English walnut (Juglans regia) is a tree nut of the family Juglandaceae. To this family belong also other tree nuts that are not present in Europe including Pecan (Carya illinoiensis), black walnut (Juglans nigra and Juglans californica) and butternuts (Juglans cinerea). Many processed foods may contain walnut including Sweets, snacks, baked goods, ice cream, soft cheeses.
Severity of walnut-induced allergic reactions ranges from slight oral allergy syndrome (itching in the mouth and/or tongue soon after chewing and ingesting a walnut) to severe and even potentially fatal systemic reactions (anaphylactic shock; hives and swelling of the throat, asthma). Severity of allergic reactions depends on which protein(s) in the walnut the patients has become allergic to.
Related foods (cross reactivity)
In general walnut-allergic subjects fall within the following categories:
Patients with birch pollen allergy that reacts to walnut (as well as to other fruits and vegetables).
In these patients walnut allergy is the consequence of cross-reactivity between pollen allergens and similar proteins in vegetable foods. This kind of food allergy is generally mild: in most cases oral allergy syndrome (OAS) is the only consequence of the ingestion of offending food. The absence of severe systemic symptoms is due to the fact that allergenic proteins are easily destroyed in the stomach. Moreover, they are heat-labile; as a consequence, patients tolerate heat-processed foods.
Patients primarily allergic to walnut and/or other tree nuts (including hazelnut, Brazil nut, macadamia nuts, etc).
Proteins responsible for this type of allergy are heat-stable and resistant to digestion in the stomach; thus, patients reacting to these proteins may experience severe allergic reactions. It is very important to be aware that severe reactions may occur on the first exposure to a nut in patients allergic to a different nut. From studies in which controlled challenges have been performed it has been calculated that walnut-allergic patients have a 37% risk of reacting to other tree nuts such as Brazil nut, cashew, and hazelnut. Unfortunately, the cross-reactivity among tree nuts are very variable both in real life and when measured in blood samples and sometimes involves unusual foods such as coconut. Thus, it is extremely important that patients with a history of severe allergic reactions to a tree nut avoid the ingestion of other nuts until the absence of reactions to walnut has been unequivocally demonstrated by properly performed diagnosis.
Cross-reactivity between tree nuts and peanut is frequently observed in skin prick tests or in blood samples due to the presence of similar proteins, but actual clinical cross-reactivity is very rare.
Patients allergic to peach (and/or other Rosaceae such as apple, pear, cherry, apricot, plum, or almond).
These patients are primarily reacting to a heat-stable protein named LTP (lipid transfer protein) present mainly in Rosaceae but also in most other plant-derived foods. Most patients have a history of peach allergy (OAS and/or more severe allergic reactions). About 50% of patients reacting to LTP react to walnut (as well as to other tree nuts) due to the cross-reactivity between LTP from botanically unrelated vegetable foods. Tree nuts (walnut, hazelnut) as well as peanut seem the most hazardous foods other than Rosaceae for LTP-hypersensitive patients. The likelihood of getting sensitized to vegetable foods other than Rosaceae is directly related to the level of IgE to peach LTP. LTP is heat-stable and resistant to digestion in the stomach; and as a consequence allergic patients may experience extremely severe reactions including anaphylaxis.
Who, when how long, and how often?
Tree nuts, particularly walnut, are one of the most frequent causes of food allergy.
Occurrence data are rather scarce for walnut allergy. In the United States and in the United Kingdom the prevalence of tree nut allergy is estimated to range between 0.2% and 0.5% both in children and in adults. Walnut allergy may appear early in childhood. The possibility exists that a percentage of patients with walnut allergy may become tolerant after some years of avoidance, but this is presently unknown.
Skin tests with fresh food are generally considered the best way to detect walnut allergy, although commercial extracts may prove equally sensitive in patients sensitized to stable allergens. Blood samples are generally equally sensitive.
As opposed to other tree nuts (e.g. hazelnut) or from peanut, walnut is rarely present as a “hidden food” in commercial preparations. Therefore, inadvertent ingestion of walnut should be rare. However, walnut-allergic patients should carefully avoid other tree nuts, such as hazelnut and Brazil nuts, unless their good tolerance has been clearly proven by reliable tests.