Hazelnuts (Corylus avellana) are among the common tree nuts that lead to allergic reactions. They can be eaten as in-shell nuts, however, the majority is cracked, and the kernels are sold to candy makers, bakers and other food processors. Hazelnuts can be found in many food products including cookies, cakes, pastries, chocolates, confectionary products, ice cream, breakfast cereals, and bread. In addition, hazelnut oil may pose a threat to patients with hazelnut allergy, depending on the method of processing with oil undergone less processing at lower temperature tended to be more allergenic.
Symptoms and related allergens
Allergy to hazelnut is often found in patients with hay fever (allergic rhinoconjunctivitis) and tree pollen allergy. These patients usually present with itching, swelling, burning in the mouth and throat after the ingestion of hazelnuts or hazelnut containing products. This so called “oral allergy syndrome” or food-pollen allergy is caused by cross-reactivity between tree pollen allergens, especially birch, alder and hazel pollen, and hazelnut proteins. In addition to the “oral allergy syndrome” severe allergic reactions to hazelnuts are reported in patients without any association to tree pollen allergy. These patients usually display hives (urticaria), swelling of the lips and face (angioedema), breezing difficulties (asthma or swelling of the throat), vomiting, diarrhoea and/or anaphylactic shock.
How much is too much?
In one study using double blind, placebo-controlled food challenges, the lowest dose inducing symptoms in hazelnut allergic patients was 1 mg of hazelnut protein. After a dose between 30 to 100 mg of hazelnut protein, corresponding to one-third of a nut, all patients had developed an allergic reaction. Most patients in this study suffered from pollen-related food allergy. No data on sensitivity (threshold dosages) are reported so far for patients with severe allergic reactions to stable non-pollen-related hazelnut allergens. Minimum provoking doses could be different in this group and might be even lower. In general, the observed threshold levels equal doses of hidden hazelnut sometimes present in food products. Therefore a precise declaration is necessary to prevent unknown exposure. Methods have been developed appropriate for use in food industry that allow the detection of <10 parts-per-million (milligrams per kilogram) of hazelnut protein. However, globally the usage of such tests is no routine yet.
Who, When how long, and how often?
Hazelnut allergy is fairly common in Europe and the United States; however, not many studies of the occurrence exist. The frequency and the type of allergic reaction seem to vary considerably from one geographic region to another depending on the presence of birch, alder or hazel trees. The cross-reactivity between these tree pollen and nut allergens can be the leading cause of hazelnut allergy. In an epidemiological study of food allergy in adults performed in Germany the prevalence rate for nut allergy was shown to be 5%. Moreover, about 18% of the population studied was reacting to hazelnut when measured by skin prick test. A strong connection was observed for hazelnut-sensitized individuals, of whom 94% also reacted to birch pollen. In contrast, in a random digital telephone survey performed in 2002 in the United States the overall occurrence of isolated tree nut allergy was only 0.4% and for both, peanut and tree nut allergy, 0.2%. About 1/3 of these tree nut allergic subjects reported to have allergic reactions to hazelnuts. Allergic reactions to hazelnut can develop at any age; however, seem to depend again on the type of symptoms. The age of onset of “oral allergy syndrome” is beyond infancy but often before school age correlating to the time allergic hayfever develops. Systemic hazelnut allergy can develop already in infancy. A close relation to other food allergies, especially peanut allergy, is suggested. Peanut and tree nut allergies are infrequently outgrown and the foods may cause severe symptoms and even death
For most patients with suspected hazelnut allergy, currently used diagnostic tests, such as blood samples or skin prick tests give no clear-cut diagnosis. Only for highly reactive patients with high levels of nut-specific IgE or large skin test reactions it can be proposed that about 95% of these patients will have clinical reactions upon ingestion of the nut. Most results from allergy tests, however, will lie in a ‘grey area’ beneath these values, but still positive where one could not be certain. These patients may be allergic or tolerant to the tested food. Moreover, variability in the composition of commercial skin prick test reagents for the diagnosis of hazelnut allergy is extreme. Sometimes, allergens important especially for severe systemic reactions to hazelnut are missing in the preparations. These shortcomings in standardization and quality control can potentially cause a false-negative diagnosis in subjects at risk of severe reactions to hazelnuts. Therefore, the gold standard is still the oral food challenge, best in a double-blind, placebo-controlled way. Recipes for hazelnut food challenges have been developed and validated.
Patients with diagnosed hazelnut allergy, especially those with systemic reactions, should completely avoid hazelnuts and hazelnut containing products. Currently, patients allergic to peanuts are advised to avoid also all tree nuts, including hazelnuts. Moreover, avoidance of all tree nuts is commonly recommended in patients allergic to any tree nut. Whether these recommendations are useful can be discussed. The elimination of all nuts is very difficult and has a great impact on the lifestyle. In addition, nuts are an important source of protein. It has recently been reported that 55% of patients allergic to one nut were able to tolerate another type of nut. Therefore the clinical relevance of hazelnut allergy should be proven before general avoidance is recommend especially since hazelnut are used in so many common food products. Moreover, infants in risk groups for developing allergy, such as infants with allergic parents or siblings, should avoid these foods in the first few years of life. The success of elimination diets depends on the recognition by the consumer of the offending food in food products. Hazelnut as a tree nut will be identifiable in food products through to the new EU labelling directive that will be used by 2005.
It is well know that heating, cooking or roasting can change the ability of a food to elicit allergy. Processed food may be either more or less allergenic. It had been shown for hazelnuts that roasting seems to reduce the reactions at least for birch pollen allergic patients due to the fact that the responsible hazelnut-allergens are destroyed by heat. However, since about 30% of the patients with food-pollen allergy still showed allergic reaction to the roasted nuts, ingestion of roasted hazelnuts or products containing roasted nuts can not be considered safe for hazelnut allergic consumers. Moreover, the risk imposed by roasted hazelnuts to patients without tree pollen allergy but severe systemic reactions to hazelnuts is not known today. Hazelnut-allergens eliciting these systemic reactions belong to the same protein family as the allergens in peanuts. At least for the peanut allergens it is known that roasting increases the allergenicity of these proteins.