While plain tuna is not toxic to cats and can be a part of a healthy and balanced diet in moderation, it is not recommended to feed cats mayonnaise. Mayonnaise is typically high in fat and contains ingredients that are not well-suited for cats, such as vinegar and onion powder, which can be harmful to them. Feeding mayonnaise to cats regularly can lead to weight gain, digestive issues, and other health problems. Therefore, it’s best to avoid feeding cats mayonnaise with tuna and stick to plain, cooked, and unseasoned tuna as an occasional treat, in moderation. Additionally, it is important to ensure that the tuna is free of bones and canned in water or brine, rather than oil or other sauces that may contain additional ingredients that are harmful to cats.