Food allergies / intolerance are caused by a reaction to a particular ingredient, which is usually a protein. Sometimes referred to as an ‘adverse reaction to food,’ it is defined as an abnormal response to a food or food additive. There are two classes of adverse reactions: those in which the immune system is involved (generally called food allergies); and those that occur without an immune component (generally called food intolerances).
Allergies may last a lifetime so the ingredient must be permanently removed from your cat’s food.
It may take months or years before your cat develops an allergic response to a particular food. However, once she's allergic, she will almost always have a negative reaction to that food. Allergic reactions are most commonly associated with protein sources – usually the meat in your cat’s food.
Food: The most common causes of food allergies and food intolerance in cats are beef, milk products and fish.
Damage: Inflammation, infection, surgery and some medications can damage the digestive system and may lead to food allergies or food intolerance.
Age: Food allergies and food intolerance can occur at any age.
Breed: Some cat breeds appear more likely to develop food allergies or food intolerance, including Siamese cats.
If your cat vomits frequently, has diarrhea, irritated skin, a poor coat condition or hair loss, then she may have a food allergy. The most common symptoms of a food allergy or food intolerance are digestive upset or skin irritation. If your cat has food allergies or food intolerance, you may notice some of the following signs:

IMPORTANT: Some symptoms of food allergies or food intolerance are similar to those of other serious conditions so consult your veterinarian if you notice any of these signs.