I have copy and pasted this for you Danielle

Tuna

If fed in excess, tuna can lead to steatitis (Yellow Fat Disease, pansteatitis). This painful inflammatory condition results from a diet high in unsaturated fatty acids & deficient in Vitamin E; over-consumption of oily fish is the main cause in cats. Tuna seems addictive to cats, but should be limited to special treats only. Tuna contains little vitamin E and the excessive unsaturated fatty acids further deplete vitamin E in the body.

Cats with steatitis develop flaky skin and a greasy, dull coat. They show signs of severe pain when touched and are reluctant to move. They also lose their appetites and develop fever. If untreated, it results in death.