Verdict: In moderation
While small amounts of plain, cooked tuna are not immediately toxic to cats, it should not be a regular part of their diet. Tuna lacks essential nutrients for felines and excessive consumption can lead to nutritional deficiencies, mercury poisoning, and thiamine deficiency if fed canned tuna packed for human consumption due to preservatives and cooking processes.
Mercury bioaccumulates and is neurotoxic. Thiaminase, an enzyme found in some raw fish (though less of a concern in cooked tuna, but can be present in some human-grade canned tuna processing), breaks down thiamine (Vitamin B1), leading to deficiency.
Mercury, Thiaminase