Why are food allergies on the rise?

Food allergies are on a steady rise, especially in the last two decades – an alarming 8 percent of kids in the US report such reactions, a third of these are severe allergic reactions.

Why would the immune system react in nasty (rashes, diarrhea, vomiting, abdominal pain) and life threatening (anaphylaxis: blood pressure drop, tightening of the airways) ways to something as peaceful and necessary as food? Why would the immune system mistake food proteins, such as tree nuts, peanuts, eggs, shellfish and soy as enemies?

You’ve probably heard of the hygiene hypothesis. Lower incidence of microbial infections in developed countries, this theory proposes, may have shifted the immune system’s attention to other targets, leading to the rise in both autoimmune and allergic diseases. 

In a new paper in the journal Cell, Yale immunologists suggest several other environmental culprits that may have triggered this crippling hypersensitivity to food.

Our “quality control” system

Natural foods contain thousands of chemicals, some of them are nutrients: fats, proteins, carbohydrates, vitamins and minerals. Some are not nutrients: they include stuff that we can’t absorb and sometimes also all kinds of toxic molecules. Our body has a way of sensing and detecting these chemicals all along our digestive tract, from nose and mouth, throughout the gut. It senses both the quality and the quantity of these substances, and is very sensitive to harmful substances. An animal’s gut “knows”; it calculates a food’s benefits in energy and nutrients against its determinants in hazardous substances, and has a mechanism to elicit reactions to it that are both learned through experience and automatic and hard-wired.

Our body’s quality control system, the authors explain, has two lines of defense in dealing with suspect food: avoiding ingestion of that food through food aversion, and minimizing its absorption.

Sensory control: why we avoid certain tastes

The gut doesn’t sense everything. It senses some but not all chemicals. Some foods may not have an unpleasant taste or smell, but after they elicit an adverse reaction the eater learns to avoid that specific taste in the future. After a negative experience we may lump together all foods that have that specific taste profile and avoid all things that taste like that, even though other foods with that taste pose no danger at all. 

Our sense of smell and taste err on the side of caution, and we may become averse to anything that reminds our senses of an offensive food. An extreme example of that may sound familiar: some women report that they first realized they were pregnant when they became acutely sensitive to the smell and taste of some foods, and were suddenly turned off by foods they previously loved, such as soft cheeses and coffee – this over-caution may to the benefit of the developing fetus.

Secondary defense: minimizing absorption

If the food’s already in the digestive system the gut can still block its entry into our cells. This may involve increasing the amount of mucus in our gut, the secretions acting as a barrier to absorption, hence the runny nose, sneezing and coughing; it also intensifies the movement of the bowel, which makes the food’s passage in the gut shorter, decreasing absorption, and making us rush to the bathroom in discomfort. 

Our body rejects food through nausea, vomiting, diarrhea, and malabsorption.

Food allergy as quality control gone haywire

Is food allergy an exaggeration of these beneficial defense mechanisms, designed to protect us from food poisoning? 

The authors postulate that in order for a food protein to elicit an allergic reaction, that protein – which is a beneficial food – has to be associated with a toxic substance. The protein and the poison can be associated by just happening to be in the gut at the same time by pure chance, or by being tied together physically. Either way, the body learns to attack the protein as if it were the toxic component of the pair. The gut, which is equipped with protections against the poison, recognizes the innocent protein as an enemy and fires in full force. 

And since this system is designed to minimize the risk from poison, it’s prone to err on the side of over-protection and caution, causing food aversions and allergic reactions that exclude perfectly good foods.

The changed environment

So what’s happening here, why are food allergies on the rise?

“One such factor could be the increasing reliance on processed foods depleted from natural compounds,” the authors explain. 

Edible plant foods naturally contain thousands of substances, among them are compounds, such as alkaloids, terpenoids, phenolics and glycosides, that are designed to protect the plant itself from munching animals, microbes and from the elements. Animal food products also contain such compounds. It’s the quantity of such compounds and the unique way they’re presented to the gut that determine how our body reacts to them. The right amount preconditions animals to accept the substance, and with it the whole food that contains it.

Large amounts and, surprisingly, amounts that are too low, cause the body to mark the food as enemy – exposure that's too small fails to desensitize the gut to the toxin.

“Another factor is the increasing consumption of artificial chemicals (food preservatives, dishwasher detergents, etc.) that we have not evolved to handle with natural defenses,” the authors continue. These chemicals that we have not evolved with and that are new to human consumption may lead to dysregulated allergic reactions.

In order to prevent and treat food allergies we need to understand the underlying biology of such disorders, and what’s quite clear is that our food environment has indeed changed dramatically. And while the hygiene hypothesis, even if completely proven, won’t convince us to revert to the days of parasite-infested food and contaminated water, the highly processed food diet is one that we should reject for many other reasons – its link to a multitude of chronic diseases is clear.

Dr. Ayala