Food allergy is widely and indiscriminately used in veterinary medicine and among dog owners as the cause of dermatological and gastrointestinal (GI) disorders.  However, two different conditions can be caused by food: food hypersensitivity/allergy and food intolerance.  Food allergy (FA) is an immunological reaction to food, which requires previous exposure to the substance causing the adverse reaction. Food intolerance (FI) does not involve the immune system.  

A food intolerance is a reaction to a food substance or additive that occurs without previous exposure to the offending food component. Food intolerances do not involve the immune system and are not mediated by IgE antibodies. Instead, they result from an inability to properly digest or metabolize specific food components.  Food intolerances typically have a slower onset of clinical signs, often occurring hours to days after consuming the offending food.

Food toxicity and poisoning are biological effects caused by an infection or the presence of toxins that can be inherent to the food or are produced by parasites or microorganisms. Examples of food poisoning by microorganisms are aflatoxicosis (aflatoxins) and botulism (exotoxins of Clostridium botulinum).

Anaphylactoid food reactions mimic true anaphylaxis but are not mediated by an immune system response. An example of this type of reaction is ingesting spoiled fish containing large amounts of histamine, which causes a non-immunological release of histamine, resulting in the dog's pruritus (itchiness)

Metabolic food reaction occurs with a particular food due to disease status, malnutrition, and congenital disabilities in the metabolic system. A typical example is lactose intolerance due to decreased lactase activity. 

Clinical signs of food intolerance within the gastrointestinal tract can include bloating, gas, diarrhea, constipation, abdominal pain, and cramping. Food intolerances are generally less severe than food allergies and are not life-threatening. However, they can significantly affect your dog’s quality of life due to chronic inflammation.

The incidence of FA has been questionable in the literature, but estimates range from 1% to 10% of all skin disorders in dogs. GI symptoms have been reported in 10-15% of FA cases. Clinical signs can occur suddenly or after months or even years of the dog consuming the diet.

A dog experiencing non-seasonal pruritus (itchy skin) that is either generalized or restricted to specific body areas such as the face, ears, paws, abdominal area, or perianal region may suffer from a food allergy.

GI disorders such as vomiting, diarrhea, and regurgitation of undigested food are also common signs of an FA.

Secondary skin problems can include bacterial infections, fungal dermatitis, and “hot spots,” self-induced injuries in response to pruritus and pain. Diseases of the outer ear are also significant as they are sometimes the only presentation of a food allergy. 

Reports of common food allergens include beef, dairy, chicken, lamb, pork, eggs, barley, soy, rice, corn, and wheat. Any food component can cause an allergy. Multiple food allergies were demonstrated in up to 35-48% of the confirmed FA cases in dogs. 

It is challenging to diagnose an FA. Food trials remain essential in diagnosing FA, as many diagnostic techniques, such as skin biopsies, intradermal skin testing, and commercial kits, are unreliable. However, a saliva-based test for allergies to 24 affinity-purified food antigens has been developed, showing promising results.

Food trials are lengthy and time-consuming for the dog’s owner.  The mislabeling of commercially available elimination pet foods often occurs, including unexpected ingredients, making an argument for homemade diets to be used.

You need to create a complete food history before beginning the food trial. You must keep a daily journal of your dog’s food consumption and clinical signs before and during the trial. Any other diseases that can cause the same clinical signs as an FA must be addressed. 

Feeding an elimination diet, either a homemade diet or a commercial diet of novel proteins, is the first step in a food trial. Studies have shown that commercial food often contains proteins not on the ingredient list, making it very difficult to determine if your dog has an allergic reaction to a specific protein. Using fresh whole foods for the elimination process ensures that you have control over what your dog is consuming and removes a lot of the guesswork because there are no fillers or other additives in the food. Some veterinarians will recommend the use of a of a hydrolyzed diet . 

However, the hydrolysis process drastically reduces the palatability of the hydrolyzed food. The nutritional value is also reduced, so it should only be used as a temporary diet during episodes of GI tract or skin disorders. Persistent diarrhea occurs in dogs fed the hydrolyzed diets long-term.

Using Fresh Whole Food as the Elimination Diet 

The elimination must contain only novel proteins, those meats, or other protein sources your dog has never eaten.  The diet should also be nutritionally adequate for your dog’s life stage.

The Real Ancestral 6X Diet™  consists of six components: raw meaty bones, muscle meat, liver, another secreting organ, seafood, and a fiber source such as vegetables or fur. During the elimination process, the source of the raw meaty bones, muscle meat, and organs must be from the same species, such as all rabbit or turkey products. Fish and fiber should not be added to the meal during the elimination phase.

Although GI signs can resolve in 2-4 weeks, the same protein should be used for at least six weeks. Skin disorders may require more than six weeks to resolve, especially if secondary conditions are involved.

The second step in the food trial is the challenge test. This step is vital in confirming a true FA. However, many owners refuse this step because they realize their dog will have recurrent problems.

You begin the challenge phase by reintroducing the original protein that is believed to cause the allergy.  The duration of the challenge can be as quick as a few hours or as long as seven days, depending on the length of the elimination diet stage and the severity of the allergy.

The final step of the food trial is the provocation test. The provocation phase establishes a more precise diagnosis of the FA and is only performed after a successful elimination diet is established. 

During the provocation phase, you challenge your dog to specific proteins.  Foods are added to the elimination diet one at a time for 1-2 weeks.  If there is no FA response, you can add different food components. 

Another aspect of food allergy testing is determining if there is an imbalance or dysbiosis in the gut microbiome. Dysbiosis can weaken the gut lining, which allows undigested food to pass through the intestinal lining and into the bloodstream, resulting in an allergic reaction. 

Gut microbiome  dysbiosis is an imbalance in the composition and function of the microbial communities residing in the gastrointestinal tract. The gut microbiome plays a crucial role in the overall health of dogs, influencing various physiological processes, including digestion, metabolism, and immune function. Growing evidence suggests a connection between gut microbiome dysbiosis and the development of food allergies in dogs.

Because the gut microbiome is critical in educating and modulating the immune system, a balanced and diverse microbiome helps train the immune system to distinguish between harmful pathogens and harmless substances, such as food proteins. The gut microbiome plays a role in developing oral tolerance, which is the immune system's ability to tolerate and not react to harmless substances, such as food antigens. Dysbiosis may interfere with establishing oral tolerance, increasing the likelihood of an allergic response to dietary proteins.

A healthy gut microbiome contributes to the maintenance of the intestinal barrier. This barrier helps prevent the entry of undigested food particles and potential allergens into the bloodstream. Dysbiosis can compromise the integrity of the intestinal barrier, allowing the penetration of substances that may trigger an immune response.

Dysbiosis can lead to an inflammatory state in the gut. Chronic inflammation may disrupt the normal immune response and contribute to the development of allergies. The immune system may start reacting to specific food proteins as if they were harmful invaders, leading to allergic reactions.

Beneficial bacteria in the gut produce short-chain fatty acids (SCFAs) as byproducts of fermentation. SCFAs have anti-inflammatory effects and contribute to maintaining a healthy gut environment. Dysbiosis can result in decreased SCFA production, potentially impacting immune regulation.

Healing your dog’s gut microbiome can actually resolve those pesky food intolerances and allergies!

The prognosis for your dog is excellent once the offending food component is identified and eliminated from the diet. Everyone in the household must be aware of the dog’s dietary restrictions, as adding table scraps or non-approved treats to the diet will cause a relapse of the clinical signs. If clinical signs reappear after the dog has been on the new diet, a repeat of the food trial may be necessary to determine if new allergies have developed.

Some studies show that the development of FA can be prevented. One study focused on the dogs' ages when placed in permanent homes, revealing that if they were adopted by the age of 12 weeks, there was a lesser incidence of the development of FA. It has also been shown that providing a variety of foods, especially good quality protein, rather than the same diet throughout the dog’s life can prevent allergies from developing.

Studies have confirmed a significant difference in the gut microbiota of kibble-fed dogs and those fed whole real food. Feeding your dog a species-appropriate diet will aid in the prevention of food allergies.

Food allergies and food intolerances are not interchangeable terms. A food allergy is an immunological reaction to food, whereas food intolerance is a non-immunological response. 

Food allergies can present as skin or GI tract disorders.

Addressing a gut microbiome imbalance can aid in the elimination of food allergies in your dog.

Diagnosing cutaneous food allergies in dogs and cats - some practical considerations 

Western blot analysis of sera from dogs with suspected food allergy   

Study of the chemical and nutritional characteristics of commercial dog foods used as elimination diet for the diagnosis of canine food allergy 

Regulatory Medicine In The Treatment Of Food Allergy In Dog A Case Report 

Critically appraised topic on adverse food reactions of companion animals (5): discrepancies between ingredients and labeling in commercial pet foods 

Allergies in Dogs: Focusing on Food

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