Your cat is scratching at her neck, or grooming a bald patch into her belly, or bringing up breakfast twice a week. Someone has said "try a limited ingredient food" and you are holding a bag that says so. Whether it helps depends on what is wrong with her, and on the ingredient list.

What limited ingredient means

A limited ingredient food is built from a short list: one named animal protein, one or two carbohydrates, and the vitamins and minerals every complete food needs. Fewer ingredients means fewer things to react to, and a clean swap when you change foods.

The words are not regulated. Any company can print them on any bag, so the ingredient list is what you read, and the label section below shows how.

What a food allergy looks like in a cat

The Merck Veterinary Manual describes it. The most common sign is itching, and in cats it very often concentrates on the face, ears and neck. Some develop miliary dermatitis, small crusty scabs, or the raised sores vets group as eosinophilic granuloma complex. Digestive trouble (vomiting or soft stool) is the most common companion to the skin signs. Merck puts onset anywhere from four months to fifteen years, most often in young adulthood, between one and four.

A 2016 review pooled the published cat cases, 78 in all. Beef was named in 14, fish in 13, chicken in 4, and wheat, corn and dairy in 3 each. With numbers that small, chicken's place in the top three rests on one cat more than wheat.

When a limited ingredient food helps

When you know, or strongly suspect, what your cat reacts to. If fish sets her off, a single-protein food with no fish in it, including no fish oil or fish broth, keeps fish out of the bowl every day. One caveat the next section explains: the label may not list everything in the bag, so if she reacts on a food that reads clean, the food is the first suspect.

When limited ingredient food does not help

A healthy cat with no symptoms gains nothing from one. Grain-free is a different claim. In the 2016 review, wheat and corn accounted for three cats each; the animal proteins did the damage, and a grain-free chicken food still feeds chicken.

The bigger catch is diagnosis. Many owners use a limited ingredient food as a home trial, and that is where the label problem bites. Any limited-antigen food, the research term for the same idea, may not match its label. In the first of two published studies, six of eleven limited-antigen wet diets contained animal proteins that were not on the label, and one contained proteins unrelated to anything it declared. In the second, 40 novel-protein and hydrolyzed diets were tested and only ten matched their labels; 23 contained undeclared species and five were missing a protein they claimed to contain. Hydrolyzed diets, with protein broken into pieces too small for the immune system to detect, did somewhat better, with fewer contaminating species found across them, but two in three still contained a species not on the label. Both studies mixed dog and cat foods, and the second had only four cat products, so the numbers show how these foods are made in general; nobody has counted cat foods on their own. Tufts' nutrition service says store-bought "allergy" and "limited ingredient" foods have high contamination rates and often more ingredients than advertised, and recommends a veterinary diet for the trial.

A trial diet uses a protein your cat has never eaten; that is what "novel" means in a novel-protein diet. Every single-protein food you try on your own uses one up, so a cat who has already had duck, rabbit and venison leaves the vet fewer options for the trial.

How an elimination trial works

If the question is "what is my cat allergic to," the answer is a trial run with the vet. Merck calls the elimination diet the reference standard and says blood and other tests cannot consistently prove or rule out a food allergy. Tufts gives the timing: eight to twelve weeks for skin signs, three to four weeks for digestive signs, and Merck notes that ten weeks may be needed to catch 95 percent of food-allergic animals. The diet is a veterinary novel-protein or hydrolyzed one, and nothing else goes in her mouth. Tufts' list of things to stop: treats, table food, dental chews, toothpaste, supplements and flavored medications. If she goes outside, whatever she catches counts too. Merck adds a last step. The allergy counts as confirmed only when the cat improves on the diet and then flares when the old food comes back. Tufts also recommends keeping a symptom journal for the whole trial.

How to read the label

Two made-up ingredient lists, the kind you will see on bags that say limited ingredient. The first reads "Duck, duck broth, peas, pea protein, sunflower oil, vitamins and minerals." One animal, start to finish. The second reads "Duck, duck broth, peas, pea protein, chicken broth, fish oil, vitamins and minerals." Two more animals arrived in positions five and six, and chicken broth is chicken to a chicken-allergic cat. Read to the end. A bag that lists "meat" or "poultry" without naming the animal has told you nothing. A hydrolyzed diet announces itself in the protein name, "hydrolyzed chicken" or "hydrolyzed soy protein". A novel-protein diet just names the animal. Wet or dry, either can be limited ingredient. And look for the "complete and balanced" statement, which means it can be the only thing she eats.

Comparing limited ingredient cat foods

If your cat's trigger is known, our limited ingredient cat food list shows each food's protein and full ingredient list side by side. If the signs are new, persist after a proper trial, or she is losing weight, see the vet first.

Best Limited Ingredient Cat Food screens every food in our database against its criteria, so you can compare ingredients, allergy risk and price side by side.

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