Here is some information that I copied and pasted from the internet while searching about the topic:
TUNA: Tuna is low in calcium and too high in phosphorous. It may cause vitamin E deficiency or yellow-fat disease. Plus it may increase susceptibility to "rubber jaw," a form of osteoporosis. Several problems are associated with feeding cats tuna. The first one is that it's highly addictive--cats love tuna oil! Additionally, "people" tuna lacks many of the essential amino acids and vitamins, especially taurine and vitamin A, B and E, necessary for feline health. Be aware that the high amount of mercury in some canned tuna is detrimental to your cat's health. Tuna should not be a staple of any cat's diet. Reserve it for an extra special treat.
WE'RE TALKING TUNA: HOW MUCH IS TOO MUCH FOR YOUR CAT?
Several cat caretakers visiting ASPCA Pet Nutrition
online have inquired about feeding tuna to their animal companions. Mindy Bough, veterinary technician for the ASPCA Pet Nutrition and Science Advisory Service, dishes out the facts on this savory feline fave:
"An occasional tuna treat for your cat is generally harmless," says Bough. "However, if a large part of the cat's diet consists of tuna--or if the cat is fed tuna exclusively--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. Felines who are fed too much tuna can develop other nutrient deficiencies, too, because most de-boned fish are lacking in calcium, sodium, iron, copper and several other vitamins.
Mercury, frequently present in tuna, also presents a potential danger. "At low levels, this may not be a concern," explains Bough, "but if tuna is fed nearly exclusively, it could pose significant problems."
The bottom line? "I recommend premium commercial food for domestic cats," Bough says. "These foods are formulated to meet all of a cat's dietary needs. Then you can feed an occasional tuna treat for your pet's enjoyment. And remember," she adds, "no more than five to ten percent of a cat's diet should ever be table food."