carcinogenic, that is, it causes cancer. Thyroid cancer is one of them. In parallel, another study showed that the incidence of hyperthyroidism in domestic dogs and cats rose dramatically in the early 70's, that is, very few years after ethoxyquin was approved for pet feeds (1969).
I did some homework too. For about six months, I contacted every person who posted asking for help regarding hyperthyroidism in the many different cat newsgroups to ask what the cat was fed and for how long the cat had been fed that specific food. Without a single exception, the answer was always the same: the cat was fed some commercial food containing ethoxyquin (many brands out there use it) for a period of at least three years. Note that ethoxyquin is sometimes labeled as E (not vitamin E!).
The brands which use ethoxyquin in their feeds claim that they have over 400 studies showing that the amount of ethoxyquin used in pet feeds has no side effects whatsoever. Well, how are these studies done?.
When a new drug is discovered, it is first tested in rats or some other guinea pig. The amount used must be many times the amount which will be recommended when the drug reaches the market, if it ever does. Why? Because the long-term side effects must be known. In other words, the only cost-effective way of learning what these side effects will be is by overdosing. Almost everything we eat or drink must be metabolized (changed) in order to be used by the body or eliminated if useless or toxic. But metabolism takes time. If we eat or ingest faster than our body can deal with the substance, then there is a build-up of that substance in our system. Take alcohol for example. Your liver can metabolize only so much alcohol in a given time. If you drink more than the amount your liver can deal with in a given time, you get drunk. This tolerance varies from individual to individual and from species to species. When an animal is given a food containing ethoxyquin, the amount ingested is way larger than the amount the animal can eliminate, therefore ethoxyquin builds up in the animal's system. After a few years, the amount built-up in the system starts causing harmful effects. Among the owners I interviewed, some pets took as little as three years of build-up to develop hyperthyroidism while others took as much as eight years. Not one of those 400 studies used higher levels of ethoxyquin than those found in pet foods and not one of them were done over a considerable period of time in order to study side effects from long term usage.
I also suspect ethoxyquin causes fatty liver disease or hepatic lipidosis. When an animal eats less than it should or when it starves, the body has to break down fat to meet its energy requirements. The fat is released into the blood and travels to the liver. The liver then breaks down the fat into glucose and ketone bodies and this process involves oxidation. Ethoxyquin interferes with the oxidation (remember, that's its duty, it's an anti-oxidant) and fat begins to accumulate in the liver because the liver cannot break it down fast enough, resulting in fatty liver disease.
You have a choice. The number of commercial diets that are preserved with vitamin E is growing and their prices are very similar to those preserved with ethoxyquin or other chemicals, such as BHT and BHA. If your cat has a special condition which requires a prescription diet, ask your vet to suggest other brands preserved with vitamin E or if you have time, to teach you to prepare your pet's food at home. Your pet will thank you.