Implementing an Elimination-Challenge Diet Trial in Cats

What is an elimination-challenge diet trial?

An elimination-challenge diet trial, also known simply as a diet trial, is a test to see if your cat is allergic to a specific food ingredient (typically proteins).

When your cat regularly has itchy skin or gut issues (such as excess gas or loose stool), sometimes an allergy to food is suspected. Your family veterinarian should perform a physical examination, because there are many causes for these clinical signs. When a food allergy is suspected, the best way to confirm is to do an elimination-challenge diet trial.

In the first part of an elimination-challenge diet trial, you eliminate all possible allergens from your cat’s food, treats, supplements, and medications. If you see the allergic response go away, you re-introduce the previous diet (challenge) and watch for the allergic response. 

Next, you challenge with the most likely allergen proteins one at a time and watch for signs of allergy. If there is no flare-up after about three to four weeks, then the suspected protein is not the problem, and other ingredients can be tested to see if they cause a flare-up.

Once you have identified the problem foods, you can avoid them, and your cat’s allergic reaction will typically settle down. During a diet trial, it is essential for your cat to avoid potential allergens that may be found in treats, rawhide chews, supplements, toothpastes, and medications given by mouth, because they often have flavors with protein. If your cat eats these items and they cause an allergic response, the diet trial will be compromised and you may need to start over.

How do I choose a diet for the elimination-challenge diet trial?

Your veterinary healthcare team will help you select a diet specifically for your cat. Look for a complete and balanced diet appropriate for your cat’s life stage. For example, kittens should eat a diet that has been formulated to be complete and balanced for kittens, or that has undergone feeding trials to confirm it is complete and balanced for kittens. If you use a home-cooked diet, it should be balanced; consult with a trained veterinary nutritionist with experience formulating balanced homemade recipes.

Most elimination-challenge diet trials involve feeding either a novel protein diet or a hydrolyzed protein diet. Ideally, the diet consists of one protein source and one carbohydrate source that your cat hasn’t eaten in the past.

• A novel protein diet ideally consists of one protein source and one carbohydrate source that your dog has never eaten before. Because the immune system has not previously been exposed to these ingredients, they are less likely to trigger an adverse food reaction. Examples may include proteins such as rabbit, venison, or duck, depending on your cat's dietary history.
• Hydrolyzed protein diets are commercially manufactured therapeutic diets made with proteins that are broken into pieces too small to be recognized by the cat’s immune system. This helps reduce the likelihood of triggering an allergic reaction. Hydrolyzed diets are available with different protein sources (e.g., soy, salmon, or chicken). Cats who don’t do well on one type of hydrolyzed food often respond better when switched to a different hydrolyzed diet, so work with your vet and be prepared for more than one trial.

Provide your veterinary team with a complete list of all foods your cat has eaten in the past, including treats, flavored medications, and table scraps. This information is essential when choosing a novel protein diet and can also help determine whether a hydrolyzed diet may be a better option.

A hydrolyzed diet is a good choice when you can’t remember or do not know what foods and treats your cat ate previously. A novel diet may be a better choice when all prior foods are known or if a pet owner has already been feeding a balanced homemade recipe. By talking with your veterinary team and providing a good diet history, you can help determine the best elimination-challenge diet.

Do I have to feed the veterinary therapeutic diet or can I feed an over-the-counter diet with the same ingredients?

During the elimination-challenge diet trial, it is best to feed the veterinary therapeutic diet chosen for your cat rather than an over-the-counter diet with similar ingredients.

Therapeutic diets (hydrolyzed protein products) are made in facilities with specific manufacturing protocols and cleaning procedures designed to reduce the possibility of contamination.

Over-the-counter diets advertised as “limited-ingredient” often contain proteins or other ingredients not listed on their label. Limited-ingredient formulas are not made in dedicated or isolated facilities; instead, they are processed directly after other over-the-counter diets. 

The food processed just before the limited-ingredient diet may leave small particles behind, causing contamination. This cross-contamination can cause a flare of allergic signs, which can lead to confusion about what your cat is allergic to. If your cat has an allergic flare while exclusively eating a therapeutic hydrolyzed protein diet, you can be confident that the flare is not due to a potential contaminant.

Once you have completed the elimination-challenge diet trial and identified a specific ingredient to avoid, you can try feeding your cat an over-the-counter diet that does not include that ingredient. If a flare occurs later, it may be due to cross-contamination during manufacturing of the over-the-counter diet or your cat may have developed a new allergy.

My cat already had allergy testing done. Why do a diet trial?

The only way to determine a true food allergy is by performing an elimination-challenge diet trial. Many tests advertise that they can identify food allergies through blood, saliva, or hair. However, numerous studies have shown that these tests are not accurate at predicting allergies. In some studies, researchers submitted non-animal samples to be allergy tested (e.g., water, stuffed animal fur), and the results indicated these “pets” had allergies to specific foods — which they clearly did not.

If you have already done an allergy test on your cat, understand that the results are not accurate or true, and you may be unnecessarily avoiding ingredients or foods. Consider testing the problem foods during the challenge portion of the diet trial to confirm suspected ingredients.

How long does the elimination phase of the diet trial last?

The elimination diet should be fed for at least eight weeks, although signs of improvement are often seen sooner. Cats with gastrointestinal signs like diarrhea usually improve sooner than cats with skin signs, but many cats see remission of signs by the fifth week.

If your cat gets better while on the elimination diet, you may choose to continue to the next step, which is the challenge phase, or you may choose to stop the trial and continue feeding the elimination diet.

If you stop the trial, it is important to understand that you have not confirmed your cat is allergic to a specific protein or food. Sometimes feeding a different diet (e.g., different protein sources, different amounts of fiber) can change a pet’s response. The only way to confirm a true food allergy is to complete the elimination-challenge diet trial.

If allergy signs do not improve after trialing a novel protein or hydrolyzed protein diet, your vet may recommend a second diet trial with a different product (there are many options). In any trial, it is important to make sure the chosen elimination diet is the only thing that crosses your cat’s lips during the 4- to 8- week trial period. If, after a second trial, there is still no improvement in allergy signs, then your cat does not have a true food allergy and you should look for other reasons for your cat’s skin or gut problems.

How do I complete the challenge phase of the diet trial?

If your cat improved significantly on the elimination diet, the next step is to feed them their previous diet and look for any signs of a flare-up. If the signs return (e.g., itchy skin, loose stool, diarrhea), they typically appear in one to three days but can take up to four weeks. If allergy signs appear, the elimination diet must be restarted. If allergy signs resolve again while on the elimination diet, you should challenge your pet with a specific protein.

This next step involves presenting individual protein ingredients to determine exactly which ones are allergens to your cat. This is done by adding a small amount of one ingredient to the elimination diet and watching for allergy signs. For example, your veterinarian may start with chicken as the first challenge. For four weeks, at each meal, you will feed your cat a small amount of chicken (about 1 to 3 tablespoons) and then watch for any recurring signs of an allergenic response. You then repeat the process for other ingredients as necessary.

Allergy signs are either gastrointestinal (e.g., soft stool, diarrhea, vomiting, bloating, excess gas) or skin-related (e.g., itchy skin, ears, paws, and bellies; reddened skin; recurring secondary skin infections). Sometimes, cats can be affected in both their gut and skin.

What should I do after we identify the problem foods?

If you successfully complete the challenge and identify one or more specific protein ingredients that are problematic, then avoidance is key. Avoiding those ingredients that cause allergic flares is necessary to prevent future flare-ups. Carefully examine everything given by mouth to avoid problematic foods, including treats, supplements, pet toothpaste, and flavored medications.

Outcomes are usually good if the problematic items are avoided. However, remember that cats can develop new allergies to foods they previously tolerated; so just because a food is tolerated now, does not mean it will be tolerated for the rest of the cat’s life.