Food is clearly key to our survival, but sometimes it can be the cause of issues. When our body has a bad reaction to food, this is called an “adverse food reaction”. Most people have heard about some form of an adverse food reaction – with food allergies being the most well-known. It’s also common to hear people talk about adverse food reactions and allergies interchangeably, but they’re not the same thing. Broadly speaking, a true food allergy involves more than one part of the body (gut, skin, lungs, etc). These reactions can be intense and life-threatening…thinking about anaphylactic reactions; whereas the effects of a food intolerance are localised to the gut and are not as severe usually.
Adverse food reactions are relatively common. In humans, 1 in 10 infants and children are estimated to have a food allergy (1,2).
In adults, it’s estimated that about 1 in 50 people, or about 2% of the population have a food allergy (1,2). It will be a higher prevalence if we're considering the broader category of adverse food reactions, not just allergies.
In pets, it has been reported that < 1% of dogs and cats visiting the veterinary clinic have an adverse food reaction (3). However, this is considering all pets visiting the vet. When thinking only about those with signs of skin disease, the percentage is much higher; in a recent study, 45% of dogs with nonseasonal atopic dermatitis also had an adverse food reaction component to their disease (4). So, in other words, adverse food reactions are not very common in pets in general, but if there are gastrointestinal or skin signs, food is a more commonly diagnosed as a trigger.
How can we determine if a dog or cat has an adverse food reaction? The gold standard is to do a food elimination-challenge trial. All other tests, including skin, blood, salvia, or hair tests have been shown to be unreliable or inaccurate (5). The exception is that a negative skin patch test can be used to pick ingredients for the food trial (5). In one classic study, researchers sent in fake dog hair from a plush toy to a company that was doing hair testing for food allergies, and they still got a result from the company (6)! So unfortunately, we don’t recommend blood, salvia or hair testing to diagnose a food allergy, even though that would make life so much easier.
What exactly is a food trial then?
In the simplest terms, a food elimination-challenge trial is where you completely stop feeding a certain food or a group of foods for a period of time in order to determine whether you (or your kid or your pet) are having a reaction to it. We do this in veterinary medicine, but it’s performed regularly in human medicine as well, especially with kids. Your family doctors may recommend stopping dairy or nuts for a period of time to see if your child's gastrointestinal or skin signs improve. If they do improve, then the next step is to challenge them with a bit of dairy or some nuts to see if their signs come back. This part is important but is sometimes overlooked. It’s not always easy to convince human parents (or pet parents) to do this step because the clinical signs have improved, so why risk it coming back? But without doing the challenge, we can’t confirm if there is a true adverse food reaction. Maybe they improved for another reason. Maybe it was seasonal and it was a change in weather. You want to be confident that there is a food reaction, because it means they can’t have that food ingredient possibly for the rest of their lives, so it’s important to be sure.
How do we do a food trial?
We start by omitting foods from the pet’s diet that they have previously eaten (as these are the foods they could be sensitised to) and we replace them with food they have never eaten before – or what we call "novel foods". Ideally, we take into account all foods that the pet has ever eaten, including meats, fish, vegetables, fruits, grains, carbs, etc, and avoid all of them. In some cases, this is really difficult as the pet has eaten everything under the sun already, so not much is still novel. It’s also really hard to recall everything a pet’s eaten over their lifetime – I have trouble remembering exactly what I ate last week! So, in these cases, we recommend avoiding at least the foods recently eaten, especially those that were fed when the pet had clinical signs. You also have the option of using special diets where the protein has been hydrolysed or broken into small enough pieces so that they will not trigger the immune system. The beauty of these diets is that it takes some of the guesswork out. But they are limited to dry and canned options.
You want to ensure the diet fed during the trial is complete and balanced because feeding a deficient diet can lead to gastro or skin issues in and of itself! You can make a complete and balanced, novel, fresh diets yourself at home (click here for dog recipes and cat recipes), or there are commercial diets with select or limited ingredients that could also be used. There is a caveat that some limited ingredient commercial diets are not reliable as studies have shown they potentially can contain undeclared proteins (7,8).
Whatever diet you choose, the main thing to remember is that your pet should have this diet with these specific ingredients for the entire time of the food trial. Nothing else, except these ingredients, should be fed. That means no other treats, handouts, leftovers, etc. You also have to be really careful they don’t get into any of your other pets’ food, or scavenge anything from the ground, including poop! It’s important to be absolutely strict about this because one slip up likely means you need to restart the trial.
Evidence shows that you need to do a food trial for 2 – 6 weeks for pets with gastrointestinal signs and 10 – 12 weeks for pets with skin signs to determine whether there is an improvement (9). Some pets may improve quicker, but for those who don’t seem to be improving, it’s important to still go through the entire trial period. This is to give their bodies enough time to right itself – if it is going to right itself!
If your pet’s signs do not improve, and you’re certain there were no whoopsies where they got something they shouldn’t have, then your pet does not have an adverse food reaction, and there will need to be further investigations to figure out what is causing their signs. If your pet’s signs do improve, then the next step is to challenge them with a small amount of another food back into their diet to see if they relapse (or in other words, if their signs come back). It can take up to 2 weeks for the relapse to occur, at least with skin signs (it may be faster with gastro signs) (10). After these two weeks, if all is well, then your pet likely tolerates this ingredient, and you can try another. It’s tedious, but so important to do this methodically and try only one food item at a time. The goal is to rule in or out which ingredients your pet can tolerate, so you can build their future diet with that information.
Ultimately, a food elimination-challenge trial is not for the faint of heart. It takes weeks if not months to do, but it is the most definitive way to determine whether your pet has a food allergy or food intolerance. The results can affect the rest of their lives though, so it’s important to do the trial right and be confident of your final conclusion: do they or do they not have an adverse food reaction? Working closely with your vet will make the process easier and they will be able to help you along the way. Also, feel free to reach out to us at VNG as well, as we’d be happy to help you on your food elimination-challenge trial journey.
1. Tang, M. L. K., & Mullins, R. J. (2017). Food allergy: is prevalence increasing? Internal Medicine Journal, 47(3), 256–261. https://doi.org/10.1111/imj.13362
2. Spolidoro, G. C. I., Amera, Y. T., Ali, M. M., et al. (2023). Frequency of food allergy in Europe: An updated systematic review and meta-analysis. In Allergy: European Journal of Allergy and Clinical Immunology (Vol. 78, Issue 2, pp. 351–368). John Wiley and Sons Inc. https://doi.org/10.1111/all.15560
3. Olivry, T., & Mueller, R. S. (2016). Critically appraised topic on adverse food reactions of companion animals (3): prevalence of cutaneous adverse food reactions in dogs and cats. BMC Veterinary Research, 13(1), 51. https://doi.org/10.1186/s12917-017-0973-z
4. Sofou, E. I., Aleksandrova, S., Chatzis, M., Samuel, E., & Saridomichelakis, M. N. (2024). Establishment of clinical criteria for the diagnosis of adverse food reactions in dogs with atopic dermatitis. Veterinary Dermatology. https://doi.org/10.1111/vde.13247
5. Mueller, R. S., & Olivry, T. (2017). Critically appraised topic on adverse food reactions of companion animals (4): Can we diagnose adverse food reactions in dogs and cats with in vivo or in vitro tests? BMC Veterinary Research, 13(1), 1–5. https://doi.org/10.1186/s12917-017-1142-0
6. Bernstein, J. A., Tater, K., Bicalho, R. C., & Rishniw, M. (2019). Hair and saliva analysis fails to accurately identify atopic dogs or differentiate real and fake samples. Veterinary Dermatology, 30(2), 105-e28. https://doi.org/10.1111/vde.12716
7. Horvath-Ungerboeck, C., Widmann, K., & Handl, S. (2017). Detection of DNA from undeclared animal species in commercial elimination diets for dogs using PCR. Veterinary Dermatology, 28(4), 373-e86. https://doi.org/10.1111/vde.12431
8. Ricci, R., Granato, a., Vascellari, M., Boscarato, M., Palagiano, C., Andrighetto, I., Diez, M., & Mutinelli, F. (2013). Identification of undeclared sources of animal origin in canine dry foods used in dietary elimination trials. Journal of Animal Physiology and Animal Nutrition, 97(SUPPL.1), 32–38. https://doi.org/10.1111/jpn.12045
9. Olivry, T., Mueller, R. S., & Prélaud, P. (2015). Critically appraised topic on adverse food reactions of companion animals (1): Duration of elimination diets. BMC Veterinary Research, 11(1). https://doi.org/10.1186/s12917-015-0541-3
10. Olivry, T., & Mueller, R. S. (2020). Critically appraised topic on adverse food reactions of companion animals (9): time to flare of cutaneous signs after a dietary challenge in dogs and cats with food allergies. BMC Veterinary Research, 16(1), 158. https://doi.org/10.1186/s12917-020-02379-3
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