This Common Type of Food Is Dangerous for Your Pet

Analysis by Dr. Karen Shaw Becker

fish based pet foods

Story at-a-glance

  • I don’t recommend feeding most fish as a protein source in cat or dog food, because most of it is contaminated with mercury, industrial chemicals, and toxic preservatives
  • Research published in 2016 revealed that many commercial dog and cat foods contain high levels of mercury, especially formulas containing fish
  • Additional contaminants found in fish used in pet food include PCBs, dioxins, and DDT
  • Much of the fish meal in pet food is heavily preserved during the manufacturing process, often with a chemical called ethoxyquin, which is known to cause cancer
  • My recommendation is to avoid feeding farmed fish (which is in virtually all fish-based pet foods) to your dog or cat; feeding small, wild caught fish can be done healthfully, as a part of varied diet
  • Pets have notable DHA and EPA requirements that are best met by supplying ocean-sourced omega 3’s; transparent companies provide third party testing for purity so you can feel confident supplying quality marine oils to support your pet’s health

Fish is a very popular ingredient in commercial pet food. But as many of my regular readers know, I’m not a fan of fish as a protein source in cat and dog food because sadly, most seafood used in pet food these days consists of leftovers from fish farming industries that research shows is loaded with toxic metals like mercury.

Mercury Concentrations Can Be Well Above Safe Levels

In a 2016 study, scientists at the University of Nevada, Reno (UNR), measured the mercury in over 100 dog and cat foods on the market.1 Their results showed that some of the pet foods had mercury levels above what is considered “safe” for other animals, and of course this was especially true of formulas containing fish.

The researchers evaluated 54 dog foods and 47 cat foods, both wet and dry formulas, across the price spectrum. The foods they tested contained from 1 to 604 nanograms per gram (ng/g) of mercury. Fourteen of the 101 samples, all of which contained fish, had mercury levels above 100 ng/g, which is considered the daily “safe” level for river otters.

For small mammals, 70 ng/g per day is considered “safe,” so the researchers considered the 100 ng/g to be a “reasonable level of concern” in dog and cat food.

Certain types of fish accumulate more mercury and other toxins — especially predatory fish such as tuna, sharks, and swordfish — because they’re higher up the food chain and eat smaller contaminated fish. The study results indicate that pet foods containing salmon and trout tend to have the highest levels of mercury.

The body of water a fish lives in also impacts the levels of mercury, other heavy metals, and pollutants it accumulates.

The researchers found that cat foods tend to have more mercury than dog food, again, especially formulas containing fish. One brand from Fussie Cat made with tuna and prawns had 604 ng/g mercury in one batch and 373 ng/g in another. Other brands with high concentrations of mercury:

  • Merrick Purrfect Bistro with tuna nicoise, 278 and 124 ng/g in different lots
  • Tiki Cat with ahi tuna and mackerel, 335 ng/g
  • Tiki Cat with ahi tuna, 234 ng/g
  • Hill's Science Diet adult 1-6 years with tuna, 282 ng/g

The UNR study authors suggest that pet food producers can avoid mercury-laden fish by understanding more about the species of fish they use in their formulas. For example, plankton-eating fish like Asian carp and other fish that eat low on the food chain are safer bets.

In a brand-new study published in the journal Science of The Total Environment,2 UNR researchers used DNA technology to test 59 dog foods (both wet and dry), 14 dog treats, 17 dry cat foods and 35 wet cat foods for not only mercury contamination, but adulteration of other kinds. I’ll review this new study in more detail in a future article, but on the specific topic of mercury, here’s some guidance for pet parents from the lead study author:

"Based on our results, I would encourage pet owners to avoid or minimize the feeding of fish-based foods to their pets to mitigate potential risks related to chronic mercury exposure.”3

More Contaminants in Seafood

Many types of fish are also contaminated with industrial chemicals like PCBs (polychlorinated biphenyls), dioxins, and pesticides like DDT (an insecticide banned in the U.S.).

As with mercury, these toxins are absorbed by the smallest ocean plants and animals at the low end of the food chain. As larger ocean dwellers come along and consume these contaminated plants and prey, the toxins accumulate and become more concentrated in the bodies of bigger fish. Consequently, the largest predators in the ocean end up heavily laden with toxins.

To make matters worse, the fish in pet food is heavily preserved during the manufacturing process, often with a chemical preservative called ethoxyquin, which is known to cause cancer. Ethoxyquin is banned in human food except in very small quantities allowed as preservatives in spices.

Ethoxyquin Is a Hidden Ingredient in Many Pet Foods

Unfortunately, ethoxyquin can still be found in many pet foods currently available on the market. It is used to preserve the fat (fat made from waste products) in almost all fish meal.

It’s important to keep in mind that if the label doesn't list exact ingredients, including the precise protein source (vs. “meat meal” or “fish meal”), you have absolutely no idea what's in there. And because ethoxyquin is added before the raw ingredients are shipped to pet food manufacturers, it doesn't get listed or disclosed on the product label.

The pet food company you purchase your animal companion’s food from may not be adding ethoxyquin, but that doesn't mean it isn't in the fish meal in that food. Don't make the mistake of assuming if the fish meal product label doesn't list ethoxyquin, it's not in there.

Unless the label specifically states the formula is ethoxyquin-free (which means they’ve done third party testing, which they will show you), or you call the manufacturer's toll-free number and are told it's not in the raw materials they purchase nor added during their own manufacturing process, you should assume the formula may contain ethoxyquin.

Fish meal made from farmed fish is one of the main pet food ingredients also contaminated with mycotoxins.4

Additional Problems With Fish-Based Pet Foods

I’m a big advocate of rotating proteins in pet diets. That's because any food that is over-consumed can create food sensitivities and nutritional imbalances over time. Fish, as it turns out, is one of the most highly allergenic foods for cats.

Allergies cause systemic inflammation. Kitties who eat allergenic foods regularly can end up with lung inflammation that can lead to asthma, which is one of the more commonly diagnosed inflammatory conditions in cats. In addition, links have been established between mercury and asthma, and ethoxyquin and asthma, so it's easy to get a glimpse of the bigger picture regarding diet-related inflammatory conditions.

Fish fed in high amounts, especially in poorly formulated and/or poor quality foods, can ultimately lead to thiamine deficiency, which can cause loss of appetite, seizures, and even death. Long-term ingestion of fish in cat food can also deplete vitamin E resources.

Seafood is a very rich source of iodine, but cats’ bodies don’t require a lot of iodine. Many animal nutritionists, including me, believe there's a link between cats consuming too many iodine-rich foods and feline hyperthyroidism. A link has also been established between pop-top cans on canned cat food and hyperthyroidism.

Pet food companies now sell “low-iodine” formulas for hyperthyroid cats. A better approach is to avoid feeding cats fish-based food in the first place. Avoiding foods high in iodine seems like a good way to prevent hyperthyroidism in kitties.

Finally, the magnesium content in fish has been linked to urinary tract disease in cats. A diet overloaded with the mineral magnesium can predispose your kitty to magnesium ammonium phosphate crystals, also known as MAP crystals or struvite crystals. Crystals are a big problem for many cats today.

My Recommendation

My advice is to be very choosy about the fish you feed your pet, and I certainly don't recommend feeding an exclusive diet of fish protein to dogs or cats. That said, fish is a rich source of omega-3 fatty acids, which are essential to your pet's well-being.

If you supplement your pet's diet with fish, I suggest you use sardines packed in water. Sardines don't live long enough to store excessive toxins in their bodies, and they're a terrific source of omega-3s. Feeding wild caught salmon in rotation with other proteins is also an excellent way to get those omega-3s into your pet’s diet.

If you choose not to feed any fish, I recommend supplementing your pet's diet with krill oil or another marine-sourced omega-3 fatty acid. Dogs and cats can’t get enough DHA and EPA from plant sources — they need ocean-sourced omega-3s, which presents a quandary. The good news is there are sustainably sourced, third party-purity tested marine oils that can safely and healthfully meet your pet’s hefty requirement for these essential fatty acids.



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