Steatitis is a painful condition resulting from a diet high in unsaturated fatty acids and deficient in Vitamin E. Tuna is the most comonly known cause of this problem.
Can I feed my cat human grade tuna?
The short answer is yes, but feeding tuna should be restricted to special treats only, and only fed extremely infrequently.
Tuna is quite addictive to cats and they can quickly develop a taste for tuna and refuse any other foods. The term "tuna junkie" has been used to describe such cats.
Feeding too much tuna can result in a condition known as Steatitis.
Raw fish also contains the enzyme thiaminase which destroys thiamine (Vitamin B1), resulting in thiamine deficiency. Cooking destroys thiaminase, thus protecting thiamine.
What is Steatitis?
Also known as Yellow Fat Disease or Pansteatitis, steatitis is caused by feeding a diet high in unsaturated fatty acids and deficient in vitamin E. Oily fish, especially red tuna are the cause of this condition.
Vitamin E acts as an antioxidant, and unfortunately tuna is an inadequate source of Vitamin E. Therefore the overabundance of unsaturated fatty acids (which also oxidize and destroy Vitamin E), combined with the deficiency of Vitamin E causes damage to body fat, which results in a painful inflammatory response. 
What's the difference between human grade tuna and tuna flavoured cat food?
Human grade tuna is just that, tuna whereas tuna flavoured cat food is not 100% tuna, and usually contains other meats and nutrients which are necessary in the cat's diet.
What are the symptoms of steatitis?
How is steatitis diagnosed?
Diagnosis is usually based on the feeding history of the cat and a biopsy of the fat.
How is steatitis treated?
If left untreated steatitis can result in death. So it is important to seek veterinary attention immediately so they can begin the cat on the following treatments:
- Elimination of fish from the diet immediately.
- Supplementation with Vitamin E.
- Force feeding if necessary.
- Some veterinarians recommend the use of corticosteroids.
*1 The Cornell Book of Cats.