강아지 가수분해 알러지 처방식 비교 — Hill's z/d vs Royal Canin HP vs Purina HA

Comparison of Hydrolyzed Allergy Prescription Diets for Dogs — Hill’s z/d vs Royal Canin HP vs Purina HA

Skin & CoatComparisonsMeongsiljang Veterinary Advisory Board

We’ve put together a clear, side-by-side comparison of the protein molecular weight, ingredients, and ideal use cases for Hill’s z/d, Royal Canin HP, and Purina HA—three hydrolyzed prescription diets. This guide highlights the key differences every pet owner should know before starting an elimination diet trial.

What Are Hydrolyzed Allergy Prescription Diets?

A side-by-side comparison of Hill’s z/d, Royal Canin HP, and Purina HA prescription diets

Hydrolyzed prescription diets for dogs are specially formulated veterinary foods in which proteins are broken down into tiny molecular fragments so the immune system doesn’t recognize them as allergens. What truly matters isn’t the brand, but identifying the specific cause of your dog’s allergies. If your dog reacts to common proteins like chicken, beef, or eggs, an elimination diet trial with a hydrolyzed formula for at least eight weeks is necessary to pinpoint the exact trigger. Rather than deciding on your own, work with your veterinarian to determine the right start date, duration, and transition method to ensure the diet is effective.

Why Use Hydrolyzed Diets Instead of Regular Hypoallergenic Diets?

The main difference between regular hypoallergenic diets and hydrolyzed diets lies in the size of the protein molecules. Food allergies occur when the immune system recognizes proteins above a certain size as foreign invaders. In human studies, proteins larger than 10,000 Daltons are typically identified as problematic. However, since this same threshold hasn’t been definitively confirmed in dogs and cats, it’s difficult to establish an exact numerical cutoff. Hydrolyzed prescription diets break down proteins into smaller peptides that are less likely to trigger an immune response, though the degree of hydrolysis varies by product. Therefore, regular diets labeled as “hypoallergenic,” “natural,” or “organic” cannot replace elimination diet trials. Only hydrolyzed prescription diets, novel protein prescription diets, and homemade restricted diets prepared with specific ingredients are recognized as valid elimination diets for diagnosis and treatment.

Hill’s z/d, Royal Canin HP, and Purina HA at a Glance

Item	Hill's z/d	Royal Canin HP	Purina HA
Main protein source	Hydrolyzed chicken liver	Hydrolyzed soy	Hydrolyzed soy
Average molecular weight	Below 3,000 daltons	Below 10,000 daltons	Below 10,000 daltons
Crude protein content	18% or higher	20% or higher	21% or higher
Crude fat content	13.5%	20%	12%
Calories (100g)	Approx. 370 kcal	Approx. 395 kcal	Approx. 352 kcal
Recommended cases	Elimination diet trial / high sensitivity	Broad food allergies	With fat restriction

Values are based on 2025 manufacturer official guidelines. Actual prescriptions should prioritize the veterinarian's judgment.

Why Does Molecular Weight Matter?

- Highly hydrolyzed proteins: Hill’s z/d features a high degree of protein hydrolysis, making it a viable option for dogs that haven’t responded to other prescription diets. However, even after hydrolysis, the immune response to the original protein isn’t always completely eliminated. In fact, one study found that a hydrolyzed chicken liver diet still triggered symptoms in some dogs allergic to chicken. - Broad-spectrum allergies: Royal Canin HP is considered a candidate elimination diet for dogs reacting to multiple food ingredients. - Low-fat formulation: Purina HA has a lower fat content, making it suitable for dogs with a history of pancreatitis or fat sensitivity. Even among “hydrolyzed” diets, the extent of hydrolysis and nutritional composition vary. Furthermore, a lack of reaction to a hydrolyzed diet doesn’t definitively rule out food allergies. Therefore, your veterinarian should help you choose the best option based on your dog’s specific concurrent health conditions.

A veterinarian explaining the differences between hydrolyzed prescription diets to a pet owner.

Elimination Diet Trials: 8+ Weeks on a 100% Single Food

The effectiveness of a hydrolyzed prescription diet depends on how strictly you eliminate all other foods. For eight weeks, you must stop giving your pet any regular treats, chews, toothpaste, or supplements so the true cause of the allergy can be identified. Even a single exposure to human food means you have to restart the trial from the beginning.

How to Choose the Right Prescription Diet for Your Pet

- History of reactions to previous foods: If itching has recurred with multiple different foods, it may be worth prioritizing z/d, which features a high degree of protein hydrolysis. However, since some pets may still react to hydrolyzed diets, this is not an absolute rule. - Concurrent pancreatitis or fat sensitivity: HA, which is lower in fat, may be a suitable option to consider. - Palatability and cost: Hydrolyzed diets generally have lower palatability and are more expensive than novel protein diets. For pets that are reluctant to eat them, consult your veterinarian to select the most appropriate product. - Reintroduction after an elimination trial: Once the trial is complete, conduct a provocation challenge by returning to the original food. If symptoms recur, work with your veterinarian to identify the culprit protein by reintroducing one protein at a time (typically for about one week per protein).

Transition Slowly Over 7–10 Days

Sudden changes to any prescription diet can cause diarrhea or vomiting. - Days 1–3: 75% current food + 25% prescription diet. - Days 4–6: 50:50 mix. - Days 7–9: 25% current food + 75% prescription diet. - Day 10 onward: 100% prescription diet. Note that for an elimination diet trial, the 8-week period begins only when the pet is eating 100% prescription diet. If itching, diarrhea, or vomiting suddenly worsens during the transition, slow down the increase over a few more days or consult your veterinarian about switching to a different prescription diet.

A gradual transition from regular food to a prescription diet.

Why a Veterinarian’s Prescription Is Essential

All three products are classified as prescription-only in Korea, so they may not be freely available at regular pet stores or online retailers. Long-term use as a daily supplement, based on a pet owner’s own judgment, can disrupt the balance of micronutrients, so it’s safest to monitor your pet’s health with regular blood tests.

Reviewed by a veterinarian

Dr. Tony — Punnawat Phongkittirak

Veterinarian

A veterinarian who majored in veterinary medicine at Khon Kaen University, Thailand, and completed the IVSA program at North Carolina State University in the United States. Drawing on clinical experience at animal hospitals, he works in the pet healthcare field and is dedicated to building a digital care environment that connects pet parents with veterinarians.

Frequently Asked Questions

Prescription diets are more expensive than regular pet food. Does my pet need to eat them for life?

Once the allergy trigger is clearly identified through an elimination diet, many pets can be gradually transitioned to a regular commercial food that simply avoids that specific protein, under your veterinarian’s guidance. Not every dog or cat needs to stay on a prescription diet for life.

Can I switch between z/d, HP, and HA in the middle of a treatment plan?

For accurate results, you must stick to a single diet throughout the entire 8-week elimination trial. After the trial is complete, it is perfectly fine to switch to a different prescription diet based on your pet’s palatability, weight, and digestive health, provided you consult with your veterinarian first.

Can’t I give my pet any treats at all?

During a dietary elimination trial, you should ideally avoid all treats, human food, and even flavored medications or chew toys. If that’s too difficult, you can lightly bake small portions of the same prescription diet to use as treats.

What if itching persists even while feeding a hydrolyzed diet?

It’s important to consider other potential causes, such as environmental allergies (atopic dermatitis). However, a lack of response to a hydrolyzed diet does not completely rule out a food allergy. If there is no improvement after feeding only the prescription diet for eight weeks, consult your veterinarian to discuss next steps, such as trying a different elimination diet or conducting allergy testing.

Compared to Nobel Protein (kangaroo, duck, etc.) food, which one is better?

Both approaches have diagnostic value for dietary elimination trials. Novel proteins must be ones the pet has never consumed, but this can be tricky because many proteins are already present in commercial diets, and some products contain undeclared ingredients. Hydrolyzed diets are sometimes chosen instead, but neither option is definitively superior. If a soy allergy is suspected, consider a chicken-liver-based z/d diet or a novel protein diet rather than a soy-based formula.