Walnut

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Code: f256
Latin name: Juglans spp.
Source material: Shelled nut
Family: Juglandaceae

The Juglandaceae family contains 2 important genera: Hickory/Pecan (Carya) and Walnut (Juglans).

Walnut is the common name for 20 species of deciduous trees in the Juglans genus (native to south-western Asia, eastern Asia, and the Americas). Those species have been separated into 4 taxonomic sections:

  • Dioscaryon (English walnut)
  • Rhysocaryon (Black walnut) 
  • Cardiocaryon (Asian butternut)
  • Trachycaryon (American butternut)

Section Dioscaryon contains a single species, J. regia, and this nut is commonly called the English walnut or Persian walnut. Section Rhysocaryon includes all of the Black walnuts: for example, the species J. nigra (Eastern black walnut), J. californica (Southern California black walnut), J. hindsii (Northern California black walnut), J. major (Arizona black walnut) and J. microcarpa (Texas black walnut).
 

Section Cardiocaryon contains J. ailantifolia (Japanese walnut), J. cathayensis (Chinese walnut), and J. mandshurica (Manchurian walnut), among others. Section Trachycaryon comprises a single species native to North America, J. cinerea, known as the Butternut or White walnut. Within some species there are also different cultivars (1). 

Allergen Exposure

Geographical distribution

Walnut is one of the most important nut crops. The leading commercial producers of Walnuts are the United States, Turkey, China, Iran, France and Romania.

The Walnut tree is up to 20 m tall, with a large, spreading, rounded top and thick, massive stem. Walnut trees are monoecious, with male flowers borne in long, unbranched, drooping catkins, and female flowers borne singly or in short spikes. The leaves and outer green husks are poisonous to fish and most animals. The flowers appear in May, and the yellow leaves turn green by mid-June.

The Walnut fruit is a nut, borne singly or in pairs and enclosed in a solid, non-splitting green husk. The fruit is oval in shape, 4 to 5 cm across. The outer, slightly soft coat, which is called a husk or shuck, is shiny and green at first. It changes to dark brown when it is ripe. Inside this fruit is a single seed, the Walnut, which has an edible, oil-rich, white, two-lobed, wrinkled kernel enclosed in a thick, hard, ridged, and brown to black shell. In mid-September the nuts can be harvested with their husks still on. By October, the husks fall off, and the hard-shelled nuts can be gathered.

Walnut shells are composed of 2 halves. Between the halves is the "shell seal", which is usually tight, to protect the nut from moisture and pests. But the shell of freshly picked Walnuts can easily be cracked open by hand. After washing and drying, the shell becomes very hard and must be broken open with a tool such as a nutcracker or hammer. A hard, fibrous membrane lines the inside of the shell, runs between the two halves of the nut, and is removed before eating. The Walnut itself is covered by a thin edible skin called the pellicle, which can be removed by blanching the nut in rapidly boiling water for 30 to 60 seconds. The pellicle contains tannins; the darker the pellicle, the more tannins it contains. Walnut has a very high polyunsaturated fat content, which makes the nut extremely perishable, requiring careful storage.

While there are numerous species of edible Walnuts, the 3 main ones consumed are the English (or Persian) walnut (J. regia), the Black Walnut (J. nigra), and the White (or Butternut) Walnut (J. cinerea). The English walnut is the most popular and features a large nut and thinner shell, which is easily broken with a nutcracker. The Black walnut has thicker shells that are harder to crack; the nuts are oilier and richer-tasting, with a much more pungent and distinctive flavour than that of the English walnut. They are harder to extract from the shell in large pieces. The much less available White walnut has a sweeter and oilier taste than the other 2 types, and very high protein content.

Environment

The nuts are rich in oil, particularly in alpha-linoleic acid (omega 3 fatty acid) and linoleic acid (polyunsaturated fatty acids), and are widely eaten both fresh and in cookery. Walnuts can be eaten directly from the shell, but more often they are eaten as an ingredient in baked goods, ice cream or other foods. They are used in salads and in meat, poultry, fish and pasta dishes. Immature young fruits are pickled in vinegar and considered a delicacy. A liqueur is made in France from the husks. Greeks and Romans used Walnuts to cure headaches, because of the shape, which resembles the two halves of the brain.

The oil is used in salads and pasta. The allergenicity of this oil and various other gourmet nut oils depends on the extraction method and the purity of the end product (2).

Oil paint often contains Walnut oil as an effective binding medium, known for its clear, glossy consistency and non-toxicity.

Walnut husks can produce a rich yellow-brown to dark brown dye that is used for dyeing fabric and for other purposes. Husks should be handled wearing rubber gloves, to avoid dyeing one's fingers.

Allergens

The following allergens have been characterised:

  • Jug r 1, a 14-16 kDa protein, a 2S albumin, a major allergen (1,3-9).
     
  • Jug r 2, a 44-48 kDa protein, a 7S vicilin globulin, a major allergen (1,3,5-7,10).
     
  • Jug r 3, a 9 kDa protein, a lipid transfer protein, a major allergen (5-7,11).
     
  • Jug r 4, a legumin-like protein. (5-6,12-13).
     
  • Jug r profilin (14).

Jug r 1, a 2S albumin, was shown to be a major allergen in a study of 20 American Walnut-allergic individuals; the allergen bound 75% of the patients' sera (4). Similarly, IgE binding to Jug r 1 was demonstrated in 12 of 16 (75%) sera from Walnut-allergic patients (3). The IgE binding inhibition study suggested that the Walnut 2S protein precursor undergoes post-translational modification into a large and a small subunit similar to Castor seed, Cottonseed, Mustard seed, and Brazil nut 2S seed storage protein allergens (3).

However, contrasting results were seen in a study of 46 Italian patients, either with oral allergy syndrome confirmed by open oral challenge, or with systemic symptoms after ingestion of Walnut; a comparison of the Walnut IgE binding profile in patients with and without pollen allergy showed that the only major allergen recognised was a lipid transfer protein (LTP) (Jug r 3), which was recognised by 36 patients (78.2%). Two other minor allergens of approximately 9 kDa, both belonging to the vicilin family, were recognised by 10 patients (21.7%). Nine patients reacted to vicilin A, and 1 patient to vicilin B (which has 90% homology with Cocoa). Two patients exclusively allergic to Walnut were sensitised to vicilin A. IgE binding to Walnut LTP was completely inhibited by Peach LTP. The study concluded that in the Italian population, the LTP is a major allergen (7).

Jug r 4 has been shown to be a major allergen and was found in sera from 15 of 23 (65%) Walnut-allergic patients. (12) rJug r 4 has been cloned and was demonstrated to bind to serum IgE antibodies from 21 of 37 (57%) Walnut-allergic patients (13).

Allergens similar to the English walnut allergens Jug r 1 and Jug r 2 have been shown to be present in Walnuts of other closely related Juglans species (1). However, although cross-reactivity between the Jug r 1 allergen and a similar allergen in other Juglans species occurs, and appears to be extensive, species other than J. regia were shown to be unable to completely inhibit binding to the recombinant form of Jug r 1. The authors suggest that perhaps the other species had less 2S albumin and left more Jug r 1-specific IgE antobodies available in solution for binding to the immobilised rJug r 1, or that binding affinities differ slightly. They also proposed that the reduction and partial linearisation of rJug r 1 by the SDS in the gel or the non-natural folding of a precursor fusion protein (the 2S albumin precursor is normally processed into large and small subunits in Walnut) introduces additional IgE epitopes that are still present when membrane-bound. The authors suggested that, in any case, subtle differences in the 2S albumins can be inferred (1).

Potential cross-reactivity

A high degree of cross-reactivity occurs among the different Walnut species (1).

Early inhibition studies suggested cross-reactivity between Walnut and Pecan when 7 tree nuts (Walnut, Pecan Almond, Hazel nut, Brazil nut, Cashew, Pistachio) and Peanut were evaluated (15). Other studies have also reported a higher risk of cross-reactivity between Walnut and Pecan (16) and among Walnut, Pecan, and Hazel nut (17). The allergens resulting in this relationship were not elucidated. Cross-reactivity among Walnut, Hazel nut and Brazil nut was reported in a patient with Walnut-induced anaphylaxis (18).

A high degree of cross-reactivity among the tree nuts is most likely to be a result of the 2S albumin and other storage proteins. Jug r 1 has been shown to have a 46.1% identity with the Brazil nut 2S albumin Ber e 1 (3). A major allergen in Cashew nut, a 2S albumin, was reported to be the possible basis for cross-reactivity with Walnut 2S albumin (19). Cross-reactivity among the 2S albumin in Almond, Walnut and Hazel nut has been demonstrated (20).

There are reports of cross-reactivity among allergens in Sesame and allergens in Walnut (21). This may occur as a result of the presence of a 2S albumin (Ses i 2) or a 7S vicilin-type globulin (Ses i 3) found in Sesame seed; both kinds of storage proteins are found in Walnut (22). Other studies have reported that the Sesame 11S globulin shows only partial immunological cross-reactivity with Walnut (23).

Vicilin allergens of Peanut (Ara h 1), Walnut (Jug r 2), Hazel nut (Cor a 11) and Cashew nut (Ana o 1) share structurally related IgE-binding epitopes, and authors have suggested that avoidance or restricted consumption of other tree nuts should be recommended to Peanut-sensitised individuals (9).  Ana o 1, a 7S vicilin-like globulin from Cashew, has a 52-62% similarity to Walnut (6). A 19 kDa protein of Buckwheat has been shown to have a weak homology to the vicilin-like protein of Walnut (Jug r 2), Cashew (Ana o 1), and to 7 S globulin from Sesame seed (24). A 7S globulin from Coconut has also been shown to be cross-reactive with Walnut and Hazel nut (25).

Linear IgE-binding epitopes identified in legumin allergens of Peanut (Ara h 3) and other allergenic tree nuts (Jug r 4 of Walnut, Cor a 9 of Hazel nut, Ana o 2 Cashew nut) were mapped.

Conformational analysis has shown that the legumin allergens of Peanut (Ara h 3), Walnut (Jug r 4), Hazel nut (Cor a 9), and Cashew nut (Ana o 2) have consensual surface-exposed IgE-binding epitopes that exhibit some structural homology, which accounts for the IgE-binding cross-reactivity observed among Peanut and tree nut allergens. IgE-binding epitopes similar to those found in 11S globulin allergens do not apparently occur in other vicilin allergens with the cupin fold, as in Peanut (Ara h 1), Walnut (Jug r 2), Hazel nut (Cor a 1), and Cashew nut (Ana o 3) (26).

Jug r 3 from Walnut is a lipid transfer protein. As LTP-hypersensitive patients experience adverse reactions after ingestion of botanically unrelated plant-derived foods as well, in view of the high prevalence and severity of the reactions, authors have recommended that Hazel nut, Walnut, and Peanut should be regarded as potentially hazardous for these patients (7,11). In a study of 20 LTP-monosensitive patients with a clinical history of allergic reactions following the ingestion of Rosaceae fruits (Apple, Pear, Peach, Cherry, Apricot, Plum, or Almond), 80% reported clinical reactivity to nuts (Hazel nut, Walnut, Pistachio); in 11 cases (55%) nuts caused systemic symptoms, with urticaria/angioedema in 9 cases and anaphylaxis in 2. Maize and beer were not tolerated by 4 (20%) and 2 (10%) respectively. After ingesting Grapes, 2 patients experienced immediate gastro-enteritis/rhinitis and oral allergy syndrome, respectively. Cashew and Pistachio induced an anaphylactic reaction in a single patient, but Cashew is rarely eaten in Italy. Only 2 patients had ever eaten Pistachio, and 1 reacted; hence the study conclusion that these 2 nuts may be significant LTP-containing foods for adverse reactions (11). The relevance of LTP in cross-reactivity among Walnut and other LTP-containing foods, e.g., Apple (Mal d 3), has been reported (27).

Jug r 4, a legumin-like protein, has been shown to have significant sequence homology to Hazel nut and Cashew legumin allergens, and in vitro cross-reactivity was demonstrated with Hazel nut, Cashew, and Peanut protein extracts (13). Legumin-like seed storage proteins have been implicated in cross-reactivity between Coconut and Walnut, Almond and Peanut (28).

Walnut profilin may result in cross-reactivity with other profiling-containing food, but this has not been established yet.

Clinical Experience

IgE-mediated reactions

Tree nuts and Peanuts are frequent causes of severe IgE-mediated food hypersensitivity and anaphylaxis. In particular, tree nut allergies are potentially life-threatening, rarely outgrown, and appear to be increasing in prevalence. Peanut and tree-nut allergic reactions coexist in a third of Peanut-allergic patients, frequently occur on first known exposure, and may be life-threatening (29).

The frequency of Walnut allergy in children with IgE-mediated food allergy has been reported as 4.2% (30). Walnut was most commonly reported to be allergenic by participants in the Food Allergy and Anaphylaxis Network (FAAN) Peanut and Tree Nut registry in the United States. A questionnaire of 5,149 patients (mainly children) found that of allergic reactions to tree nuts, 34% of reactions were to Walnut (31). In 1,000 British patients allergic to nut, 30 cases of severe allergic reactions to Walnut occurred (32). Random telephone surveys found that the importance of Walnut allergy and other nut allergies is related not only to the severity of the allergic reactions but also to the high prevalence in the general population, estimated to be around 0.2% to 0.7% (33-34). In a SPT study of 1,286 allergic patients, aged 2-79 years, in the southeast part of Iran, it was found that 30.33% of the subjects were sensitised to Hen's egg, 29.16% to Walnut, 21.46% to Cow's milk, 19.21% to Beef, and 15.32% to and Hazel nut (35).

As Walnuts are promoted as part of a healthy diet because of their polyunsaturated fat and antioxidant content, consumption of Walnuts and the addition of them to processed foods can be expected to increase.

In a study based on a patient questionnaire, hypersensitivity to Walnuts was shown to be more common in patients with Birch pollen allergy (26%) than in patients without Birch pollen allergy (6%) (36). Similarly, in a study of grass pollen allergy in Northern Italy, where this allergy is the most common, IgE antibodies to Bet v 1 were more strongly associated with nuts and legumes, while Bet v 2 was more strongly related to allergy to fresh fruits and vegetables (37).

Anaphylaxis to Walnut has been frequently reported. In a case series, 10 of 32 fatal anaphylactic reactions to foods resulted from ingestion of a tree nut, and Walnut was named in 3 cases (38). In the United Kingdom register of anaphylaxis deaths between 1992 and 1998, 37 food-induced fatalities were recorded, 5 caused by Walnut (39-40). Of 55 severe or fatal food-allergic reactions reported in a British paediatric population, 1 near-fatal reaction was caused by Walnut (41). Nevertheless, the prevalence of severe allergy may vary according to population groups. In a retrospective review of 213 Australian children with Peanut or tree nut allergy, there was only 1 reported case of anaphylaxis to Walnut alone, and 3 to a mixture of nuts (42).

Walnut-induced anaphylaxis was reported in an 11-year-old boy who developed gastric pain followed by severe asthma, generalised urticaria and collapse 15 minutes after eating home-baked cookies made with Walnuts, Pistachios, dried Figs and raisins. Skin reactivity and IgE antibodies were detected to Walnut and Hazel nut (18). Significantly, accidental intake of foods containing small amounts of Walnuts not listed on the food labels can induce anaphylaxis (43).

In particular, patients with an initial diagnosis of "idiopathic" anaphylaxis may have in fact reacted to Walnut. A hundred and two patients with an initial diagnosis of idiopathic anaphylaxis were evaluated with 79 food allergens using SPT; only those patients were included whose episodes consisted of at least 2 of the following: angioedema with or without urticaria, laryngeal oedema leading to severe dyspnoea, hypotension, and loss of consciousness; 32 patients (31%) had positive tests to 1 or more foods. Walnut was among the foods implicated. The authors concluded that some instances of "idiopathic" anaphylaxis are not truly idiopathic (44).

Importantly, delayed anaphylaxis to Walnut following epinephrine administration may occur, as documented in a 7-year-old girl with known Walnut allergy. Five minutes after the accidental ingestion of a bite of a Walnut-containing salad, the patient's mother injected her with 0.15 mg epinephrine, although no clinical reaction was noted prior to administration. After 90 minutes of observation, she suddenly developed diffuse pruritus, cough, wheeze, erythema, and urticaria. Further treatment resulted in resolution of symptoms (45).

Walnut food-dependant exercise-induced anaphylaxis has been reported (46-47).

Cross-reactivity may have severe consequences. An 18-year-old male experienced generalised urticaria, angioedema of the face, wheezing, dyspnoea and hypotension a few minutes after eating a cake containing Walnut. He had previously experienced episodes of urticaria and oral itching after eating Walnuts. During SPT evaluation for Walnut and Brazil nut allergy, a severe episode of anaphylaxis occurred immediately after pricking with fresh Brazil nut. The subject had never eaten Brazil nut before. The authors hypothesise that this adverse reaction was a result of cross-reactive mechanisms (48).

Anaphylaxis to Walnuts and Pine nut induced by angiotensin-converting enzyme inhibitors (ACE) has been described (49).

Although the storage proteins are in general major allergens responsible for adverse reactions to Walnut, in Italian patients the lipid transfer proteins are major allergens and responsible for oral allergy syndrome. In a study of 46 Italian patients either with oral allergy syndrome confirmed by open oral challenge or with systemic symptoms after ingestion of Walnut, the only major allergen recognised was a lipid transfer protein that bound serum of 37 patients. Two other minor allergens, both belonging to the vicilin family, were recognised by 10 patients. IgE binding to Walnut LTP was completely inhibited by Peach LTP, and the authors suggested that the sensitisation to this protein seemed to be secondary to the sensitisation to Peach LTP, which would act as the primary sensitiser. Symptoms of oral allergy syndrome were reported in 24 patients, gastrointestinal symptoms in 1, glottic edema in 13, urticaria in 6, angioedema in 8, shock in 10, atopic dermatitis in 2, and asthma in 1 (7). The importance of lipid transfer proteins in Walnut-allergic patients has been reported in other studies (11,18).

Walnut allergy may occur as part of a true multifood allergy. A 4-year-old male, who first experienced urticaria and asthma related to the ingestion of eggs at the age of 2, in subsequent years developed rhinitis, angioedema, headache, and gastroenteritis. Symptoms started between a few minutes to 2 hours after the ingestion of a number of foods. SPT  and IgE antibody determinations were positive for a number of foods, including Walnut, and this was confirmed by DBPCFC (50).

Walnut may also exacerbate atopic dermatitis. Over half of paediatric patients with atopic eczema were found to be have IgE antibodies to Walnut (51).

Occupational contact urticaria from Walnut associated with hand eczema has been reported (52).

Other reactions

Juglone is the active ingredient of the green flesh of Walnuts and is known to be a strong irritant. Two young nursery-school playmates developed contact pigmentation and acute irritant contact dermatitis as a result of contact with the "juice" of green Walnut husks (53).

Compiled by Dr Harris Steinman, harris@zingsolutions.com

References

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As in all diagnostic testing, the diagnosis is made by the physican based on both test results and the patient history.