My vet wants to do a diet trial. What does that mean?
When your cat has skin or gut issues on a regular basis, sometimes an allergy to food is suspected. The best way to confirm an allergy is to do an elimination-challenge diet trial.
An elimination-challenge diet trial is a test to see if your cat is allergic to a specific food ingredient. You first eliminate all possible allergens (typically proteins) from the diet until you see the allergic response go away. Then, you re-introduce the possible allergens, one at a time, and watch for the allergic response to flare up again.
Once you have identified the problem foods, you can avoid them, and your cat’s allergic reaction will typically settle down.
Other potential allergens to avoid during a diet trial include toothpastes, medications given by mouth (as these often have flavors with protein), and supplements. If your cat eats these items and they cause an allergic response, it may complicate the diet trial.
How do I choose the diet for the elimination-challenge diet trial?
Your veterinary healthcare team will help you select adiet specifically for your cat. Look for a complete and balanced diet appropriate for your cat’s life stage (e.g., kittens should be on a diet that has been formulated or undergone feeding trials to confirm it is complete and balanced for kittens). If you use a home-cooked diet, it should be balanced. Consult a Board-Certified Veterinary Nutritionist for guidance on how to create balanced home-cooked meals for your cat.
Ideally, the diet will be novel or hydrolyzed. A novel diet consists of protein sources (and ideally carbohydrate sources that your cat hasn't eaten in the past. Provide your veterinary team with a list of all the foods (including treats and human food) you have fed your cat previously, as this can help in choosing the right foods for the diet trial.
"Your veterinary healthcare team will help you select adiet specifically for your cat."
A hydrolyzed diet is a commercially available food that is specially made with proteins that are broken into pieces too small to be recognized by the cat’s immune system, hopefully preventing the immune allergic response. There are many different types of hydrolyzed diets, including soy, salmon, and chicken, so if one protein type does not work, your vet may decide to try another protein during the trial.
A hydrolyzed diet is a better choice when you can’t remember or do not know what foods your cat had previously. A novel diet may be a better choice for reasons such as price, availability, or palatability (tastiness). By talking with your veterinary team and providing a good diet history, you can help choose the best elimination-challenge diet.
Do I have to feed the prescription 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 prescription diet chosen for your cat rather than an over-the-counter diet with similar ingredients. Prescription diets are made in facilities specifically designed for each special allergy food, similar to human foods that advertise “made in a peanut-free factory,” which indicates there is no possibility of contamination and those with peanut allergies can enjoy the food safely.
Studies show that over-the-counter diets advertised as “limited-ingredient” (similar to veterinary novel diets) contain proteins and other ingredients not listed on their label because these diets are processed in a facility one after another. The food processed just before the “limited-ingredient” diet may leave small particles behind, causing contamination. The contaminants 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 on a prescription diet, you can be confident that the ingredients in that diet are a problem for your cat, and the flare is not due to a potential contaminant."
Veterinary prescription diets are made in facilities specifically designed for the allergy food and undergo extensive testing to ensure no contamination occurs. If your cat has an allergic flare on a prescription diet, you can be confident that the ingredients in that diet are a problem for your cat, and the flare is not due to a potential contaminant. With an over-the-counter diet, there is no way to know for sure.
Once the challenge portion of the diet trial is complete and you have identified problematic food(s), you may switch to an over-the-counter diet that does not include the problem food. However, if a flare occurs later, remember that it could be due to a contaminant in that diet or a newly developed allergy to the currently tolerated food.
My cat already had allergy testing done. Why do an elimination diet?
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, and 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 may not be correct, and you may be unnecessarily avoiding foods. Consider testing the problem foods during the challenge portion of the diet trial to confirm the allergy.
How long is the elimination portion of the diet trial?
The elimination diet is typically fed for at least eight weeks, although signs of improvement are often seen much sooner. Cats with gastrointestinal signs like diarrhea usually improve sooner than cats with skin signs, but many cats see remission of signs by the sixth week.
If your cat gets better while on the elimination diet, you may choose to continue to the next step of the trial (the challenge) or, if you are feeding an appropriate food for your cat, you may choose to stop the trial and continue feeding the elimination diet.
If you stop the trial, remember you have not confirmed that your cat is allergic to a 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 an allergy is to complete the elimination-challenge diet trial.
How do I complete the challenge portion of the diet trial?
If your cat has improved significantly on the elimination diet, then the next step is to feed your pet their previous diet and look for the signs of allergy to flare again. If the signs return, they typically appear in 1 to 3 days but can take up to 2 weeks. If allergy signs return, you re-start the elimination diet. If allergy signs resolve again while on the elimination diet, a food allergy can be confirmed.
Ideally, the last step is to challenge individual ingredients to determine exactly what foods are allergens to your cat. This is done by adding small amounts of one ingredient at a time to the elimination diet and watching for a repeat of the allergic responses.
For example, your veterinarian may start with chicken as the first challenge. For two weeks, at each meal, you will feed your cat a small amount of chicken (about 1-5 tablespoons, depending on cat size), and then watch for any recurring signs of allergy in your cat.
If allergy signs do not improve at all on one diet, your vet may decide to try another novel diet or hydrolyzed protein diet.
Allergy signs are typically gastrointestinal/GI signs (e.g., soft stool, diarrhea, vomiting, bloating, excess gas) or cutaneous/skin signs (e.g., itchy skin, ears, paws, and bellies; reddened skin; recurring secondary skin infections).
If allergy signs do not improve at all on one diet, your vet may decide to try another novel diet or hydrolyzed protein diet. After a second diet, if there is still no improvement in allergy signs, then your cat does not have a food allergy and you can look for other reasons for your cat’s skin or gut problems.
What should I do after we identify the problem food(s)?
If you successfully complete the challenge and identify one or more specific foods that are problematic, then avoidance is key. Avoiding the foods that cause allergic flares is necessary to prevent future flare-ups. Carefully examine everything given by mouth to avoid the problematic foods, including diet, treats, supplements, cat toothpastes, and flavored medications.
Outcomes are usually good if the problematic food is 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 forever.
