Corn (maize) Allergy

Corn (maize) is a plant belonging to the family of the grasses (Poaceae) and is cultivated globally being one of the most important cereal crops world-wide. Maize belongs to the genus Zea. Maize is not only an important human nutrient, but also a basic element of animal feed and a common ingredient for industrial products. Maize is found all over the world, mainly due to its easy processing, ready digestion, and cheaper cost than other cereals. It is also a versatile crop, allowing it to be grown across a range of agro-ecological zones. Today, the major maize production areas are located in temperate regions of the globe. The United States, China, Brazil, and Mexico account for 70% of the global production. In Europe, maize production is concentrated in the Mediterranean area (France, Italy and Spain) and in Eastern Europe (Romania and Hungary). According to FAO data, 589 million tons of maize was produced worldwide in 2000.

The maize seed is called the kernel, which contains starch, protein, oil and other nutritionally valuable substances such as carbohydrate, protein, iron, vitamin B, and minerals. Corn oil contains polyunsaturated fatty acid and vitamin E. Corn allergy is a so-called IgE-mediated food allergy. IgE (Immunoglobulin E) is the allergy antibody. Allergy to corn is caused by proteins in the kernels. Only one of these proteins has firmly been established as an allergen involved in corn allergy. This protein is called the lipid transfer protein (LTP). LTP has first been identified as an important allergen in fruits, but it is also present in nuts, various vegetables and in cereals. LTP is an extremely stable protein. It is resistant to food processing, including heating, but also to gastro-intestinal digestion. These properties make LTP a strong food allergen that can cause severe reactions. Other potential allergens in corn are storage proteins that have also been identified as allergens in other cereals like wheat. Some allergens in corn pollen are also present in the kernel. It is unlikely that these proteins play a role as allergens in food allergy to corn.

Symptoms

Corn allergy can lead to symptoms of the skin like atopic dermatitis or eczema and urticaria or nettle rash. It can also cause swelling of skin, lips or throat (angioedema), itching in mouth and throat, symptoms of the stomach/gut (diarrhoea, nausea, abdominal pain and vomiting), runny or stuffed nose and asthma, and in severe cases anaphylactic shock. All these symptoms have been observed in patients with allergy to corn LTP. Whether other allergens have a more limited pattern of symptoms is not known.

Related foods (cross-reactions)

Corn is related to other cereal grains like wheat, barley, oat, rye and rice. The only well-studied group of corn allergic patients is allergic to corn LTP. Of this group of 29 patients, around 1/3 (n=10) reported allergy to other cereals, the most commonly implicated being rice (8 times) and barley (5 times). This suggests that corn allergy caused by LTP is frequently accompanied by allergy to other cereals because they contain similar LTP. Such reactions based on similarity are called cross-reactions.

Who, when, how long and how often?

Very little is known about the occurrence of corn allergy. There are no studies that have investigated how often corn allergy occurs. The only clear risk factor for corn allergy so far identified is having IgE antibodies to LTP. This is most frequently seen in (young) adult patients with fruit allergy. The most common fruit implicated is peach. Almost all patients with allergy to corn linked to LTP also had peach allergy. Allergy to LTP seems to be life-long, although this has not really been studied.

How much is too much?

For corn, it has not been established how little is enough to trigger an allergic reaction. For those patients that are allergic to corn on the basis of LTP, a minute amount (milligram range) might already be sufficient. Threshold studies are needed to confirm this.

Diagnosis

Diagnosis of corn allergy starts with recording a clear clinical history to establish a link between allergic reactions and corn. Skin prick tests and measurement of specific IgE levels are used to support a history-based suspicion of IgE-mediated corn allergy. Corn shares similar allergens with other cereals but notably also with grass pollen. A positive skin test or serum IgE test for wheat can easily be based on cross-reactivity to grass pollen. Positive test results based on similarity between allergens from grass pollen and cereal proteins is a frequent cause of false-positive diagnoses. Corn also shares LTP with many plant-derived foods including fruits, nuts and vegetables. On the basis of similarity, IgE antibodies can cross-react between different LTPs. Although LTP is a potentially severe allergen, not every (cross-reactive) IgE response to corn LTP is of clinical relevance. To distinguish whether cross-reactions have clinical relevance, the only definitive method is a so-called double-blind placebo-controlled food challenge. In this procedure, increasing doses of corn are administered to the patient as well as placebo meals not containing corn. Both patient and doctor are unaware of the meals with and without corn. Effective blinding of the taste of corn is essential for such challenge procedures.

Where do I find corn?

Whole kernels of corn are easily recognisable and are eaten separately, or in mixtures of (canned) vegetables, on pizza’s etc. Corn derived ingredients are however found in a broad variety of processed foods. About 15% of corn harvested is transformed into derivatives, like alcohol, oil, starch, corn syrup and dextrose. Whether corn-derived alcohol or corn oil contains enough protein to induce allergic reactions depends on the level of purity and the sensitivity of the individual patient. Corn oil is commonly used as salad or frying oil and in margarine. Cornstarch is primarily used to thicken and stabilize other ingredients in foods such as baking powder, prepared mixes, candies, baking goods, and puddings. Corn syrup is mainly used to replace sugar in confectionery, but also in bakery and dairy products, sweet beverages, candies, ketchup, pickles and other condiments. High fructose corn syrup (HFCS) and dextrose are used in confectionery, baked foods, table syrup, sweet beverages, ketchup, pickles and other condiments. The grits fractions are used in brewing processes, to produce corn flakes or are directly consumed boiled. Corn meal is a long shelf-life product used to produce a range of chemically leavened baked products, mainly bread; it is also used to make snack foods (corn chips and taco’s) and ready-to-eat breakfast cereals. In Italy it is used for making a dish called polenta.

Non-food products

Corn oil is sometimes used in tooth paste. The likelihood of an allergic reaction depends on the level of purity (absence of protein) and the sensitivity of the patient. Corn syrup is often used as texturizer and carrying agent in cosmetics. Adhesives for envelopes, stamps and stickers might contain corn-starch. Medication can also be a source of hidden corn.

Avoidance

For the treatment of food allergy, at present avoidance is the only solution. Since corn is used in many common food products, also as an “invisible” ingredient, avoidance is difficult. Indications on food labels that point towards (possible) use of corn as an ingredient include baking powder, caramel colour, confectioners sugar, dextrin, dextrose, fructose, hominy, invert sugar, invert syrup, lactic acid, malto-dextrins, manitol, sorbitol, starch, vanilla extract and descriptions like vegetable broth and vegetable starch.

Fortunately, according to recent new legislation (EU Labelling Directive 2003/89/EG and list of Codex Alimentarius Commission on mandatory labelling of pre-packaged food) all products containing corn-derived ingredients should be labelled as such.