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Soy allergy is a type of food allergy. It is a hypersensitivity to dietary substances from soy, causing an overreaction of the immune system which may lead to severe physical symptoms for millions of people. The Asthma and Allergy Foundation of America estimates soy is among the eight most common food allergens for pediatric and adult food allergy patients. It is usually treated with an exclusion diet and vigilant avoidance of foods that may be contaminated with soy ingredients. The most severe food allergy reaction is called anaphylaxis and is a medical emergency requiring immediate attention and treatment with epinephrine.
Signs and symptoms
Food allergies can have fast onset (from seconds to one hour) or slow onset (from hours to several days) depending on mechanism. Symptoms may include: rash, hives, itching of mouth, lips, tongue, throat, eyes, skin, or other areas, swelling of lips, tongue, eyelids, or the whole face, difficulty swallowing, runny or congested nose, hoarse voice, wheezing, shortness of breath, diarrhea, abdominal pain, lightheadedness, fainting, nausea and vomiting. Symptoms of allergies vary from person to person and may vary from incident to incident. Serious danger regarding allergies can begin when the respiratory tract or blood circulation is affected. The former can be indicated by wheezing, a blocked airway and cyanosis, the latter by weak pulse, pale skin, and fainting. When these symptoms occur the allergic reaction is called anaphylaxis. Anaphylaxis occurs when IgE antibodies are involved, and areas of the body that are not in direct contact with the food become affected and show severe symptoms. Untreated, this can proceed to vasodilation, a low blood pressure situation called anaphylactic shock, and death (very rare).
Non-IgE mediated reactions are slower to appear, and tend to manifest as gastrointestinal symptoms, without cutaneous or respiratory symptoms. Within non-IgE reactions, clinicians distinguish among food protein-induced enterocolitis syndrome (FPIES), food protein-induced allergic proctocolitis (FPIAP) and food protein-induced enteropathy (FPE). Common trigger foods for all are soy infant formula, and also cow's milk formula. FPIAP is considered to be at the milder end of the spectrum, and is characterized by intermittent bloody stools. FPE is identified by chronic diarrhea which will resolve when the offending food is removed from the infant's diet. FPIES can be severe, characterized by persistent vomiting 1–4 hours after an allergen-containing food, to the point of lethargy. Watery and sometimes bloody diarrhea can develop 5–10 hours after the triggering meal, to the point of dehydration and low blood pressure. Infants reacting to soy formula may also react to cow's milk formula. International consensus guidelines have been established for the diagnosis and treatment of FPIES.
Sources of soy protein
Many fast-food restaurants commonly use soy protein in hamburger buns (soy flour), hamburger meat (soy protein) and hydrolyzed vegetable protein (HVP) in sauces. On their respective websites, McDonald's and Burger King list soy flour as an ingredient in their hamburger buns. Multi-grain breads, doughnuts, doughnut mix and pancake mix commonly contain soy flour. Ice cream can also contain soy ingredients. Canned tuna may contain vegetable broth which contains soy protein. Some food contains soy-based ingredients that are not considered allergens under national regulations, and thus not labeled, for example, foods cooked in highly refined soy oil, which is considered safe due to absence of soy protein.
Products containing soy protein include:
- shoyu sauce
- soy (soy albumin, soy fiber, soy flour, soy grits, soy milk, soy nuts, soy sprouts)
- soybean (curd, granules)
- soybean butter
- soy protein (concentrate, isolate)
- soy milk
- soy sauce, tamari
- textured vegetable protein (TVP)
- hydrolyzed vegetable protein (HVP)
The following food additives may contain soy protein:
- flavoring (including natural and artificial)
- prepared broths, including chicken broth, vegetable broth, and bouillon cubes
Non-allergenic soy derivatives
Highly refined soybean oil contains very little amounts of soy proteins, and is usually safe for people with soy allergy. They are exempt from being labelled as a major allergen under US FDA regulations. Note that soybean oils extracted using mechanical means (pressed or extruded) should be avoided.
Soy lecithin, usually derived from soybean oil, likewise presents minimal amounts of soy protein. However, the US FDA only exempts a few soy lecithin products from its mandatory allergenic source labeling requirements.
Cross-reactivity with dairy
Infants - either still 100% breastfeeding or partially/entirely on infant formula - and also young children - may be prone to a combined cow's milk and soy protein allergy referred to as "milk soy protein intolerance" (MSPI). A US state government website presents the concept, including a recommendation that nursing mothers discontinue eating any foods that contain dairy or soy ingredients. In opposition to this recommendation, a published scientific review stated that there was not yet sufficient evidence in the human trial literature to conclude that maternal dietary food avoidance during lactation would prevent or treat allergic symptoms in breastfed infants.
A review presented information on soy allergy, milk allergy and cross reactivity between the two. Milk allergy was described as occurring in 2.2% to 2.8% of infants and declining with age. Soy allergy was described as occurring in zero to 0.7% of young children. According to several studies cited in the review, between 10% and 14% of infants and young children with confirmed cow's milk allergy were determined to also be sensitized to soy and in some instances have a clinical reaction after consuming a soy-containing food. The research did not address whether the cause was two separate allergies or a cross-reaction due to a similarity in protein structure, as which occurs for cow's milk and goat's milk. Recommendations are that infants diagnosed as allergic to cow's milk infant formula be switched to an extensively hydrolyzed protein formula rather than a soy whole protein formula.
Many people with soy allergy can tolerate small or moderate amounts of soy protein: the typical dose needed to induce an allergic response is about 100 times higher than for many other food allergens.
Conditions caused by food allergies are classified into three groups according to the mechanism of the allergic response:
- IgE-mediated (classic) – the most common type, manifesting acute changes that occur shortly after eating, and may progress to anaphylaxis
- Non-IgE mediated – characterized by an immune response not involving immunoglobulin E; may occur hours to days after eating, complicating diagnosis
- IgE and non-IgE-mediated – a hybrid of the above two types
Allergic reactions are hyperactive responses of the immune system to generally innocuous substances, such as proteins in the foods we eat. Why some proteins trigger allergic reactions while others do is not entirely clear, although in part thought to be due to resistance to digestion. Because of this, intact or largely intact proteins reach the small intestine, which has a large presence of white blood cells involved in immune reactions. The heat of cooking can help make protein molecules less allergenic.
In the early stages of acute allergic reaction, lymphocytes previously sensitized to a specific protein or protein fraction react by quickly producing a particular type of antibody known as secreted IgE (sIgE), which circulates in the blood and binds to IgE-specific receptors on the surface of other kinds of immune cells called mast cells and basophils. Activated mast cells and basophils undergo a process called degranulation, during which they release histamine and other inflammatory chemical mediators into the surrounding tissue causing effects, such as vasodilation, mucous secretion, nerve stimulation, and smooth-muscle contraction. This results in runny nose, itchiness, shortness of breath, and potentially anaphylaxis. Depending on the individual, the allergen, and the mode of introduction, the symptoms can be system-wide (classical anaphylaxis), or localized to the respiratory system (asthma) or skin (eczema). After the chemical mediators of the acute response subside, typically 2–24 hours after the original reaction, late-phase responses referred to as non-IgE mediated can occur due to the migration of other types of white blood cells to the initial reaction sites.
Allergenic proteins from soy (Glycine max) are named under a nomenclature decided by IUIC, which is also responsible for numbering many of the proteins. Proteins numbered by IUIC include:
- Gly m 1, a hydrophobic protein
- Gly m 2, defensin
- Gly m 3, profilin
- Gly m 4, PR-10
- Gly m 5, vicilin, a cupin
- Gly m 6, legumin, a cupin
- Gly m 7, seed biotinylated protein
- Gly m 8, 2S albumin
These proteins are recognized by the immune system as antigens in susceptible individuals. As many as 8 other soy allergenic proteins has been reported in literature. Peanut, a legume, shares many similar proteins with soy, so individuals with soy allergy may have peanut allergy too.
Diagnosis of soy allergy is based on the person's history of allergic reactions, skin prick test (SPT), patch test and measurement of soy protein specific serum immunoglobulin E (IgE or sIgE). A negative IgE test does not rule out non-IgE mediated allergy, also described as cell-mediated allergy. SPT and sIgE have sensitivities of 55% and 83% respectively, and specificities of 68% and 38%. These numbers mean that either test may miss diagnosing an existing soy allergy, and that both can also be positive for other food allergens. Confirmation is by double-blind, placebo-controlled food challenges, conducted by an allergy specialist.
Treatment for accidental ingestion of soy products by allergic individuals varies depending on the sensitivity of the person. An antihistamine such as diphenhydramine (Benadryl) may be prescribed. Sometimes prednisone will be prescribed to prevent a possible late phase Type I hypersensitivity reaction. Severe allergic reactions (anaphalaxis) may require treatment with an epinephrine pen, i.e., an injection device designed to be used by a non-healthcare professional when emergency treatment is warranted. A second dose is needed in 16-35% of episodes.
Society and culture
Whether food allergy prevalence is increasing or not, food allergy awareness has definitely increased, with impacts on the quality of life for children, their parents and their immediate caregivers. In the United States, the Food Allergen Labeling and Consumer Protection Act of 2004 causes people to be reminded of allergy problems every time they handle a food package, and restaurants have added allergen warnings to menus. Soy is one of the eight foods with mandatory labeling required. The Culinary Institute of America, a premier school for chef training, has courses in allergen-free cooking and a separate teaching kitchen. School systems have protocols about what foods can be brought into the school. Despite all these precautions, people with serious allergies are aware that accidental exposure can easily occur at other peoples' houses, at school or in restaurants. Food fear has a significant impact on quality of life. Finally, for children with allergies, their quality of life is also affected by actions of their peers. There is an increased occurrence of bullying, which can include threats or acts of deliberately being touched with foods they need to avoid, also having their allergen-free food deliberately contaminated.
Genetically modified food
There are concerns that genetically modified foods, also described as foods sourced from genetically modified organisms (GMO), could be responsible for allergic reactions, and that the widespread acceptance of GMO foods may be responsible for what is a real or perceived increase in the percentage of people with allergies. One concern is that genetic engineering could make an allergy-provoking food more allergic, meaning that smaller portions would suffice to set off a reaction. Of the food currently in widespread GMO use, only soy is identified as a common allergen. However, for the soybean proteins known to trigger allergic reactions, there is more variation from strain to strain than between those and the GMO varieties. The same review quoted a 2016 U.S. National Academy of Sciences report that concluded, “The committee did not find a relationship between consumption of GE foods and the increase in prevalence of food allergies."
A second concern is that genes transferred from on species to another could introduce an allergen in a food not thought of as particularly allergenic. Research on an attempt to enhance the quality of soybean protein by adding genes from Brazil nuts was terminated when human volunteers known to have tree nut allergy reacted to the modified soybeans. At present, prior to a new GMO food receiving government approval, certain criteria need to be met. These include: Is the donor species known to be allergenic? Does the amino acid sequence of the transferred proteins resemble the sequence of known allergenic proteins? Will the transferred proteins be in the part of the plant to be eaten (as opposed to non-consumed roots, stems or leaves)? Are the transferred proteins resistant to digestion - a trait shared by many allergenic proteins? Finally, there are requirements in some countries and recommendations in others that all foods containing GMO ingredients be so labeled, and that there be a post-launch monitoring system to report adverse effects (much there exists in some countries for drug and dietary supplement reporting). According to a 2015 report from the Center for Food Safety, 64 countries require labeling of GMO products in the marketplace.
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