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Digestion: An Intolerance vs. Allergy


Digestion. It’s the process by which foods are broken down into component molecules, mechanically and chemically speaking.

When it goes wrong, the symptoms can be expressed in a variety of ways, but essentially boil down to two things: intolerance and allergies.

Both are inconvenient, frustrating, and pass on as much judgment and weird looks as, I don’t know, someone wearing fur in a vegan restaurant. (For the record, it was faux fur…but I suppose my server didn’t realize that.)

Here’s a quick breakdown of what separates the two:

Food intolerance:
Gastrointestinal discomfort caused by a sensitivity to certain foods. An intolerance is NOT a result of an immune system reaction. An intolerance is synonymous with a sensitivity.

Food allergy:
An allergic reaction to food, caused by a reaction of the immune system.


This poses another question – what causes an immune system reaction?

An immune system reaction is when immune cells release chemicals that cause either limited (localized) or systemic (whole-body) inflammation.

Said another way, the routes of an intolerance versus a food allergy are different. Below is a quick breakdown of the separate routes, but before we get there, this is the underlying mission/why any of this happens:

Food. It contains energy and nutrition for your body to maintain and perform. To convert food into usable energy, food passes through three bodily stages:
(1) digestion, (2) absorption, (3) elimination.

(1) Digestion breaks food into molecules small enough to be transported into enterocytes (absorptive cells in the villi of the small intestine).

(2) Absorption is the process of taking food molecules out of the GI tract (mouth, esophagus, stomach, small intestine, and large intestine) and into the circulation system. A majority of absorption takes place in the small intestine.

(3) Undigested food and waste products are then removed from the body via elimination.


With that said,

In the case of a food intolerance:

1. Food molecules have passed through the mouth, esophagus, and stomach, and now enter the small intestine.

2. Cells lining the small intestine typically produce an enzyme in response to the food molecule’s encounter. If a certain enzyme is not present, the undigested food molecule passes into the large intestine.

3. Bacteria housed in the large intestine ferment the food molecules for their energy/survival purposes, producing excess* gas and acid. Acid production draws water into the bowel, causing the intestines to expand (leading to discomfort via bloating, cramping, constipation, and diarrhea).

*Note: Feeding bacteria in your large intestine, specifically the “good” kind is essential for optimal gut health. In addition to probiotics, prebiotics (such as fiber) are encouraged. The key word to an intolerance is excess* gas and acid. This (feeding bacteria, gas, acid) is a normal process, but when overdone (excess), is deemed an intolerance. How will you know its excess? Answer: Bloating, cramping, constipation, and diarrhea.

In the case of a food allergy:

1. Food molecules might not get far at all, depending on the severity of the allergy. The proteins in a food allergen stimulate immune cells throughout the body to release inflammatory chemicals – reactions being either localized or systemic.

2. Localized inflammation may include itchiness in the throat, swelling of the face or mouth, skin rashes, vomiting, and wheezing.

3. Widespread/systemic inflammation can constrict airways (leading to respiratory collapse), and cause blood vessels to dilate, thus plummeting blood pressure levels (leading to circulatory collapse).

4. Food allergies can also localize to the intestines and cause IBS symptoms (like those of food intolerances). However, in an allergic reaction, the body’s immune system response causes damage to the lining of the small intestine (specifically the villi, or fingerlike projections along the small intestine’s wall).*

*Note: This is especially harmful because a food allergy response in the small intestine inhibits the absorption of nutrients by preventing or destroying access to enterocytes (absorptive cells in the villi of the small intestine). It can do this by flattening the finger-like villi. For this reason, a gluten intolerance versus a gluten allergy (Celiac disease) is less severe, but nevertheless, important to not ignore.

Commonality: IBS.

Food intolerances are sometimes associated with Irritable Bowel Syndrome (IBS).

IBS is a long-term condition that can cause stomach upset by way of cramps, bloating, diarrhea, and constipation. Though poorly understood, IBS seems to be triggered by stress, lifestyle, and certain types of foods. IBS is one of the most common disorders, affecting as many as 1 in 7 people (Monash University). While IBS does not discriminate, it is typically more common in females than males.

IBS is also associated with food allergies, meaning the presence of IBS is not unique to only those with food intolerances.

About food allergies.

Food allergies are less common than food intolerances and can be far more serious. According to 2012 stats reported by the FDA, about 30,000 consumers require emergency room treatment, and 150 Americans die each year because of allergic reactions to foods.

In the USA, there are eight (8) allergenic foods (requiring labeling) – selected from the more than 160 identified food allergies. The identified eight are highlighted because they account for over 90% of all documented food allergies in the USA:

1. Wheat
2. Peanuts
3. Eggs
4. Milk
5. Soy
6. Fish
7. Shellfish
8. Tree nuts

In short, food allergies are more severe than food intolerance, though neither should be ignored. Food allergies are deemed more severe for the reasons they can (1) lead to death immediately following consumption, and long-term (if left untreated, or ignored), (2) inhibit the absorption of nutrients by damaging or preventing access to the absorptive cells of the small intestine (leading to nutritional deficiencies, which spiral into other, worse things).

Action items.

The descriptors "good" and "bad" for foods is seldom tolerated by science - the role of dietitians is to instead to teach consumers about the makeup of food and potential affects on the body, allowing them to make educated decisions when it comes to purchasing and consuming food.

Treatment is not a one-size-fits-all approach. In addition to visiting a primary care doctor or gastroenterologist, a registered dietitian can help create an individualized treatment plan, targeting both prevention of and alleviation of symptoms. Each body and digestive tract is unique!​

To test for an intolerance,