For a French Bulldog with a food allergy, a prescription hydrolyzed diet or a prescription novel protein diet from your vet is the most reliable choice, and a store-bought "limited ingredient" bag is the least reliable. The problem with the store bag isn't the protein on the front. It's everything that isn't on the label. And if your Frenchie already reacts to chicken, pick a hydrolyzed food that isn't made from chicken, or a protein he has truly never eaten.
Two posts in our Facebook group this week asked versions of the same thing. One rescue Frenchie reacts to chicken and has terrible gas every evening. Another is scratching nonstop, and his owner is shopping for a salmon food. Here's how to choose between the three types of food instead of between fifty brands.
What's the difference between hydrolyzed, novel protein, and limited ingredient food?
All three try to keep the allergen out of the bowl. They just go about it differently.
- Hydrolyzed protein. The protein is broken into pieces "too small to be recognized by the dog's immune system," according to VCA's elimination diet guide, written with the Canadian Academy of Veterinary Nutrition. These come from your vet.
- Novel protein. One protein and one carbohydrate your dog has never eaten before. You can get these from your vet or off the shelf.
- Limited ingredient. A store-bought food with a shorter ingredient list. That's all the name promises.
Why isn't a store-bought limited ingredient food good enough?
Because the label is often wrong. In 2018, veterinary dermatologists Thierry Olivry and Ralf Mueller pulled together the studies that lab-tested pet foods for ingredients that weren't on the label. Across those studies, undeclared ingredients turned up in a median of 45% of the foods tested. For the "novel" or "limited" ingredient foods people buy specifically for allergy trials, it was 33% to 83%. Their conclusion, in the BMC Veterinary Research review, was that mislabeling "appears rather common."
VCA explains why. Limited ingredient formulas "are not made in dedicated or isolated facilities," so they can pick up traces of whatever ran on the line before them. That's how eight careful weeks of "chicken-free" food can turn into eight weeks of chicken anyway, and a Frenchie who never gets better looks like proof that food wasn't the problem.
Hydrolyzed diets did better in that same review. Six studies tested them, and only one found possible mislabeling.
Does a hydrolyzed diet work if my Frenchie is allergic to chicken?
Often, but the source protein matters. In a 2010 study by Ricci and colleagues in Veterinary Dermatology, 12 dogs whose skin reacted to chicken were switched to a hydrolyzed chicken diet, and 11 of the 12 scored far lower than they did on regular chicken. Good news, but not a guarantee.
A systematic review by Olivry and Bizikova the same year found that up to 50% of food-allergic dogs in three controlled studies got worse eating partially hydrolyzed versions of a food they were allergic to. The authors concluded that these diets are "probably best used" in dogs that aren't suspected of reacting to the ingredients they're made from.
Translation: if you already know chicken is a problem, ask your vet for a hydrolyzed diet that isn't made from chicken. Several are made from hydrolyzed soy instead.
Is salmon a safe swap if my Frenchie is allergic to chicken?
Maybe. Don't assume it. There are two reasons to be careful.
First, chicken and fish share some proteins. In a 2019 lab study, Bexley, Kingswell and Olivry tested blood from dogs with antibodies to both chicken and fish, and found antibodies that cross-reacted between chicken and salmon in 4 of the 7 dogs they tested closely. They identified nine proteins the two meats have in common. The authors were clear that nobody has proven yet whether that cross-reaction makes a dog itch. It's a reason for caution, not a verdict.
Second, allergic dogs often react to more than one food. In a 1996 study of 25 food-allergic dogs fed one ingredient at a time, the average dog reacted to 2.4 different foods, and 64% reacted to two or more. So "allergic to chicken" can easily mean "allergic to chicken and something else."
If you do try salmon, make sure it's actually new to him. Read every treat bag, chew and topper in the house for fish before you count it as novel.
So which one should I actually pick?
It depends on whether you're still trying to find out what he reacts to, or you already know.
- Still figuring it out: a prescription hydrolyzed diet (not chicken based, if chicken is a suspect) or a prescription novel protein diet. VCA's guide says it is best to feed the veterinary therapeutic diet your vet chooses, because those foods are made under specific manufacturing protocols. Then run a proper trial. We walked through exactly how in our post on telling a food allergy from an environmental one.
- Want to cook: a home-cooked novel protein diet can work, but VCA says it has to be balanced. Get the recipe from your vet or a board-certified veterinary nutritionist, not a forum.
- Already know the trigger: this is where a store-bought limited ingredient food makes sense to us. Once you know he's fine on duck and he stays fine on a particular bag, that bag has earned its spot. Our take: it's a gamble as a diagnostic tool, not as a daily food that's already working.
And if eight strict weeks change nothing, food probably isn't the main driver. That's when the itch medications come in, which we compared in Apoquel vs Cytopoint for Frenchies.
Want the checklist version?
Drop your email and we'll send the one-page Frenchie Itch-Relief Checklist. What to track, what to cut, and what to bring to your vet appointment, in the order that matters.
FAQ
How long before I know if a new food is working?
Give it at least eight weeks of nothing but that food. Stopping at three or four weeks is how people end up deciding "it's not food" when it is.
Can my Frenchie have treats during a food trial?
Only ones your vet has signed off on. Every chew, dental stick, flavored pill and lick of the plate is a new ingredient, and one of them can undo the whole trial.
Are prescription allergy foods worth the extra money?
For the trial, we think so. The research above found fewer labeling problems in hydrolyzed diets than in "limited" store foods, and two wasted months on the wrong bag cost more than the price difference.
What we'd use
For the food question: your vet and a strict eight weeks. Nothing we sell replaces that, and please don't add a new supplement in the middle of a trial, ours included. Once you know what your Frenchie can eat, Frenchies vs Mushrooms is our daily powder to support immune health. If your dog has a known food allergy, show your vet the full ingredient list first. Give your itchy gremlin a belly scratch from us.