Appropriate diets for cats: Part 2 - Cats and fish

It is a common myth that fish is good food for cats. In reality, our domestic cats are descended from african wild cats that evolved eating terrestrial mammals - not fish - and while fish is high in protein, it is a very poor source of other nutrients for cats and is implicated in a range of health problems.

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Fish is also responsible for a large percentage of food allergies in cats. In one study of 56 cats with identifiable food allergies, fish was a responsible ingredient in 13 (23%) of those cases. This puts fish in third place for potential allergic reactions behind only beef and dairy products (16 cases [29%] each).

Freshwater fish should not be fed raw as it contains high amounts of the enzyme thiaminase which can produce a deficiency by rapid destruction of dietary thiamine. The heat associated with canning or cooking is sufficient to destroy thiaminase. Thiamine (Vitamin B1) converts glucose to energy and therefore is important for muscle and nerve cell function. Deficiency generally does not develop in cats fed properly prepared, commercial, complete and balanced diets.

Large carnivorous fish such as some types of tuna can absorb a lot of mercury, thanks to human pollution of their habitat. Mercury is not just bad for cats, but every living species. Note that smaller species of fish such as trout, cod or halibut have negligible amounts of mercury.

In cats, steatitis or yellow fat disease results from a diet high in Polyunsaturated Fatty Acids, particularly from marine fish oils when these are not protected with added vitamin E (α-tocopherol) or other antioxidants. Kittens or adult cats fed exclusively on fish can develop anorexia and muscular degeneration; accumulated body fat becomes brown or orange coloured. The high levels of polyunsaturated fatty acids in a heavy tuna diet can also deplete a cat's supply of vitamin E. Inadequate vitamin E leads to 'Brown Bowel Syndrome' - the main symptoms of which are ulceration, hemorrhage and degeneration of the bowels. The first sign of this is a cat that seems to have abdominal pain if picked up. Other kinds of fish (e.g. trout) are too low in fats.

When choosing an Omega 3 supplement, make sure that it also contains Vitamin E. It is useful to mix fats in the diet, animal fat and liver are good sources of Vitamin E. Cod liver oil is also not a suitable supplement for cats because it contains high levels of Vitamin A, which can cause bone disorders

When analysed, tuna and most other fish is too low in Vitamins A,D & E, Calcium, Iron, Zinc and Taurine and is too high in Phosphorus, Sodium and Magnesium. The calcium to Phosphorus ratio is also wrong, Phosphorus being too high and calcium far too low. When there is too much phosphorus, calcium leaches from bones to normalise the ratios, causing secondary hyperparathyroidism. Very high phosphorus levels also damage the kidneys.
When comparing phoshorus levels, tuna is high, Salmon even higher and sardines highest. This is mainly due to the small bones found in sardines. Sardines are also very high in purines, which acidify the urine. Highly acidic urine can contribute to kidney stone formation and in rare cases, cats form urate stones. Fish bones can also damage or obstruct a cat’s throat or gastrointestinal tract. Pet food also tends to be made with a high proportion of offal, that is: fish heads and fins, which greatly increases the mineral content of cat foods if they contain fish. While it can be tempting to feed human-grade fish instead, it is important to remember that the nutrient balance is still wrong for cats.

Fish diets have long been suspected as a cause of hyperthyroidism in cats, and it was thought that this was due to the low quality of the fish ingredients in the cat foods or that chemicals (BPA) used to line the cans might be involved. While hyperthyroidism has been linked to flame-retardant chemicals in the home, a Japanese study in 2016 also linked hyperT to fish-flavoured cat foods. While more research is needed to clarify these findings, you can read something about this study here http://veterinarymedicine.dvm360.com/research-review-does-fish-flavoring-contribute-hyperthyroidism-cats
Some other schools of thought around hyperthyroidism in cats relate it to iodine levels in cat food. Fish heads, skin and fins can be extremely high in iodine. Too little iodine is known to cause hypothyroidism, and too much iodine can cause hyperthyroidism. Diets that control hyperT must be low in iodine and another possibility to consider is the use of kelp supplements, which can vary wildly in their iodine content. Supplements are notorious for inconsistencies in the ingredients and great care should be taken when using these for cats.

Although this list makes fish sound deadly, fish is an excellent source of protein and can be fed in small amounts as treats and is often very useful when tempting a sick cat to eat, especially tuna and sardines. Regular feeding of fish-only canned foods is best avoided. In many countries, fish-only cans are correctly labelled as "treat food only" and "incomplete food not for exclusive consumption", but this warning isn't supplied by all pet food manufacturers. Fish shouldn't constitute more than about 10% of a cat's diet.