Watermelon – refreshing but allergenic
Watermelon (Citrullus lanatus) – like melon, cucumber, courgette and pumpkin – belongs to the cucurbitaceae family (Cucurbitaceae) [1]. It consists of approximately 68% flesh, 30% skin and 2% seeds [2]. It is exceptionally rich in water, accounting for as much as 93% of its weight [3]. However, it is not lacking in nutritional properties. The main bioactive component of watermelon is lycopene, which has anti-inflammatory and anti-cancer effects [2]. In addition, the fruit contains other carotenoids.
But is watermelon allergenic? Allergens of plant origin are the main culprits of allergic reactions to foods. These allergens are present, among others, in the rind, pulp and seeds of fruit and vegetables. They can be found in many products, including fruit juices. Watermelon is no exception. Although it consists mostly of water, it is not free of allergens. So far, five have already been identified. The most common allergen is malate dehydrogenase, which is responsible for 96% of reactions [1].
Watermelon rash and other symptoms of an allergic reaction
The predominant allergic reaction in people sensitive to this fruit is oral allergy syndrome (OAS), which may be accompanied by a watermelon rash (urticaria). With OAS, localised itching and tingling of the lips and throat as well as swelling of the tongue occur [4]. Oral complaints may progress to gastrointestinal symptoms (nausea, vomiting, diarrhoea, abdominal pain) and, in rare cases, even to systemic reactions (anaphylaxis).
In general, symptoms are mild, which may be due to the fact that watermelon allergens are rapidly broken down in the gastrointestinal tract [5]. Sometimes, however, the spectrum of complaints is broader. A case report in Spain describes a 54-year-old man with allergic rhinoconjunctivitis caused by grass pollen. He developed voice disorders (dysphonia), pruritus of the mouth and throat and, in addition, swelling of the genital organs. This occurred approximately 3 hours after eating watermelon. The same symptoms appeared several more times in response to the same fruit before the man discovered the cause of them [1].
Watermelon allergy and cross-reactivity
Watermelon allergens are rarely a direct source of sensitisation. They usually cause symptoms due to a cross-reaction: either pollen-food syndrome or latex-fruit syndrome.
Pollen-food syndrome
Is a reaction that occurs in up to 30% of people with pollen allergy after eating certain fruit and vegetables [6]. It results from the similarity of common allergens, the source of which can be both pollen and food. This similarity causes the immune system to misrecognise as allergenic those allergens that are not the primary source of sensitisation.
In Australia, a group of children with seasonal allergic rhinitis were tested for pollen-food syndrome. It was found that children allergic to birch (Bet v 1) reacted mainly to Rosaceae fruits (apple, peach), in addition to fennel, peanuts and walnuts. In contrast, those with grass allergy (Phl p 2) were significantly more likely to report reactions after Cucurbitaceae fruit (melon, watermelon) and kiwi [7].
Thus, cross-reactions to watermelon mainly occur when grasses are pollenised. In Poland, this period lasts a very long time. It starts in late April and early May, while it ends in September and even early October [9]. Significantly, the peak of grass pollen is when watermelon reigns supreme on shop shelves.
In addition, this fruit cross-reacts with ragweed and mugwort pollen [10], which are also in the air during summer. Therefore, the risk of developing a watermelon rash is greatest during the holiday season.
Latex-fruit syndrome
In addition, watermelon allergy is easily confused with the symptoms of latex-fruit syndrome, which affects about 40% of people with latex allergy [11]. Bananas, avocados and kiwis are most commonly involved in this syndrome, but watermelon is also on the list of potential risks [12,13]. It can produce a variety of symptoms: on the skin (mainly at the site of contact with the fruit – mouth, hands), on the respiratory system (coughing, breathlessness). As a result of this cross-reactivity – in extreme cases – even anaphylactic shock may occur [14].
Watermelon allergy versus fructose intolerance
In addition, watermelon causes complaints in people with fructose intolerance, which can accompany intolerance to other nutrients, such as lactose [15]. As a fresh, small-seeded fruit, it contains large amounts of the fruit sugars fructans [14]. In the case of genetically determined disorders of the metabolism and absorption of these sugars in the intestine, a number of bothersome symptoms occur. There is gas and bloating, abdominal pain, nausea, vomiting, constipation and diarrhoea [16]. The causes of these gastrointestinal complaints often remain unexplained [17].
