I think it is Steatitis that is the problem when you feed a fish only diet, as something in the fish prevents the absorption of Vit E and can make them seriously ill. Found this on a website: Cats fed a diet containing excessive amounts of tuna can
develop steatitis, also known as yellow fat disease."
Tuna fish, and many other fish species, contain relatively large amounts of unsaturated fats. Although health-minded people eat fish to decrease their consumption of saturated fats, the excessive unsaturated fat in a cat’s diet may be harmful.
Tuna and certain other fish possess very little vitamin E. Vitamin E is an important antioxidant. When a cat’s diet consists mostly of tuna fish that is not commercially formulated as cat food, the cat becomes deficient in vitamin E. Dietary unsaturated fats from the fish are oxidized by a biochemical called peroxidase into a substance called
ceroid. Since the affected cat has low vitamin E levels, this oxidation process is not restrained. Ceroid, an abnormal, pigmented, yellow-brown breakdown product of unsaturated fat oxidation, is formed and deposited in fat cells. The result is yellow fat disease(steatitis).
Ceroid triggers an inflammatory response by the immune system as if it were a foreign invader. The subcutaneous fat of cats affected with yellow fat disease causes pain; these cats become hypersensitive and will resist handling and petting. The muscles of affected cats will atrophy and become weak; these cats do not want to move. As the
disease process progresses, the body fat degenerates and is replaced by fibrotic tissue, leaving the skin hard and nodular. Affected cats may also develop fevers unrelated to infection.
Yellow fat disease occurs most commonly in young, overweight male and female cats with inappropriate diets. Treatment includes discontinuing the inappropriate diet and administering therapeutic doses of vitamin E. Corticosteroids may also be prescribed to relieve the inflammatory response.
Even if a tuna-fed cat receives prophylactic or supplemental doses of vitamin E, there are other problems besides steatitis that make feeding tuna unwise. Some believe that tuna contains specific substances (allergens) that stimulate allergic-like disorders in cats.
Cats should be fed a balanced, commercially prepared diet to avoidthese problems."
Blackcat - you are thinking of Taurine, fish does contain small amounts of it, but a cat fed on no meat at all (or mainly dog food) will have a Taurine deficiency, which can cause blindness and heart problems, and killed a lot of cats up until around 30 years ago when they realised that while dogs can make enough themselves, cats can't, hence why cats can't have a vegetarian diet without a lot of supplements, whereas a dog can.
