man holding takeout box and clutching stomach

What is a Food Intolerance?


Written by: CDHF

Updated: December 1st, 2022

If you are someone who consistently experiences unpleasant symptoms such as bloating or abdominal pain a few hours after eating a specific type of food, you could potentially have a food intolerance, and you are not alone! Food intolerances are very common. In fact, a 2021 report from Dalhousie University suggests that an estimated 6.8 to 7.4 million Canadians suffer from at least one food intolerance. (1)

CDHF understands it can be frustrating to try and pinpoint the food intolerance specifically. Oftentimes, people may not even realize that their symptoms are due to food intolerance, and they may believe the symptoms are stemming from something else. (2) 

By definition, a food intolerance is an adverse reaction to a food substance or ingredient. It affects the digestive system, as it is the inability to digest or absorb certain foods.

Typically, symptoms occur within a few hours after eating, and can include: 

It’s important to note that a food intolerance is generally not life-threatening, and it is often unclear why a person is sensitive to certain foods. 

How does a food intolerance differ from a food allergy?

Rather than a digestive system response, a food allergy is the reaction that involves your immune system. Your immune system is reacting to a food protein or the allergen that you have ingested, one that your body thinks is harmful. Unlike a food intolerance, with food allergies, symptoms occur within minutes of consuming even a small amount of food or trace amount of allergen. Symptoms can range from mild to severe, and may include:

Food allergies can lead to anaphylaxis, which can be a life-threatening condition. There are many different types of food allergies, and the most common are shellfish, nuts, fish, eggs, peanuts, and milk. In a 2021 Dalhousie University report, they found that an estimated 2.5 to 3.1 million Canadians have at least one food allergy. (1) CDHF recommends seeing a doctor to help determine if you have an allergy or intolerance and establish a plan to help control symptoms. 

While we don’t have all the facts surrounding food intolerances and sensitivities yet, scientists are working hard to understand them. In this article, we will be discussing what we do know about the 7 key food intolerances. Let’s get into it!


Lactose intolerance is the most common food intolerance! People who are lactose intolerant have unpleasant symptoms after eating or drinking milk or milk products. Normally, when someone eats a food that contains lactose, an enzyme is produced in your small intestine called lactase, which breaks down lactose into its simpler components (which are simple sugars called glucose and galactose). These simple sugars are then able to be absorbed in the bloodstream and give you the energy you need. 

If you have lactose intolerance, the body doesn’t make enough lactase to break down lactose and as a result allows undigested lactose molecules to pass to the lower parts of the intestine. At this point, the undigested lactose encounters bacteria which begins digestion through a process of fermentation. The outcome of this fermentation (hydrogen, carbon dioxide, methane gasses, and short-chain fatty acids) leads to many of the telltale symptoms for lactose intolerance – like gassiness, bloating and diarrhea. (3)

While there is no actual cure for lactose intolerance, most people manage their symptoms by decreasing the amount of lactose in their diet or using commercially available lactase enzymes.

Luckily, there are many lactose-free dairy products available in grocery stores such as lactose-free milk, yogurt, cheese, and ice cream that are great tasting and suitable for those with lactose-intolerance! 


Gluten is a protein that can be found in wheat, rye, and barley. Gluten intolerance (or gluten sensitivity) is different from celiac disease. If you have celiac disease, eating gluten damages the villi (small finger-like structures) that make up the lining of your small bowel. This injury prevents nutrients from being properly absorbed. Some people test negative for celiac disease and show no signs of the intestinal damage. However, they still react negatively to eating wheat with symptoms such as abdominal pain, fatigue, and headache. Such people are said to be gluten-sensitive or intolerant. Gluten sensitivity exists on a spectrum of severity, so not all people react to the same amounts or types of gluten-containing foods in the same way.

Individuals with gluten intolerance may find relief by reducing the amount of gluten in their diet, or eliminating gluten from their diet entirely. There are a lot of great gluten-free food products on the market, such as gluten-free breads, baked goods, sauces, soup mixes, dairy products, and beverages to consume as an excellent alternative to traditional wheat flour products. (4)


That’s right, sugar! Sucrose is a disaccharide, meaning it is two individual sugar molecules that are linked together. For the body to absorb it, the linked sugars need to be pulled apart and broken down. Enzymes are required to facilitate this breakdown and make the sugar molecules into monosaccharides, which are small enough for the body to absorb. If a person is missing or low in the enzymes needed to digest sucrase, they are sucrose intolerant.

People with sucrose intolerance are typically diagnosed early in life, while others are diagnosed later in life, even in adulthood. Sucrose intolerance can manifest in two ways – one, which is known as congenital sucrase isomaltase deficiency CSID (from birth) and the other is acquired from damage to the gut and can be seen with Irritable Bowel Syndrome (IBS), celiac disease, SIBO and other digestive conditions. The most common symptoms of sucrose intolerance include diarrhea, smelly stools and gas, abdominal pain, and bloating and usually occur after eating.  

Sucrose is found in a variety of foods such as: 

The tolerance of these foods is dependent on the amount eaten. Small amounts may be tolerated, but with larger quantities consumed symptoms begin to occur. (5) People with sucrose intolerance often use enzyme replacement therapy, as well as dietary measures such as the disaccharide-free diet to manage symptoms. Many times, this isn’t all-or-nothing, but rather figuring out how much of these specific carbohydrates you can tolerate – so work with a registered dietitian to find what works best for YOU. 


Maltose is another disaccharide, similar to sucrose. Maltose intolerance is similar to sucrose in that it manifests as a result of lacking an enzyme (maltase, in this case) that would normally split the molecule into single monosaccharides which can be successfully processed and digested in the gut. If someone is maltose intolerant, consuming grains and starchy vegetables that contain maltose will present symptoms such as diarrhea, bloating, excess gas, and abdominal pain.  

Some foods high in maltose include:

Maltose is also often used as a food and beverage sweetener that provides additional texture as well as sweetness. Currently, food manufacturers are transitioning to the use of maltose syrup to replace high-fructose corn syrup. 


Histamine is a chemical that affects many of the bodily systems, like the digestive, immune, and neurological processes. Histamine intolerance is a little different than the rest of these other intolerances. It’s not necessarily an intolerance to histamine, but a bodily reaction indicating that there is an excess of it. Histamine is a chemical response from within the body because of an allergic reaction, but it can also be ingested in foods. (6, 7) Symptoms of histamine intolerance are similar to those of traditional allergies and include hives and itching, nasal congestion, headache, lower blood pressure, runny nose, and more. 

Some histamine-rich foods include: 

Some medical conditions like Crohn’s diseasegastroesophageal reflux disease, or an imbalance in the microbiome can lead to a greater risk of histamine intolerance. (6) A histamine intolerance is complex, and it can take a lot of investigation to understand your unique histamine tolerance limit. CDHF recommends working with a registered dietitian experienced in food intolerance to learn about histamine contents of foods and to ensure you are well-nourished.


Tyramine is a naturally occurring amino acid present in many common foods. Tyramine is typically safe to consume, however if you have experienced headaches, or take monoamine oxidase inhibitors (MAOIs) you may have already been told to avoid or limit the amount of Tyramine in your diet. Other symptoms of tyramine intolerance include sweating, hives, lightheadedness, chills, increased heart rate, and high blood pressure. 

Some foods high in tyramine include:

A low tyramine diet can be a simple solution to alleviate this intolerance. Contact your physician or registered dietitian before implementing this. 


Monosodium Glutamate (MSG) is made of water, sodium and glutamate. Glutamate is an amino acid (protein). MSG is added to many foods such as meat, soups, poultry, stews, casseroles, seafood, snacks and vegetable dishes to enhance their natural flavour. It can also be found naturally in foods like corn, green peas, corn, and tomatoes. Some people may have a sensitivity to MSG, and have reported temporary symptoms such as tingling and/or burning sensation, headache, nausea, and pain in the chest. 

Some foods that MSG is commonly added to is:

Some foods that have natural occurring MSG include: 

According to Health Canada, and the World Health Organization (WHO), MSG is not considered a health hazard when consumed in small amounts. Always speak to a registered dietitian if you are concerned about MSG in your diet. (8)

So what do I do next?

Treating a food intolerance involves reducing your intake of problem foods and treating symptoms when you do eat a problem food. Monitoring your symptoms and the foods you eat can help determine what types of foods are causing symptoms. Finding professional support such as a registered dietitian that is knowledgeable in this area can help you manage these conditions over time.


(1) Dalhousie University. 2021. Dalhousie Report Allergies.

(2) Zopf, Y. 2009. The differential diagnosis of food intolerance.

(3) Cleveland Clinic. 2015. Food problems: Is it an allergy or an intolerance.

(4) Roszkowska, A. 2019. ¬Non-celiac gluten sensitivity: A review.

(5) Burkhart, A. n.d. Sucrose intolerance: Sugar and Stomach Problems.

(6) Osborn, C. Low-Histamine Diet.

(7) Anthony, K. 2019. Histamine intolerance.

(8) Dietitians of Canada. 2021. The truth about MSG.

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