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  • Writer's pictureMarianne Trevorrow ND

Irritable bowel syndrome, Food Sensitivity Tests (FST) and where to start.

Updated: May 31, 2022

(Updated June 2022)

As a naturopathic doctor, I frequently see patients with irritable bowel syndrome or IBS. Many of these patients have tried various types of diets; dairy free, grain free/keto, elimination or various others, and have had a variety of diagnostic tests, often with no clear answer. These patients then end up wondering what they can do to fix the problem, and frustrated with all the time, energy and money they may have spent with little to no result.

In reality, most food adverse reactions that underpin IBS are not allergies, even though we might think of them that way. To explain, classic food allergy has very distinct symptoms—an immediate, severe type of response that can include hives, shortness of breath and/or fainting. Without immediate treatment, it can be very dangerous, even fatal. Thankfully, however, true food allergies are not that common—recent guidelines have placed the incidence at only 2% of adults and 8% of children in the developed world.

On the other hand, food sensitivities or intolerances are much less severe, but also much more frequent. They also show up in many different ways; for example, they might appear within 30 minutes to an hour after eating, or may take several hours or even days to appear. Some of these food-related symptoms may mimic conditions more commonly associated with environmental allergies; sinus congestion, headaches, eczema or asthma. Or they might be purely digestive: patients may complain of bloating, abdominal cramps, constipation or diarrhea. A common characteristic of food sensitivities is that symptoms may be triggered by a food at one occasion, but not on another.

Research tells us that most adverse reactions to food are of mixed antibody type, or a combination of sensitivity and intolerance, but usually somewhere along a spectrum of both. Usually food triggers are a commonly consumed part of the diet; one or a combination of gluten, egg, dairy, soy, corn or pea protein. The initial triggering event may occur following a gastrointestinal (GI) or other infection, a course of antibiotics, or by processes we don’t yet understand. Microscopically, what is happening is that the digestive immune system begins to recognize these foods as 'foreign' bodies, so the system loses tolerance to that food as something to be digested and assimilated into the bloodstream. Once this process begins in the gut immune system, constituent amino acids (the smallest breakdown products of proteins) are then ‘tagged’ with antibodies, creating antibody/antigen complexes. In small quantities, these complexes have no health effects but when deposited into small blood vessels in the gut, lungs, nasal passages or other areas of the body, can potentially cause or increase the symptoms listed above.

With food intolerances, the problem is different; in this case, the person lacks an enzyme in their digestive tract border that can break down the larger food particles into molecules that can be absorbed into the body. This occurs before the food can even be absorbed. The result is that on all or almost all occasions that food is eaten, the patient will experience gas, bloating, reflux, GI pain or similar symptoms. Lactose or gluten intolerance fall into this category.

Several studies have actually shown a poor correlation between FST IgG testing and clinical symptoms, whether they are true sensitivities or not, which is why I no longer recommend FST testing in my practice, and instead start with modified FODMAP diets for IBS (with additional restrictions of dairy and egg proteins, which are known to be inflammatory proteins). I work with plant-based dieticians or nutritionists to help people implement FODMAP elimination and reintroduction phases, so that the process is as stress free as we can make it, and that re-introduction goes as smoothly as possible.

Please note however, that reintroduction is not appropriate for known food allergies; for those cases, I recommend patients work with an Allergist (MD/DO specialty) if they want to evaluate whether reintroduction is possible for them due to the possibility of severe, life threatening reactions.

With more benign adverse food reactions, an important point to remember is that eliminating foods by itself rarely solves problems like IBS. The reason for this is that many food reactions are in fact a messenger for the breakdown of proper immune balance and tolerance, and of the orderly digestion of food and elimination of food waste. As research is increasingly telling us, the real problem is a combination of inflammation, loss of tolerance, and loss of microbial diversity to prime the gut mucosal immune barrier and enable good digestion.

As I guide patients along the journey to managing and healing IBS, we discuss the importance of supporting hormonal and microbial health in addition to what should be done with nutrition. To truly solve the problem, we need to decipher why the gut is treating harmless foods like bacteria or viruses, at the point at which the immune system begins to overreact. We certainly explore whether the loss of microbial diversity or physical stressors are contributing to this process. However, we are also learning that simply adding probiotics to the GI tract via supplements only partially solves the problem, similar to what often happens when patients follow one particular type of diet (e.g. FODMAP-elimination phase). To replete microbial diversity, diet therapy with a variety of high fibre foods (primarily fruits, vegetables and legumes) is emerging as a more sustainable and effective strategy than simply eliminating foods long term.

Additionally, figuring out where lifestyle factors such as stress, sleep and exercise are making things better or worse with IBS is crucial to the healing process. IBS can be a condition of improvements and relapses. Still, there is hope for healing IBS with a multi-pronged strategy and good, long term team-based support with NDs and nutrition professionals working together.

A previous version of this article appeared in Life As A Human E-Magazine: August 2014.

For more information:

1. Blaser, M. Missing Microbes: how the overuse of antibiotics is fueling our modern plagues. Harper Collins, 2014.

2. Waltner-Toews D. Food, Sex and Salmonella: Why Our Food Is Making Us Sick. Greystone Books, 2008.

3. Zutavern A et al. Timing of Solid Food Introduction in Relation to Eczema, Asthma, Allergic Rhinitis, and Food and Inhalant Sensitization at the Age of 6 Years: Results From the Prospective Birth Cohort Study LISA Pediatrics Vol 121, no1, Jan 2008.


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