Most of this information is already here in The Food Group, but I thought I would put it all together in one discussion for easy access. I hope that you regular Food Groupies will remember that it's here and use it when a newbie pops in with questions about food allergies.
Fact 1: Food Allergies Are Relatively Rare.
There is a 1 in 10 chance that any dog will have some kind of allergy. There is a 1 in 100 chance that any dog will have an allergy to any type of food. Food allergies are the least common type of allergy in dogs, ranking behind flea allergies and Atopic Dermatitis, (often called "skin allergies", "seasonal allergies", or "environmental allergies")
Fact 2: Food Allergies Take Time to Develop.
It is extremely rare for a puppy under the age of 7 months to have a food allergy. It usually takes 6 months to a year of eating the same food repeatedly for a dog to develop an allergy to it, with symptoms most often appearing between 1-2 years of age. This is one reason why it's a good idea to rotate different protein sources in your dog's diet.
Fact 3: Vomiting And/Or Diarrhea Are NOT Symptoms of a Food Allergy.
The symptoms of any allergy in a dog take the form of itching and scratching; rubbing the face, chewing, biting, or licking the paws, itching, scratching & biting the groin, axilla, and base of the tail, red inflamed eyes, skin and/or ear infections. The most common, and usually the first, sign of allergies in dogs is licking or chewing the paws. I have seen one reliable article stating that in a very small percentage of dogs with food allergies, GI symptoms may also appear, but even there, they are always accompanied by dermatological issues.
Gastrointestinal symptoms such as vomiting, regurgitation, loose stools or diarrhea may be signs that your dog has an intolerance to a particular food or combination of foods, but a food intolerance has nothing at all to do with an allergy. An allergy is an incurable disease of the immune system. A food intolerance means something does not agree with your dog's digestive system. There is a big difference.
(Note: Ongoing or severe GI symptoms can also be indicative of a digestive disease, so in those cases, please consult your vet.)
Fact 4: Dogs Are Allergic to A Specific Food, Not a Brand of Food Or A Type of Feeding.
If there is a food allergy, this means that the dog is allergic to one or more specific ingredients, not to all of the ingredients in a particular type of food. The most common food allergens for dogs are corn, wheat, beef, chicken, and soy. If your dog is allergic to chicken, he is allergic to all chicken in every form, sold by every manufacturer....raw chicken, cooked chicken, the chicken in Purina and the chicken in Orijen. It is the particular combination of protein molecules within a specific food that trigger the allergic reaction, not the brand of dog food or the method of cooking.
Also, food allergies are very specific. Your dog can be allergic to chicken but not to turkey, because the protein molecules (and therefore the antibodies) are very different. And your dog cannot be allergic to something that he has never eaten. There has to have been prior exposure to develop an immune response to something.
Fact 5: There is No Medical Test for Food Allergies, and Switching Brands Won't Help.
Blood tests for food allergies have been proven to be extremely unreliable. Scratch tests are not done for foods.
In order to determine if your dog has a food allergy, you must do food trials. This means you eliminate corn, wheat, beef, and chicken from your dog's diet and feed a limited ingredient diet with one "novel" protein source (something the dog has not been eating on a regular basis, like rabbit or duck) and one starchy carbohydrate (often oatmeal, potatoes, or some type of legumes) for a period of 3 months. (It is not possible for a dog to be allergic to a food he has never eaten.) This also includes all treats, everything that the dog consumes. If the symptoms go away, you then gradually add the chicken back in. If that's okay you add the beef back in, etc., until you have determined what the allergens are. Food trials take many months and are very restrictive.
If you just change your brand of foods, and the dog's symptoms happen to go away, what does that tell you? Which ingredient was the culprit? What can he eat now?
Fact 6: Certain Types of Inhalant Allergies Can be Triggered by Poor Quality Food.
Storage mites are dust-mite type micro-organisms that are believed to be as much to blame for asthma attacks in children as dust-mites, and storage mite allergies are increasing in children...and dogs. Storage mites live in dry cereal-type foods with a lot of particulate matter. The longer the shelf life of a product, the greater the chance that it contains storage mites. The more tightly sealed and impermeable the packaging, the less likely it is to contain storage mites. A dog who is eating a grocery store brand that sits for a year on a warehouse shelf in a paper bag might well show great improvement in itching and scratching if you switched him to a premium dog food. Did he have food allergies...or storage mite allergies?
A poor Omega 6:3 ratio can also contribute to allergy symptoms, although it won't cause them. You want a food that is high in Omega 3 fatty acids DHA and EPA from fish, and an Omega 6:3 ratio of not more than 5:1. 3:1 or lower would be best.
Fact 7: Rx Diets Prescribed by the Vet Are Garbage and Should Be Avoided at all Costs.
Rx diets do not contain any magic ingredients that will cure your dog's allergies...if he even has any allergies. They are outrageously priced crap in a can/bag that your dog doesn't need..you can feed an alternative diet cheaper, and with healthier ingredients. Please use this link to learn more about why you should say no when your vet prescribes an Rx diet.
Fact 8: Seasonal Allergies Are Never Caused by Food.
This one should be self-explanatory, but let's go over it to make sure. If your dog is allergic to his food, he would be having symptoms all year round, right? So if he suddenly develops allergy symptoms, especially if it's spring or late summer/early fall, don't listen to all the people who tell you to change his food. Chances are great that if he's allergic, it's an allergy to pollen or mold...which are ten times more common than food allergies. Try conservative treatments as advised by your vet and wait until there's a pattern in terms of time of year. Severe allergic reactions always require immediate treatment.
Hope I covered everything. Bottom line, food allergies are nowhere near as common as people think they are, and misinformation is abundant. I hope this helps.