Wheat 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. Wheat belongs to the genus Triticum. There are different species of wheat; the most widely cultivated are common wheat (T. aestivum) and so-called durum (T. durum), the former accounts for approximately 95% of world production and the latter for approximately 5%. Wheat provides over 20% of the calories for the world population and in 1997/98 world wheat production totalled 579 million tons; in 2000 it was approximately 571 million tons; about 35% of the world’s wheat is produced in Asia, 17% in Europe and 16% in the United States. It is estimated that nearly two-thirds of the wheat is used for human nutrition and a little less than one-third is used livestock feed and non-food applications. The majority of the wheat produced is consumed in the country in which it is grown.
The wheat kernel is the seed of the wheat plant. Each tiny seed has three parts: the endosperm, the bran, and the germ. The endosperm is the largest part of the kernel and is where the starch is stored and where the largest amount of flour comes from. It also contains protein, iron, and B-complex vitamins. The wheat bran is the outer coat of the kernel and contains mostly fibre, but also contains protein and B-complex vitamin. It is only present in whole-wheat flour.
Adverse reactions to wheat may be explained by different mechanisms. The most common hypersensitivity to wheat is caused by the gluten-fraction of wheat and some other cereals. Gliadins in gluten play an important role. This hypersensitivity is called celiac disease and is characterized by chronic diarrhoea, abdominal bloating, pain, weight loss, iron deficiency and evidence of nutrient malabsorption. At this place, however, we only deal with the less common form of hypersensitivity to wheat, i.e. IgE-mediated or type I wheat allergy. Immunoglobulin E (IgE) is the allergy antibody. Wheat proteins of the kernel are responsible for IgE-mediated food allergic reactions. Several proteins have been identified as allergens amongst which is also gliadin, albumins and globulins but very little is known about their relevance. Some wheat allergens are the same proteins as allergens in grass pollen. Their relevance as food allergens is unlikely.
Wheat allergy often leads to symptoms of the skin (atopic dermatitis or eczema and urticaria or nettle rash). It can also cause swelling of skin, lips or throat (angioedema), symptoms of the stomach/gut (diarrhoea, nausea, abdominal pain and vomiting), runny or stuffed nose and asthma, and in severe cases anaphylactic shock.
Exercise-induced wheat allergy (WDEIA) occurs within minutes to hours after ingestion of wheat. The duration of exercise needed to spark off a reaction is between 10 and 60 minutes. The symptoms typically range from nettle rash (urticaria) and swelling of the skin (angioedema) to shortness of breath (dyspnea), drop in blood pressure, collapse and shock.
Occupational allergy to wheat flour in bakers is usually limited to asthma.
Related foods (cross-reactions)
Wheat is related to other cereal grains like barley, rye, corn (maize) and rice. Very little is known about the risk that these related cereals cause allergy in patients with wheat allergy. Corn and rice are usually well-tolerated by wheat-allergic patients. Barley, however, has been reported to cause symptoms. Similar proteins are present in all cereals. Theoretically wheat allergic patients might react to several of these related proteins. Such reactions based on similarity are called cross-reactions. For cereals this problem of multiple sensitivities appears to be of limited importance.
Who, when, how long and how often?
Although wheat is generally mentioned together with milk, egg, shrimp, fish, peanut, soy, and tree nuts as common cause of food allergy, data supporting this are missing. In children, wheat allergy is possibly more frequent than in adults but the prevalence is certainly much lower than to those other foods. A recent study among young adults in Australia resulted in a prevalence of about 0.25%. Among 486 Danish families, 1834 subjects were investigated for food allergy. These included 486 3 year old children, 111 < 3 year, 301 older than 3 year and 936 adults. In none of these subjects, wheat allergy could be confirmed. There are no data available on the prognosis (persistence or outgrowth) of wheat allergy.
A specific type of wheat-related allergy is linked to physical exercise after wheat consumption, so-called wheat-dependent, exercise-induced anaphylaxis (WDEIA). This type of wheat allergy is mainly reported for adults without a history of wheat allergy in childhood.
Overestimation of the importance of wheat as a source of food allergy is possibly related to confusion with celiac disease. Additionally, wheat is also an important source of allergy amongst bakers. Although bakers usually suffer from asthma induced by breathing flour dust, the perception of wheat as an important allergenic food might partially arise from this phenomenon.
How much is too much?
For wheat it has not been accurately established how little wheat causes a reaction. For some foods like milk, egg and peanut these levels have been established to be in the milligram range, i.e. 1/1000 of a gram, for very sensitive patients. For wheat such data are not yet available. A challenge protocol recently reported by a group of German paediatricians used a range between 4 mg and 3.5 gram of wheat flour. Although it was not communicated at which quantities positive reactions were observed, it clearly indicates that low quantities of wheat flour, i.e. in the milligram to gram range are enough to induce symptoms.
Diagnosis of wheat allergy starts with recording a clear clinical history to establish a link between allergic reactions and wheat. This will also distinguish IgE-mediated allergy from celiac disease. Skin prick tests and measurement of specific IgE levels are used to support a history-based suspicion of IgE-mediated wheat allergy. Wheat 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. 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 wheat are administered to the patient as well as placebo meals not containing wheat. Both patient and doctor are unaware of the meals with and without wheat. Effective blinding of the taste of wheat is essential for such challenge procedures.
Where do I find wheat?
Common wheat is used for preparing soft wheat flour products such as bread, biscuits, crackers, cookies, cakes, pastry, thickening agents, and breakfast cereals; it is also used for the production of alcoholic beverages such as beer and whiskey, and for industrial alcohol; bran from flour milling is used for livestock feed. Durum wheat is used principally for the production of pasta. Italian legislation does not allow the use of other wheat species for the making of pasta.
Wheat is also present in products where it is less obvious like battered foods, bouillon cubes and soups, corn starch, breaded meats and vegetables, coffee substitutes, surimi, couscous, gravy, hot dogs, ice cream, processed meats, milk shakes, seasonings and spices like pepper, liquorice, chewing gum, candies, chocolate bars, salad dressings, soy sauce, ketchup, mustard and other sauces.
Hair cosmetic and hygiene products as well as pharmaceutical products like medications and vitamins may contain wheat. In some cases corn starch is an ingredient which in its turn contains wheat. Pet foods can also contain wheat.
For the treatment of food allergy, at present avoidance is the only solution. Since wheat 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 wheat as an ingredient include bulgur, bran, couscous, farina, gluten, kamut, MSG (monosodium glutamate), protein, semolina, and spelt.
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 wheat-derived ingredients should be labelled as such.