Walnuts – the healthiest of all?
In many countries, such as Spain, the UK and Poland, the most popular and widely consumed nuts are walnuts. High in unsaturated fatty acids, protein and the vitamins B and vitamin C, it is not surprising that they are considered to be of great importance in the human diet [1,2].
Among all nuts, these are the ones with the highest concentration of valuable acids (69%) in a very favourable omega-6 to omega-3 ratio (5:1) [1,2]. Among other things, they contain a lot of α-linolenic acid (ALA n-3), which is a powerful antioxidant. Therefore, nuts are recommended for the prevention and dietary therapy of cardiovascular diseases [2]. They have been proven to reduce the risk of death from cardiovascular disease and stroke [3]. At the same time, however, they are on the list of foods that cause severe allergic reactions [4].
Walnut allergy – dangerous and not at all uncommon
Tree nuts (including walnuts) are included in the so-called big nine food allergens, which account for 90% of allergic reactions to food. They are made up of many proteins (molecules), and some of these molecules are capable of sensitising. Sensitising walnut proteins vary in their properties. In allergology, they are labelled with various symbols – from Jug r 1 to Jug r 8. The specific protein to which you are allergic (and to what degree), for example, determines the type and severity of your symptoms. You can check your allergy profile with molecular diagnostics.
A study carried out at the Department of Allergology and Pneumonology in Rabka-Zdrój showed sensitisation to walnuts in 33% of the atopic children (aged 0-18 years) treated there. In comparison, the percentage of those allergic to peanuts and hazelnuts in the same group was as high as 52% [6].
Walnuts contain reserve proteins (Jug r 1, Jug r 2, Jug r 4, Jug r 6), which are associated with a high risk of anaphylaxis [5,6]. In addition, there are other nsLTP molecules (Jug r 3, Jug r 8) that are almost as dangerous. They can induce shock particularly when contact with them coincides with the action of factors that enhance the course of the allergic reaction. These include so-called cofactors, e.g. exercise, drinking alcohol [6].
Walnut allergens that can induce anaphylaxis are thermostable, meaning that they do not lose their properties when exposed to temperature [9]. Thus, in an allergic person, a reaction can be expected even after eating roasted or cooked food with their content.
Allergy symptoms – mild and severe
Symptoms that may indicate a walnut allergy are:
- oral allergy syndrome (swelling of the mouth, itching, burning) – most commonly reported,
- urticaria,
- angioedema,
- allergic asthma,
- allergic rhinitis,
- allergic conjunctivitis,
- gastrointestinal complaints (nausea, vomiting, diarrhoea),
- anaphylactic reaction (including anaphylactic shock) [7,11].
These usually occur within 2 hours of ingestion.
In the case of a severe reaction, we may have to deal with:
- dysphagia (swallowing disorders),
- dysphonia (voice disorder),
- lower respiratory tract symptoms (laryngeal oedema, bronchospasm), cardiovascular (sudden drop in blood pressure below acceptable levels) or neurological symptoms [11].
According to a large study conducted in various research centres in Europe, mild to moderate symptoms affect about 70% of people with this type of allergy, while severe symptoms affect about 30% [11].
A serious problem is the presence of nuts (walnuts and others) as hidden allergens in processed products [8]. They can enter food completely by accident, e.g. at the preparation stage in a production facility or when preparing a meal in a restaurant. Unfortunately, even trace amounts of nuts in highly allergic individuals can lead to anaphylaxis [9].
Walnut allergy and cross-reactivity
However, this is not all. Molecules resembling the structure of walnut proteins are found in other products, which is why cross-reactions occur.
What does this mean? Let us use an example. A high degree of similarity (more than 90%) has been shown between the stock proteins of walnut and pecan, which belong to the same botanical family (Juglandaceae) [5,6]. The immune system can therefore easily ‘confuse’ these proteins and, in a walnut allergy, react with the same symptoms to pecans.
By an analogous mechanism, walnuts can also cross-react with: peanuts, hazelnuts, pistachios, sesame, lentils, soya, peach, tomato, apple, kiwi [6,11].
Jug r molecules show strong associations not only with proteins found in plant foods, but also with those hidden in pollen: birch, grasses, mugwort, plantain [6,11]. Consequently, they enter into a particular type of cross-reaction with them, called pollen-food syndrome. This is especially true for profilin (Jug r 7) and the proteins PR-10 (Jug r 5) and nsLTP (Jug r 3, Jug r 8) [6].
It is cross-reactivity with pollen that is the main cause of allergy symptoms after walnuts (as far as Europe is concerned) [11]. Walnut allergy associated with birch pollen is predominant in northern and central Europe. The Jug r 5 protein is very similar to Bet v 1, the main birch allergen [11].
Sensitisation to nsLTP (Jug r 3) associated with the main peach allergen is mainly diagnosed in southern Europe. In contrast, sensitisation to walnut profilin (Jug r 7) occurs with similar frequency throughout Europe [11].
Interestingly, research has shown that allergy to mugwort pollen increases the risk of severe walnut allergy by almost four times [11].
