I got an Iams booklet with my kitten when I got him three weeks ago and it says:
"Never feed the following people foods to your new cat:
-Chocolate
-Onion Powder
-Cow's Milk
-Raw Eggs
-Tuna - low in calcium and too high in phosphorus; feeding exclusively may cause a vitamin E deficiency (yellow-fat disease) or rubber jaw, a form of bone demineralization."
So, what I gather is that tuna is okay every-now-and-then but if that's all you feed your cat, it's bad news.
tanyuh