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Why Do a Food Trial For Dogs and Cats? How is a Food Trial Effectively Done?

Dog Food TrialYour vet might have suggested a food trial for a couple reasons. (photocredit:

If your dog has chronic ear infections and/or an itchy rear end, an allergy to food is top of the suspect list.

If your cat has itchy spots, pulls out her hair, or has scabby areas, it could be her food she’s allergic to.

Unfortunately, there is no test that is reliable for food allergies. We can send a robot to Mars and drive it around, but we can’t develop a way to diagnose food allergies in pets that’s any better than the good ol’ food trial. Bummer.

I often call it “food trial & error” because that’s more or less what it is! The theory is we are eliminating any food the pet can have a reaction to for 2-3 months. If the pets improves (less itchy, less infections, etc) during these months, then we know he or she was allergic to the food. But then what? We do what’s called a “challenge” where we offer a food the pet has not eaten at all during the food trial, and if the ears or skin flare up again, we know that is the problem! Very low tech and simple, but NOT easy.

Food trials are hard. Really hard. It sure sounds simple, but if you’ve lived one, you know it’s tricky.

For two-three months your pet must eat only the prescription food. And yes, it ideally needs to be prescription. What pets are most often allergic to is the protein (beef, chicken, liver) or carbohydrate (rice, rarely corn) source in the food.

Simply changing brands will not change the allergens.

Switching to holistic or organic does not make it hypo-allergenic.

Think of the kid with a life-threatening peanut allergy. Can he eat an organic peanut? No way! A peanut is a peanut! And does he need to eat a handful of peanuts? Nope – one can be enough to send him to the ER!

Pets with food allergies might not go to the ER, but that all-or-nothing concept is the same. They need to eat a protein and a carbohydrate source their body has never seen. If they’ve never seen it, they can’t be allergic to it. The common proteins used in food trials are duck, venison, whitefish, and rabbit, and the common carbohydrates are potato, yam, and green pea. Nothing else.

Over-the-counter foods may list these same ingredients, but they are not LIMITED ingredients. Some will have chicken fat, poultry by-product, etc on the label! Obviously not a single protein food! Others will not have it on the label, but the other protein and carbohydrate sources still make it into the food. Often these foods are manufactured in facilities where lamb food is made one day, beef the next, and then the salmon food you want to buy. Guess what – the equipment does not get cleaned all that well between batches. One study showed 40% of OTC dog and cat foods contained protein sources that were NOT on the label! (Here’s a link to the study if you want to check it out.)

OK, so you’ve bucked up for the prescription food. We now have to think about everything else that goes through your pet’s mouth. Cats are much easier, as they don’t chew like dogs do, and are less accustomed to treats. However, if you have multiple cats, don’t think for a second you can leave the “regular” food down for your other cats and the problem child on the special food won’t eat it. Sometimes we’ll put every cat in the house on prescription food, just to make sure there’s no tomfoolery.

It only takes ONE bite of non-prescription food to trigger an allergic reaction. And it brings your food trial back to Day 1. You thought you were on day 20. Not any more!

(Think back to the peanut analogy).

Dogs are harder. What heartworm preventative do you give every month? Guess what – those are a protein source, and they absolutely can trigger an allergic reaction! I suggest topical heartworm preventatives during the food trial, like Revolution or Advantage Multi. Both are liquids applied to the skin – nothing going through the mouth.

What about his flavored chew toys? Yep, those get picked up too. His treat for when he potties? Nope, need to find a replacement! Rawhide chews or dental chews are all out for the next couple months as well. Life for a dog on a food trial can be downright lame for a few months.

Most prescription foods come in both dry and canned. Some people use the canned version as treats during the food trial. It’s also a good way to give medications (what WERE you using??) Others will offer veggies. For instance, if your pet is on a duck and green pea diet, then you may want to try peas (fresh or frozen) as treats. Again, a pea is a pea, so those would be safe during a food trial.

Hopefully, if you’ve successfully kept your dog out of the trash, and kept gramma from slipping the cat a treat when she comes, your pet’s condition should begin to noticeably improve. This is what makes it worth it! No more ear infections! No more spinning on the carpet! No more watching your cat pull her hair out!

Some people, if their pet IS food allergic and they experience drastic relief, are afraid to rock the boat. I get it! Others would like to have their dog get his favorite dental chew. Once you and your vet decide the food trial is over, and your pet is feeling better, then you can “challenge” them. Give them that dental treat. If your dog’s ears smell or other symptoms return over the next day or so, you’ll know!

What if you did everything right? Your pet got NOTHING except the prescription food. You interrogated the kids and no one broke. Yet, your pet is not better. Here’s the bummer: 20% of food trials fail on the first attempt, because even though the food should be hypo-allergenic, the pet found a way to react. So yes, sometimes we’ll have to start over with a different food. A second food trial – lucky you!

But don’t abandon hope! If your pet truly is food allergic, getting him on a diet that doesn’t cause him to itch 24/7 is the most loving thing you can do for him. So tough it out – it’s worth it!

Even if he swears he’s abused because he doesn’t get his favorite rawhide any more.

Hopefully this is temporary. And if that rawhide ends up being the cause of his discomfort and infections, it’s not worth it anyway! We can find a substitute!

Web-DVM guest blogger Dr. Karen Louis is a practicing small animal veterinarian.  See more of her articles at her blog at

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