Pet Food Intolerances

(by Dr. Iuliana Mihai, DVM)

More and more pet owners are having to help their pets find a great food option that doesn’t upset their stomach, cause itching or hair loss, or leave them sluggish.


Studies show that the rate of food intolerance in pets has increased more and more in recent decades, which worries animal owners and lovers. This adverse reaction of the body may be due to food indiscretion, food allergies, or food intolerance.


Many people, including the most devoted pet owners, often confuse food intolerance with food allergy – an easy mistake to make. So we thought we’d take some time to break down the difference between the two, how to spot one or the other, and what to do if your pet struggles with food problems.


This confusion can occur because both have a common denominator - the negative reaction of your pet to certain foods/ingredients. Food intolerances are different from food allergies; the latter has more serious medical implications and involves the immune system.


Today we’ll focus on food intolerance, and next time I’ll dive further into food allergies. In this article, we will present the common ingredients that can cause food intolerance, the clinical signs that may occur, and what to do if you conclude that your pet is suffering from food intolerance.


What are Food Intolerances in Pets?

Food intolerances do not involve the immune system, but are actually caused by a response of the digestive system. Intolerance covers a wide range of food adverse effects, too. For example, most adult cats and some people are lactose intolerant because they lack the enzyme lactase. It can cause stomach upset, nausea, and GI problems. Similar situations can occur with other ingredients that pets are sensitive to.


A more medical definition for food intolerance would be that any abnormal physiological response to a food or food additive that is believed to be non-immunological is defined as food intolerance.


These food intolerance reactions are not sex or age-dependent but are dependent on how much of the ingredient is consumed by the pet.


In dogs, food indiscretion and intolerance are more common than true food hypersensitivity.

In veterinary medicine, the terms - food intolerance or food allergy - were first used in dogs to describe any adverse food reaction.



What Causes Food Intolerance in Pets? Common and Not-so-common Pet Food Ingredients that Cause Food Intolerances


As studies show, food intolerances can be caused by a variety of factors such as:


  1. Food toxicity


Toxicity can be caused by microbial contamination or spoilage, nutrient excess, plant-derived toxins, other toxins, specific foods, etc.


  1. Pharmacological reactions

These are defined as adverse reactions of the organism to various biologically active food chemicals (natural and/or added).


For example, ingesting (eating) onions, garlic, leek, chives, nuts, grapes, macadamia nuts, or raisins will produce pharmacological reactions (to dogs and cats).


Onion, garlic, leek, and chives are part of the same genus, Allium. The compound responsible for their toxicity is called organosulphoxide, which is converted to a complex mixture of sulphur compounds. And cooking, drying, or processing won’t have any affect in lowering their toxicity. Therefore, eating onion, garlic, and chives regularly will lead to anemia in pets.


Other substances are histamine (mostly found in ageing or mature fish and meats), salicylates, theobromine from chocolate, and xylitol (artificial sweetener). Theobromine and caffeine can be toxic to the central nervous and cardiovascular systems of dogs. Theobromine is eliminated very slowly from the organism, increasing the risk of toxicity by repeated ingestion of low doses. Caffeine in large doses can become fatal for your pet.

Xylitol can be found in candy, gum, and some children’s medications to sweeten the taste.


  1. Metabolic reactions

Examples include carbohydrate intolerance/lactose intolerance. The inability of the small intestine to digest certain types of carbohydrates (due to a lack of digestive enzymes) is called carbohydrate intolerance. In general, people with carbohydrate intolerance find it very difficult to digest some or all of the carbohydrates correctly, especially bread, pasta, and fruit. The most common clinical signs are bloating and flatulence.


This can also happen to pets, especially dogs.


Lactose intolerance is the inability to digest milk sugar, and it’s caused by the lack of a specific enzyme, lactase.


  1. Physical effects caused by dietary indiscretion – i.e. eating things that aren’t food!


Dogs are famous for ingesting everything from toys, sand, wood, soap, and clothing to wire sponges. The ingestion of indigestible objects is a common cause of improper food reactions, and in some serious cases, it can lead to intestinal obstruction and possibly death.


Scavenging or dietary indiscretion can lead to ingestion of toxins, excessive fats, indigestible substances, or other compounds that can cause gastric fermentation. For example, sand ingestion or any other indigestible substances can lead to the obstruction of the small intestine, causing ulcerations of the intestinal mucosa, hemorrhages and other changes.



If we were to talk about the ingredients found in pet food, theoretically, all the ingredients can cause allergies or food intolerances, but in this article, we will discuss the most common ones.


These ingredients are also called food allergens.  The most common are:


- Animal protein – from lamb, beef, chicken, chicken eggs, fish (especially in cats) or dairy products;

- Food additives: artificial colors, dyes, preservatives;

- Soy;

- Gluten (from wheat);

- Corn.


As the numbers show, beef, dairy products, and wheat represent 69% of all reported cases of allergies or food intolerances in pets, and 25% for lamb, chicken, eggs, and soy. 


Another study from 2016 revealed that the most frequently reported food allergens in dogs are:

  • beef (34 %),
  • dairy products (17 %),
  • chicken (15 %),
  • wheat (13 %)
  • lamb (14, 5 %).
  • soy (6 %),
  • corn (4 %),
  • egg (4 %),
  • pork (2 %),
  • fish and rice (2 %).
  • barley, rabbit, chocolate, kidney bean, and tomato.


Regarding corn, studies have shown that cats and dogs that were previously sensitized to corn are less reactive to cornstarch. As a result, pet food containing cornstarch is preferred as a source of carbohydrates for dogs and cats that are suspected of corn allergy/intolerance.


When it comes to food additives, the adverse reactions caused by those are not as frequent in pets as in humans. But one food additive that can cause food intolerance in pets is called disulfide, which is also found in onions. Disulfides can cause damage to the red blood cells and lead to anemia.


Common and Not-so-common Symptoms in Food Intolerances

An important feature of food intolerance is that it occurs on initial exposure to that food ingredient (including food additives). The reactions of the immune system (food allergies) generally require more exposure before the clinical signs can be observed.


In other words, food intolerance can occur immediately after ingestion.  By contrast, food allergies depend on the immune system’s response, where the organism must be subjected to the allergen several times until symptoms occur.


Signs of food intolerance or sensitivity can occur at any time, sometimes a few hours or days after consuming the causative food, and they can last for hours or days.


Most often, food intolerance manifests as indigestion - abdominal pain, bloating, noisy stomach or intestines, flatulence, and sometimes diarrhea or altered and irregular stools.


If your dog or cat begins to have episodes of diarrhea, soft stools, or he or she is straining to defecate, it may be a sign of food intolerance. It is a good idea to contact your veterinarian to establish a correct diagnosis and administer appropriate treatment.


Depending on the ingredient or mix of ingredients that led to the food intolerance, your pet may also experience vomiting. Repeated vomiting, especially after each meal, can be a sign of food intolerance. For a better overview of the problem, it is best to contact your veterinarian as soon as your dog or cat starts vomiting, as the causes for vomiting can be multiple.


Another sign of food intolerance is represented by changes in the energy levels. This can happen due to diarrhea or vomiting - two actions that exhaust the body. If you see your pet having a low energy level, make an appointment with your current veterinarian for a better insight of the problem.


Loss of appetite is commonly seen in food intolerances. While some pets can become picky eaters at some point (and has nothing to do with food intolerance), others will present a lack of interest in food.


If you notice that your pet has a lack of appetite associated with other clinical signs (vomiting, diarrhea, etc.), I recommend going to a veterinarian for a consultation.


Now, of course, the causes for gastrointestinal issues in dogs and cats are multiple – viruses, bacteria, parasites, liver or pancreas problems, spoiled food, etc. And it is important to observe if the clinical symptoms occurred after eating a certain diet or ingredient (for example, food with too much/little fiber, other ingredients that your pet doesn’t tolerate).


When it comes to itchy skin, ears, and skin rashes, pet owners often confuse food intolerances with food allergies because adverse reactions to food in pets (especially dogs) often mimic food allergies. There are only so many ways in which the body can demonstrate a food problem.


Studies have shown that food intolerances affect the most the following dog breeds: Westie, Cocker Spaniel, Irish Setter, Shar-Pei, and German Shepherd.

Regarding cats, Siamese and Siamese crossbreeds are known to develop frequently itchy skin and other skin problems.

What to do if Your Pet Has a Food Intolerance

-         If your pet is prone to food intolerances, you may want to consider rotating the food types to prevent your pet from being constantly exposed to the same ingredients.


-         Consider that treats and some medications could cause your pet adverse food reactions.


-         And most importantly - always consult your veterinarian before deciding to change your dog or cat diet or if the clinical signs persist.


Dr. Iuliana Mihai, DVM, Masters In Small Animals And Equines Pathology

Iuliana graduated from the University of Agronomical Sciences and Veterinary Medicine in 2012, Romania. She has a Master’s degree in Small Animal and Equines Pathology and a strong affinity for Veterinary Parasitology and Laboratory. In 2013 she started her Ph.D. in epithelial cancer in dogs and cats. She volunteered at the faculty’s clinic at her 3rd year of study, and continued her career in small animal pathology and laboratory. She has one cat and eleven rats. Her interests outside of work include traveling, writing, and crafting.





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