Can Cats Be Fed Tuna?

My friends feed their cat canned tuna fish. I have heard this is bad for the cat and can cause may health problems including heart failure. Is this true?

An occasional tuna treat for your cat is generally harmless and very welcome, but if a large part of the cat’s diet consists of tuna some problems are likely to arise. Tuna does not contain significant amounts of vitamin E, for example, so too much of the fish can lead to vitamin E deficiency, resulting in yellow fat disease, or steatitis. Symptoms include loss of appetite, fever and hypersensitivity to touch, due to inflammation and necrosis of fat under the skin. Most de-boned fish are lacking in calcium, sodium, iron, copper and several other vitamins too, which may all lead to other nutritional deficiencies. In addition, mercury is frequently found in tuna and presents a potential threat, especially where tuna is fed exclusively.