Animals can produce some of the fatty acids they need, but not all of them. Those fatty acids which they can not produce themselves, but must be obtained through their diet, are called 'essential' fatty acids. Interestingly, what is 'essential' for one species of animal is not necessarily essential for another. For example, the fatty acid, arachidonic acid is essential for cats but not for dogs.
In some disease conditions, certain enzymes which convert one fatty acid to another may be deficient, or the animal may not be able to adequately absorb fatty acids from the intestine. In animals with these conditions, some of the 'nonessential' fatty acids actually become 'essential,' that is, required in the diet, and in higher amounts. Deficiencies of fatty acids may also occur with the use of fat-restricted diets in overweight dogs.
Fatty acids in foods are subject to degradation. Overcooking can destroy fatty acids. Improper storage or a suboptimal amount of antioxidants in dry food may result in rancidity and a subsequent deficiency in fatty acids.