Diet trials 101

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Chronic pruritus is one of the most frequent dermatologic complaints in general practice, and food allergies are responsible for roughly 15 to 35% of non‐seasonal cases.^1,2 Accurately identifying food‐induced dermatitis requires a systematic approach, since most itchy dogs suffer from environmental allergens, parasites, or infections rather than dietary triggers. This was the topic in focus at a panel discussion at Fetch Nashville 2025 titled “Don’t dread the diet trial! Demystifying food allergies and their diagnosis,” featuring Joya Griffin, DVM, DACVD, Julia Miller DVM, DACVD and Natalie Marks, DVM, CVJ, CCFP, Elite FFCP-V. The trio came together to share practical insights on when and how general practitioners should implement diet trials.³

When to consider a diet trial

Early in the conversation, Griffin described her typical approach to newly pruritic patients. She said she rarely brings up diet in a first‐time offender unless there is a clear nonseasonal pattern. She illustrated this point with a hypothetical example: “If it’s June and it’s a Labrador and it’s the first time they’ve ever had issues, I don’t talk about food…I treat that dog like an atopic [dermatitis] case, because atopy is more common.”

The panel agreed that environmental allergies, parasites, and infections are far more likely in a first‐time itchy dog than food. In many general practice settings, Marks noted that general practitioners often feel pressed for time and said, “We don’t need to do a diet trial on every itchy dog.” She encouraged ruling out infections and parasites first, managing pruritus, and monitoring the patient’s response before considering elimination diets.

A diet trial becomes reasonable when pruritus is persistent and nonseasonal or when standard atopy treatments fail over several months. Griffin explained that once you establish that itching persists into colder months, when environmental allergens should be less active, it makes sense to discuss diet.

Framing the diet trial as a diagnostic test

A recurring theme was how to position the elimination diet as a finite, diagnostic tool rather than a lifelong prescription. Griffin stressed that it’s important to communicate clearly with clients due to the challenge of compliance with the prescribed diet, stating it “takes a lot of work” to get the client on board. “If I’m going to do a diet trial, I want to do it once and do it well,” she said

She typically asks clients to commit to at least 6 weeks on a novel or hydrolyzed‐protein diet, with a plan to taper pruritus medications around week 4. In her words:

“Give me 6 weeks. If you can give me 6 weeks with this, I’m going to try to get your dog off of Apoquel right around week 4 or 5, and then we’ll see where we are…By week 8, you are going to get dramatic improvement in greater than 95% of your food allergic dogs.”

At the mid-point (week 4–5), Griffin will withdraw medications such as oclacitinib (Apoquel; Zoetis) or lokivetmab (Cytopoint; Zoetis)—provided she knows the patient’s Cytopoint injection schedule—so she can judge whether the diet alone maintains comfort. If itch returns immediately, she discontinues the trial; if the dog remains comfortable, she continues to week 8, she said.

The idea is that describing the trial this way helps owners see it as a limited study, not a permanent expense.

Choosing the appropriate diet

The panel next addressed diet selection, emphasizing the pitfalls of over‐the‐counter limited‐ingredient foods. Griffin debunked the grain‐free myth:

“Grains are great. They’re great for the heart…There is the occasional wheat or soy allergic dog, but they are really, truly the low end of the list.”

Griffin explained that “cross‐contamination” in commercial kibble is rampant. Even novel diets often contain trace amounts of common proteins. Marks cited a 2011 study showing that 10 of 12 over-the-counter “limited‐ingredient” foods labeled free of mammalian, fish, or dairy contaminants nonetheless tested positive for those proteins.⁴ This evidence led all 3 veterinarians to recommend prescription diets containing a verified novel protein (e.g. rabbit, alligator) or a hydrolyzed protein (commonly salmon).

Miller reminded attendees that in cats, beef, dairy, and fish top the allergy list. She agreed that choosing a diet involves a thorough review of every protein or treat the patient has consumed, including flavored medications and occasion‐specific snacks.

Common pitfalls and how to avoid them

Throughout the discussion, several pitfalls emerged:

Hidden allergens in treats and supplements

Griffin warned that owners may inadvertently sabotage a trial by offering flavored pill pockets, rawhides, or table scraps. “Cheese for pills … flavored medications … they forget that.” She recommended alternatives like vegan cheese or specialized “pill wrap” products to ensure no animal protein is introduced.

Not counting everything they eat

In urban practices, Marks has seen dogs flare because doormen or neighbors slip them treats. She advised, “Ask exactly what goes in the mouth…even treats outside the home count.”

Automatic refills without rechecks

Griffin described how some owners simply reorder prescription food indefinitely, too often without re‐evaluation. She suggests restricting refills to a three‐month supply. “Your front desk staff will call you and say, ‘Myrtle needs her refill.’ And you'll look back [think], ‘Oh my god, I haven't talked to Myrtle in 3 months. Is that [hydrolyzed‐protein diet] even working?

Skipping the mid‐trial follow‐up

Both Miller and Griffin stressed that a recheck—ideally at week 4 or 5—is non‐negotiable. Owners need guidance on tapering medications, troubleshooting appetite issues, and confirming compliance.

Interpreting outcomes and next steps

At the end of 8 weeks, Griffin expects a clear reduction in pruritus, measured via a Pruritus Visual Analog Scale (PVAS) recorded at baseline and week 8. “If there’s been no change or only a teeny change by week 8…that diet has not made a difference,” she said. Successful diets not only allow withdrawal from medications but also provide a basis for a formal rechallenge: re‐introducing the original diet or suspect protein to confirm a true food allergy. If a dog’s itching returns immediately upon rechallenge, the diagnosis is confirmed, the panel said

Griffin acknowledged that some patients have mixed atopic and food allergies. In such cases, even after confirming a food trigger, clinicians must address environmental allergens while maintaining dietary restriction for the offending protein.

Takeaway

Diet trials don't have to be scary or arduous. By following a structured approach that starts with ruling out more common causes first, framing elimination diets as limited diagnostic tests, choosing verified prescription diets, and scheduling timely rechecks, general practitioners can efficiently identify those few patients who truly require lifelong dietary restriction.

References

1. Carlotti DN. Cutaneous adverse food reactions in dogs and cats. Vet Dermatol. 2003;14(5):269-277.
2. Olivry T, Mueller RS. Critically appraised topic on adverse food reactions of companion animals (3): prevalence of cutaneous adverse food reactions in dogs and cats. BMC Vet Res. 2017;13:51.
3. Griffin J, Marks N, Miller J. Don’t dread the diet trial! Demystifying food allergies and their diagnosis. Presented at: Fetch dvm360 conference; Nashville, TN. May 30, 2025.
4. Raditic DM, Remillard RL, Tater KC. ELISA testing for common food antigens in four dry dog foods used in dietary elimination trials. J Anim Physiol Anim Nutr (Berl). 2011;95(1):90–97. doi:10.1111/j.1439-0396.2010.01016.x.