Food allergy is an immune system response in which the body mounts an attack on food proteins it mistakes for being harmful. In a process called "sensitization," immune cells generate antibodies specific to that food protein (called IgE), so they'll recognize the offending food the next time it comes calling.
When it does, IgE antibodies trigger a cascade of chemical reactions throughout the body that are experienced as allergic symptoms; these range from mild to life threatening. Allergic reactions can occur in the skin (rash, hives, itching); in the digestive tract (vomiting, diarrhea, cramping); and/or in the respiratory tract (sneezing, congestion, itchy mouth or inner ear).
The most serious allergic reaction is called anaphylaxis: It's an acute condition in which a person has difficulty breathing due to severe throat swelling and/or a life-threatening drop in blood pressure.
Chances are, you know someone who's allergic to milk, egg, peanuts, tree nuts, fish, shellfish, wheat or soy. These eight foods account for 90 percent of all food allergies in this country. But 10 percent of allergies are attributable to other foods - some of which are so low in protein that it's hard to imagine they could provoke a reaction. In some rare cases, the circumstances under which a food is consumed can determine what type of reaction takes place.
Indeed, the allergies that make up this minority often defy logic or belief. If you think you've heard it all when it comes to food allergies, read on:
-- Fruit: During spring and fall allergy seasons, I see a spike in patients with Oral Allergy Syndrome (OAS) - localized allergic reactions to fruits, veggies and nuts botanically similar to environmental allergens from birch, ragweed or grass pollen. In OAS, seemingly innocent fruits and veggies like kiwi, apples, bananas, celery or carrots can "cross-react" with pollens due to structural similarities between their proteins. In other words, the immune system is tricked into thinking the fruit or veggie is, in fact, the pollen, and it mounts an allergic response. Different fruits and veggies cross-react with different environmental allergens.
OAS reactions can be more severe when the burden of environmental allergens is high. For the majority of sufferers, it results in an uncomfortable but generally benign itchiness in the mouth, nose, eyes or throat. It's unusual, but not impossible, for the reaction to be more severe or even life-threatening.
In many - but not all - cases, cooking the offending fruit or vegetable changes the offending protein's structure enough to deactivate its allergenic potential. For these people, baked apples or applesauce would be well tolerated, but raw apples wouldn't; similarly, poached pears, peach pie or cooked carrot soup may be fine even if their raw ingredients weren't.
-- Red meat: For years, the scientific literature contained isolated case reports of mystery anaphylaxis to which a specific food trigger could not be traced. Often, these puzzling episodes were noted to occur a full three to six hours after the person consumed a meal - far longer than the typical 30 minutes or less in which an anaphylactic food reaction generally occurs.
Recently, epidemiologists appear to have solved the mystery and, in so doing, identified one of the strangest food allergies to date: red meat allergy. The cause? Bites from a critter called the Lone Star tick.
In the case of red meat allergy, the reaction is not actually to meat protein itself but rather to a carbohydrate nicknamed "alpha-Gal" that's attached to the red meat's protein or fat. As it turns out, Lone Star tick bites can sensitize people to the IgE antibody for this very same carbohydrate by the same mechanism responsible for food allergy.
When the offending carbohydrate shows up again - upon eating meat, in this case - the body launches a full-scale allergic attack. The reaction appears to take place in response to mammalian meat, which would include beef, pork and lamb (and presumably game meats like venison as well). Protein from non-mammals like chicken, turkey and fish should not cause a reaction.
The Lone Star tick is endemic to the Southeastern United States and, as a result, case reports of red meat allergy tend to cluster there as well.
-- Food + a co-factor: In a rare phenomenon called "Food and Exercise Induced Anaphylaxis" (FEIA), an anaphylactic reaction to a food allergen only takes place if that food was consumed within about two hours of exercise. Other so-called "co-factors" to anaphylaxis have been documented, including non-steroidal anti-inflammatory drugs (NSAIDs) and alcohol.
The scientific literature documents multiple case reports of FEIA, most of which appear associated with allergies to plant-based foods like fruits, vegetables or grains. There are reports of patients with OAS who experience mild itchy mouth when consuming their forbidden fruits in isolation but full-blown anaphylaxis when eating it right before exercise.
Wheat appears to be a particularly common trigger of this uncommon reaction. Allergenic foods consumed in close proximity to taking aspirin or other NSAIDs (e.g., ibuprofen) may also be more likely to induce an anaphylactic reaction in some people, both with or without exercise.
It's unclear why exercise should have an amplifying effect on allergic reactions in an unlucky few, but scientists have offered several possible explanations. Some have postulated that exercise increases allergen absorption from the GI tract.
Recently, researchers in Canada have offered a different hypothesis: that exercise reduces the secretion of stomach acid. Since stomach acid alters the signature shape and configuration of a food protein before it reaches the intestines, they posit that it may reduce the likelihood that intact allergenic proteins come into contact with the immune cells that line the gut walls. In the absence of sufficient stomach acid, more such proteins may be available to provoke a reaction. More research, however, is needed to understand the precise mechanism.
Unusual as they may be, these food allergies are every bit as real - and every bit as serious - as more mainstream ones. If you experience unusual allergic-type symptoms in connection to mealtime, consult an allergist for testing. And if you've been diagnosed with a food allergy and prescribed an epiPen for use in emergencies, be sure you know how to use it - and carry it with you everywhere. Mild past reactions are no guarantee of mild reactions in the future.
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Tamara Duker Freuman, MS, RD, CDN, is a NYC-based registered dietitian whose clinical practice specializes in digestive disorders, Celiac Disease, and food intolerances. Her personal blog, www.tamaraduker.com, focuses on healthy eating and gluten-free living.