Allergy to sorrel? A childhood memory

Weeds of the genus Rumex number about 200 species, of which about 25 occur in Poland [1,2,3,5]. Some of them are considered nuisance weeds, while others are used for their edible leaves. They grow both wild (in meadows and pastures) and in cultivation. They are widespread in temperate climates, especially in the northern hemisphere. They primarily grow in lowland areas [1]. In Poland, two species are most common: Rumex acetosa (common sorrel) and Rumex acetosella (field sorrel) [5]. The name Rumex probably comes from the Greek word for spear or arrow and refers to the shape of the leaves [6].

In soup and in salad

The seeds of sorrel are eaten by birds, and the leaves or whole plants by some rodents and ungulates (rabbits, deer, sheep – hence the English name sheep sorrel) [1]. Sorrel is also a valuable part of the human diet (although in excess it can even prove poisonous, as discussed further below) [1,2]. It is edible both cooked (e.g. in soup) and raw (e.g. in salad). Older leaves are sometimes bitter and hard, but are suitable for cooking [6].

Sorrel has a distinctive sour taste, which is attributed to its high oxalic acid content. It goes well with meat and dairy products. The subtler-tasting sorrel sprouts are also used in cooking, with which, for example, sandwiches can be sprinkled [6]. The disadvantage of sorrel is that its leaves quickly lose their quality after harvesting. Therefore, this plant is rarely found in markets [6].

Studies of the phytochemical properties of sorrel have shown that it contains many bioactive compounds and is also a good source of macro- and micronutrients. It has almost as many antioxidants as Japanese green tea. However, the high level of oxalic acid significantly reduces the bioavailability of some minerals, especially calcium and iron. That is, it hinders their absorption [6].

Oxalates and oxalic acid mean that eating the leaves of this plant raw in large quantities carries certain risks. The compounds in question can harm children and people over 55 years of age in particular – by upsetting the mineral balance. Their excess is one of the factors contributing to health problems such as kidney stones and anaemia [2,6,7]. Potassium oxalate, by binding with calcium, forms an insoluble compound that is not excreted from the body [8].

Not necessarily allergy

An overdose of oxalates results in diarrhoea, nausea, vomiting and the appearance of skin lesions. These are indicative of poisoning, although they overlap with symptoms characteristic of a food allergy (although they usually have nothing to do with it). A dangerous consequence of excess oxalates in the body is kidney damage [2].

Oxalates are not only found in sorrel – they are present in most plants. Spinach and rhubarb, among others, are characterised by their significant content. Very high concentrations of oxalic acid in plants make them inedible (e.g. hyacinth, the lily of the valley-like coconut) [7]. The good news is that cooking reduces the acid concentration in sorrel to a negligible amount [6].

We already know how the consumption of sorrel can affect us. But what about its pollen, which becomes airborne during the flowering of this plant? The pollen season for sorrel in Poland starts in April and ends in September. Its pollen accounts for 1-1.9% of the total pollen in the air. The highest concentration occurs in May and June. One plant can produce 400 million grains [3].

The influence of Rumex species on the occurrence of seasonal allergy symptoms is probably greater than believed. Its importance is considered secondary. However, it probably remains underestimated because of the overlap between sorrel pollen and grass pollen [1]. Oxalis precedes other weeds, most of which (such as mugwort or ragweed) only release pollen in late summer [4].

Allergy to sorrel or cross-reactivity?

The few studies carried out on the subject show that reactions to sorrel occur in 20% of people with pollen allergy. This is the result of cross-reactivity between grass and sorrel pollen allergens, which are similar in structure [9]. Cross-reactions are said to occur when an allergic person develops symptoms after contact with something to which they are not allergic. However, the body has nevertheless recognised the molecule in question as a threat. The reason for such ‘confusion’ is the similarity in structure of the proteins found in pollen and food (especially of plant origin).

Aleksandra Lipiec 

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translation: Julia Majsiak