Food Trial. Words that no one—not the veterinarian, their staff, and certainly not the pet owner—enjoys hearing.
A properly conducted food trial means 8 long weeks of saying “no” to the dog dad who wants to offer just a little bite of cheese, “no” to those sad brown bulldog eyes staring at the dinner table, and “no” to the cat mom who thinks the food recommended by Google is less expensive and better.
The patient will not only survive but thrive. Their humans, however, are going to suffer. For these reasons and more, it’s imperative that veterinarians conduct a food trial that is definitively diagnostic.
Patients diagnosed with recurrent otitis externa, superficial pyoderma, or Malassezia dermatitis may experience food allergies. Dogs with ventrally distributed and/or pedal pruritus may be excellent candidates for food trials. Cats exhibiting miliary dermatitis, head and neck pruritus, eosinophilic granuloma complex, or self-induced alopecia are also food trial nominees.
The challenge isn’t recognizing the patient who needs a food trial, it’s determining how to best execute the test.
No Substitute for Controlled Food Trial
No serologic, intradermal, saliva, or hair tests are currently considered dependable for diagnosis. Some dogs may respond to their food allergens via type I hypersensitivity reactions and be eligible for serum IgE testing, but a larger proportion of food allergic canines respond with type IV (cell-mediated) hypersensitivity reactions. As such, serum testing for the type IV reactors is invalid. Lymphocyte proliferation assays, which are currently commercially unavailable, would be required to diagnose these patients and identify their allergens.
At present, the only diagnostic test for food allergy in dogs and cats is a highly restricted, 8-week diet trial during which the patient is fed only one, carefully selected diet with no other foods, treats, chew toys, or flavored medications, including parasite preventatives.
In dogs, approximately 95% respond by 8 weeks. In cats, over 90% respond by 8 weeks. Short-term use of anti-inflammatory medications during the initial phase of the diet trial may reduce the time needed to observe improvement to 4 to 6 weeks.
Prednisolone (0.5 mg/kg once daily) or oclacitinib (Apoquel®) (0.4–0.6 mg/kg once daily) can be administered in 2-week increments with periodic withdrawal for 1 to 3 days to assess relapse of clinical signs.
Recommended Trial Diets
Diet selection for the food trial can be challenging. Proteins with molecular weights between 15 and 40 kilodaltons are the most common food allergens in both dogs and cats. In dogs, the most frequently implicated allergens include beef, dairy products, chicken, wheat, corn, and soy. In cats, beef, fish, and chicken are the likely culprits.
Recent advances in molecular allergology indicate that dogs allergic to chicken may be at increased risk for cross-reactivity with fish proteins, and dogs allergic to beef have a higher likelihood of cross-reacting with other ruminant proteins, including lamb, venison, and bison. Hence, novel protein diets remain acceptable but are increasingly difficult to implement due to concerns regarding cross-reactivity of proteins, unknown dietary histories (particularly in rescue animals), and contamination of commercial diets.
Hydrolyzed protein diets have become increasingly popular and successful. Although the ideal molecular weight for hydrolyzed proteins is unknown, proteins less than 5 kilodaltons are unlikely to cross-link IgE and are therefore hypoallergenic for most patients. Only a limited number of diets available in the United States meet or closely approach this criterion. Examples include Royal Canin Ultamino®, Purina Elemental®, and Farmina VetLife Derma HF®. Dogs and cats that have complied with an 8-week trial utilizing one of these diets are considered to have successfully completed a food trial. Other diets may be used on a case-by-case basis if the patient experiences palatability issues or has multiple co-morbidities.
Definitive Diagnosis
The food allergy diagnosis should ideally be confirmed through a dietary provocative challenge. The original diet is reintroduced for up to 2 weeks while all other variables remain constant. Recurrence of prior clinical signs confirms food allergy. Type I reactions may recur within hours to a few days, whereas Type IV reactions may not appear until 4 to 12 days after exposure.
Throughout the food trial process, pet owner compliance can be enhanced by educating family members on the food trial rules. Adequate preparation, empathy, and assurance that, if done correctly, a food trial must only be completed once, can go a long way to achieving a successful outcome for everyone involved.
By Stephanie Bruner, DVM, DACVD
Recommended Reading
Fernandez-Lozano C, Mas-Fontao A, et al. Evaluation of a direct lymphocyte proliferation test for the diagnosis of canine food allergies with delayed reactions after oral food challenge. Vet Dermatol. 2025; 36: 433–442. doi.org/10.1111/vde.13312
Jackson HA. Food allergy in dogs and cats; current perspectives on etiology, diagnosis, and management. J Am Vet Med Assoc. 2023 Mar 18;261(S1):S23-S29. doi: 10.2460/javma.22.12.0548
Jackson HA, Dembele V. Conducting a successful diet trial for the diagnosis of food allergy in dogs and cats. Vet Dermatol. 2024; 35: 586–592. doi.org/10.1111/vde.13274
Olivry T, Mueller RS. Critically appraised topic on adverse food reactions of companion animals (7): signalment and cutaneous manifestations of dogs and cats with adverse food reactions. BMC Vet Res 15, 140 (2019). doi.org/10.1186/s12917-019-1880-2
Olivry T, Mueller RS. 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 Vet Res 16, 158 (2020). doi.org/10.1186/s12917-020-02379-3
