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The Case Against Cod Liver Oil

Updated: Thursday, May 23, 2019 05:58 PM
Published: Friday, November 26, 2010 03:09 PM
Written by Margaret Gates

I have a question about the ground raw cat food recipe in "Feline Nutrition's Easy Homemade Cat Food Recipe." You say in it that cod liver oil should not be used. Can I ask why? I've been using salmon oil and I'm just curious why cod liver oil shouldn't be used.

Coco has been scratching excessively since starting on raw cat food. Considering all the foods she ate previously had no fish and knowing that fish can be an allergen in cats, the fish oil in the raw recipe I'm using will be the next likely suspect I investigate. If she is allergic to fish, are the fish oil capsules a problem?

Fish oils are added to raw cat food for the omega-3 and omega-6 fatty acids they provide. Salmon, small fish and cod liver oil are generally comparable in omega-3s and omega-6s. But they also provide vitamins A and D. Salmon and small fish oil generally have low to moderate amounts of A and D. Cod liver oil can have very high amounts of vitamin A and sometimes be higher in vitamin D. Vitamin A can be toxic if given in large amounts, especially to kittens.¹ Toxic levels of more than 20,000 micrograms, which is approximately 66,600 IU, per kilogram of body weight are unlikely to be reached in normal feeding. But since we are adding a natural source of vitamin A with the chicken liver in the recipe, a second large source is not a good idea. One ounce of raw chicken liver has about 5800 IU of vitamin A.²

As you can see from the chart, the amount of vitamin A in fish oil can vary greatly.³ Vitamin A amounts in salmon and small fish oil usually range from 200 to 700 IU per 1000 mg. In cod liver oil, you can find amounts anywhere from 200 to 3200 IU, or more, per 1000 mg. The warning to avoid it comes from this variability.

A fish allergy is a reaction to ingesting fish proteins. Fish oil is made from fat, so in theory it should be free of proteins and not be causing an allergic reaction. You could safely give fish oil to a cat with fish allergies as long as it is truly free of proteins, and your veterinarian agrees. Some manufacturers do make an effort to have a protein-free product, as fish allergies are also a problem in people. A recent study showed that while non-refined oil does contain protein molecules, refined oils are free of proteins. Check with the manufacturer of the product you use to see if it is protein-free, and always consult your veterinarian.

There have been small studies in humans that indicate that fish oils do not cause allergic reactions in people with fish allergies. Whether this is because it is free of proteins or because of the small amounts used is unclear. It is also unclear whether cats that are allergic to fish will have a problem with fish oil.

Try eliminating the fish oil altogether from her diet and see if her scratching improves. If it does, try re-introducing either a protein-free oil or just a different brand back into her diet and monitor closely. If eliminating it doesn't improve her scratching, you should consult your veterinarian and look for other possible allergens in her diet or environment.

Note: Feline Nutrition provides feline health and nutrition information as a public service. Diagnosis and treatment of specific conditions should always be in consultation with your own veterinarian. Feline Nutrition disclaims all warranties and liability related to the veterinary advice and information provided on this site.

Margaret Gates is the founder of the Feline Nutrition Foundation.

1. K. C. Hayes, "Nutritional Problems in Cats: Taurine Deficiency and Vitamin A Excess," Canadian Veterinary Journal 23, no. 1, January 1982, 2-5.

2. Obtained using the USDA National Nutrient Database for Standard Reference, Release 28, September 2015.

3. Some examples of the vitamin and omega variation among different brands: Land of Pure Gold Salmon Oil, Spectrum Fish Oil, Spectrum Cod Liver Oil, Carlson Cod Liver Oil, Swanson Cod Liver Oil.

4. Scott H. Sicherer, MD, "Food Allergies," eMedicine, May 7, 2010.

5. Carmen Martin-Hernandez, Sylvie Benet and Ladislav Obert, "Determination of Proteins in Refined and Nonrefined Oils," Journal of Agricultural and Food Chemistry 56, no. 12, 2008, 4348-4351.

6. B.J. Mark, A.D. Beaty and R.G. Slavin, "Are Fish Oil Supplements Safe in Finned Fish-Allergic Patients?" Allergy and Asthma Proceedings 29, no. 5, Sep-Oct 2008, 528-9.