Food Intolerance 101


Written by: Heather Gerrish, RDN

Try to remember the last time you had a stomach ache or felt “off “ after eating a certain food. It’s not hard for some of us, and even easier to imagine if you have a food intolerance. There are specific differences between food intolerance and a food allergy. An Intolerance does not have an immune-mediated response, meaning the body does not go into an immune response when exposed to the food, but instead results in discomfort and feeling...well, pretty crummy. Allergies can also be life-threatening and without medical attention after exposure, can further lead to need for medical intervention. Peanut allergy is a common potentially life-threatening allergy that is flagged (even in kindergarten!) to be aware of. . Allergies should not be taken lightly, and respecting those that do have food allergies is the best way to ensure that everyone is happy and healthy and there is no need for an emergency trip to the hospital.

There are actually common allergens that are required to be disclosed in any packaging and food labels - they termed the “Big 8”. These include milk, eggs, fish, shellfish, tree nuts, peanuts, soy (soya) and wheat. These allergens are so common that on the back of nutrition facts labels you can often read below the ingredients list “This food may contain….” or “This food was manufactured in a factory containing….” The next time you have a moment, see if you can find this disclaimer yourself! If you would like to find out more about common food allergies visit: for a great overview.

Let’s move on to intolerances.

Now that we have the distinction of what an allergy is compared to an intolerance, we can dive right in!

An intolerance can be built, brought down and change over time. For example, lactose is a common intolerance. When someone who does not have an allergy to milk consumes a small serving, let’s say a glass of milk - nothing really happens. BUT let’s flip this and say a person who has a mild lactose intolerance consumes the same amount the milk and some intestinal distress might result.

Building from this example, let’s say the individual who does not have any signs of a lactose intolerance decides to stay away from all dairy products for a whole year - that would be tough, no cheese! If this same person consumes a glass amount of milk (the same serving as before) they too might find themselves with acute intestinal distress.

Why does this happen?

Well, lactose has a funny thing about it’s with digestion - it requires an enzyme (produced in our intestines) to be broken down. Lactose is actually two glucose molecules linked together (one molecule of galactose and one molecule of glucose) and they need lactase to break them down - no lactase, no breakdown. When we do not consume dairy products, our body reads that signal as “no need for lactose breakdown, sure - I’ll reduce the production of lactase!” and we run into the same problem as before. Because our body is not producing enough lactase to digest lactose found in dairy products, our body runs into trouble digesting it. And knowing that lactose is a carbohydrate, this then travels undigested through the intestines, attracting water and causing cramping, bloating, gas and other adverse symptoms.

So the next time you feel like a glass of milk, try to remember your current regular intake of lactose and gauge this with your intake - no need for intestinal distress around here!

Other common intolerances include wheat (when there is no immune-mediated response and/or no immune-mediated response to Gluten aka Celiac Disease) and FODMAPs (Fermentable, Oligosaccharides, Disaccharides, and Polyols - read more here).

The best way to determine an intolerance is to speak with a medical professional and monitor your symptoms after eating your hypothesized “trigger” foods. Recording these can be an efficient way to devise a plan of attack to leave you (and your gastrointestinal system) happy.


(Photo): Allergenic Foods and their Allergens, with links to Informall. (n.d.). Retrieved from

Heather Gerrish