Today a question was posted on our Facebook page that I’ve been asked many times…is an all fish diet harmful to a cat?
It’s actually a fairly complicated question to answer. But here it goes.
First of all, we are talking about actually formulated cat food diets, not fresh fish. Fresh fish, such as Tuna, can be very harmful to cats if given in excess. Cat foods are treated to destroy an enzyme that can cause anemia in cats.
1) Allergies: Fish is a fairly allergenic protein in cats. It can lead to food allergies, especially in the gastrointestinal tract. Beyond that, when fish is left out too long, bacteria can begin to spread. These bacteria release histamine which can cause allergic reactions. Histamine is not broken down by cooking or processing into pet food.
2) Addiction: Some cats adore fish and it can be difficult to convert them away from it if a specialized diet becomes required.
3) Fish is higher than other proteins in magnesium and phosphorus. This can increase risk for struvite crystals which can lead to urinary disease. With added water content canned food poses less risk than dry.
4) Heavy metal toxicity: No this is not an 80′s hair band. Fish can accumulate heavy metals such as lead, mercury, and PBPE’s (PBPE has been linked to hyperthyroidism in cats). Top predator fish such as tuna, swordfish, shark, and mackerel have the highest concentration of metals. For a nice diagram of heavy metals and fish go to: http://www.sierraclub.org/sierra/201111/Whatfish_graphic.pdf
So I advise not to give fish based diets in cats with a history of allergies (especially vomiting/diarrhea), urinary tract disease, and kittens or cats that will be or are currently pregnant.
Overall, I don’t mind fish based foods as a portion of a cat’s diet that don’t have the above problems. But I would generally recommend against feeding a purely fish based diet. I am also a big believer in keeping canned foods as a part of a cat’s diet as well, due to benefits to their urinary tract.