Cats can be addicted to tuna, whether its packed for cats or for humans. Tuna-based pet foods make up about 5 percent of canned pet foods, and a surfeit of formulations exist. The tuna used may or may not be human-grade; though it usually is in upscale gourmet lines.
Sometunanow and then probably wont hurt. But a steady diet of tuna prepared for humans can lead to malnutrition because it wont have all the nutrients a cat needs. And, too much tuna can cause mercury poisoning.
Dark muscle meat, byproducts, chemically altered fish protein and even endangered tuna species are used in cat foods.