Login 
Food allergies compromise approximately 10% of all dog allergies and, because of this low percentage, they may have historically been dismissed or only mentioned in passing by many vets. However, as more and more pet parents become proactive about their dogs’ health, conversations about food allergies are happening more frequently in vets' offices.
Most often, food allergies are caused by protein in the pet’s food eliciting a reaction in the immune system. Often, the protein source is animal-based (meats, eggs, or dairy), but sometimes a carbohydrate source can be involved too. The most common ingredients that dogs have allergies to include beef, chicken, eggs, milk, fish, horsemeat, potatoes, soy, corn, wheat gluten, or additives.
Symptoms
The most common clinical signs of a food allergy are severe itching, scratching, and chewing. Small red bumps, pustules, and infection can sometimes accompany itching. In dogs, the paws, flank, groin, neck, and ears are commonly affected. Dogs with food allergies often have recurrent ear infections. It's important to note that while some common foods like potatoes are usually safe, the question "can dogs eat potatoes?" becomes more complex for dogs with food allergies. But, it is a different conversation for puppies as it is rare for a puppy to develop true food allergies, as the pet has to be exposed to the allergen repeatedly.
Food allergies can also present with gastrointestinal components. GI signs include chronic vomiting (typically not with an acute onset), diarrhea or loose stools, belching, and frequent bowel movements and/or flatulence. GI signs do fluctuate a bit more than skin problems, but a long history of “troubled GI system” is not uncommon. Pets may suffer from both skin and GI symptoms. Unlike canine atopy and flea allergy dermatitis, food allergies are not seasonal, however GI symptoms may wax and wane to some degree.
Diagnosing Food Allergies
No single specific test can diagnose a food allergy. Allergen blood testing is available, but it is more appropriate for canine atopy and is not a reliable way to identify what food allergen your pet may be allergic to.
If you were to pursue a food allergy diagnosis, it is made by placing your pet on a strict “elimination diet” in which you introduce a new, highly-digestible protein source, and/or carbohydrate source, and no food additives. You can make this diet at home yourself, but be sure to consult with a veterinarian with expertise in nutrition before doing so. Usually it is advised to select a single novel protein (such as duck, rabbit, kangaroo, or other protein your pet has never eaten before, even once), a novel carbohydrate source (such as snow peas or potato), and a source of fat. During the first initial trial it is not as important that the diet be balanced. If your pet responds and the itching decreases, it will be crucial long-term (more than 2 months) to ensure the diet is adequately balanced.
Commercial diets with novel food sources are available, but in my opinion, if you want a commercially-prepared diet, it is wiser to choose a hydrolyzed diet because it is possible your pet may have been exposed to a novel food source that you weren’t aware of. Hydrolyzed protein diets contain proteins that are broken down into pieces too small to fit into the receptors that stimulate the pet’s immune system. These diets are already complete and balanced for adult dogs. Royal Canin has a new food called Anallergenic that shows results in 4 weeks which is a great improvement. Other prescription hypoallergenic diets are also appropriate but do take a little longer for a food trial.
A reduction in itching may be seen with a couple weeks, but in most cases it does take longer (usually around 6 weeks). The test diet should be fed for two to four months (except with the Anallergenic food). For dogs that respond positively, the diet should be continued provided it is balanced and complete. You must remember that table scraps, treats, chews, and flavored medications (such as heartworm meds) must not be given while the pet is on the trial diet and no offending allergens can be given the rest of the pet’s life after the offending allergen is identified.