Fruit and vegetables

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Booklet: Fruits & Vegetables.
Allergy - Which allergens? Phadia AB, 2010.

 

The botanical term “vegetable” means either all plant material or all the edible parts of plants, compared to “fruit”, which denotes the ripened ovaries of flowering plants and surrounding tissues (1). True fruits, therefore, are developed from the ovary in the base of the flower and contain the seeds of the plant (though cultivated forms may be seedless). Thus, many botanical fruits are not edible at all, and some are actually extremely poisonous (2).

But in practice the definitions of the both words are traditional rather than scientific, and somewhat arbitrary and subjective, being determined by local customs of food selection and preparation. In general, vegetables have traditionally been regarded are those plant parts suitable for savoury or salted dishes, rather than sweet dishes. However there are many exceptions, such as the Pumpkin, which is eaten in both savoury and sweetened dishes. In contrast to this, some edible botanical fruits, including the Tomato, the Eggplant, the Bell pepper, and the Bean pod, are classed as culinary “vegetables”. The term “fruit” can refer to a part of a plant which is not technically a fruit but which is used in sweet cooking: Rhubarb, for example. Mushrooms belong to the biological kingdom Fungi, not the plant kingdom, and yet they are also generally considered to be vegetables. Some vegetables, such as Carrot, Celery, and Bell pepper are eaten either raw or cooked, while others, like the Potato, are eaten only when cooked. Most fruits are eaten both raw and cooked.

Fruits are a staple of the human diet predating agriculture by millions of years, if the evidence of primate diets is relevant. Preference for fruit-like sweetness in food is generally considered by anthropologists to be programmed into homo sapiens through natural selection and to serve as a signal that a new food is edible and not harmful. The human “sweet tooth” is probably heavily indebted to the prominence of fruit in the environment and nutrition of early hominids.

Fruit is experiencing a renewed importance in the human diet, as an indirect result of this very “sweet tooth”. With a very abundant food supply and wide choice among foods in the present-day developed world, the preference for sweetness has run amok, one result being widespread overindulgence in candy, pastries and other processed sweet foods. This has contributed to the obesity epidemic and unprecedented rates of diabetes and other obesity-related illnesses. But with health awareness growing, there is, at least in industrialised countries, a partial return to human dietary roots, with more and more “natural”, less-processed foods being eaten, and this trend includes the replacement of fattening sweets with non-fattening fruits.

Fruits and vegetables form a large part of the average middle-class diet. Fixed numbers of portions per day are recommended by dieticians, and the recommendations are widely heeded, especially by parents and schools. In the modern era, fruits can be cheap, abundant “convenience foods”. Apples, Oranges, Bananas, Lemons and Limes have for most of living memory been easy to transport and store and so very readily accessible. Commercial fruit juices, often available in single-portion servings, have expanded in variety from Apple and Orange juices and lemonade to a bewildering array, including exotic mixes. Some fruits such as Mangoes, which because of their tropical or semi-tropical origins and difficulty of transport and storage were seldom if ever tasted in temperate zones by previous generations, are now growing familiar because of advanced storage and transport technology. South African and Israeli fruits, for example, are transported by plane to Europe within a few hours, and boxed juices (many with shelf-lives of from six months to a year) are a growing industry in a number of tropical and semi-tropical countries. Additionally, fruit is often used as a flavourant or other additive in processed foods. Finally, there is broader fruit exposure through greatly increased travel opportunities. The increasing availability of vegetables is less dramatic but is still notable. Some of the same factors in transport and retail have been influential, and under consumer pressure, prepared salads have become common convenience foods.

Allergen exposure
It is not surprising that fruit and vegetable allergy is increasing: two very clear causes are the increasing availability of fruit and vegetables in general and the movement of some fruits and vegetables into regions where they were almost never eaten before. This movement complicates the increase in allergy, however, as some methods of processing and storage can activate or de-activate certain fruit allergens. Heat-lability and heat-stability, for instance, are important in fruit allergen metamorphosis.

Fruit and vegetable allergy symptoms span a wide range, from atopic dermatitis to urticaria to oral allergy syndrome (itching lips, tongue and throat, and sometimes swollen lips, tongue, throat or palate) to anaphylaxis (A claim put forth is that anaphylaxis-prone allergy does not exist in conjunction with oral allergy syndrome, but this is debatable). Some authors have suggested that “pollen-food syndrome” is a more apt description of reactions involving cross-reactive pollen and food allergens and encompassing symptoms of oral allergy syndrome.

Considered in isolation, fruit and vegetable allergy might seem to create relatively little danger or trouble. Fruits and vegetables, when compared to Peanut or fish, were initially not known for commonly causing anaphylactic reactions in very tiny amounts; and it was relatively easy to avoid one or several fruits or vegetables in a diet. However, severe reactions have been recorded to both (3-4). It also must be kept in mind that fruit and vegetables, like Soya, Wheat, and Cow’s milk, may now commonly be added to processed food, and that, like these allergens, fruits and vegetables are not always properly labelled. A fruit or vegetable extract called a “flavourant”, for example, can act as a hidden allergen (5).

More importantly still, the role of fruits and vegetables in cross-reactivity (see below) sets them in the middle of some of the major dramas of allergy. Fruit-fruit cross-reactivity in itself creates the danger of incomplete diagnosis and allergens unexpectedly encountered later on. Also, hay fever is on the increase, giving great importance to the role that fruits play in pollen-fruit cross-reactivity. Latex-fruit syndrome (or Latex-food syndrome) links fruit and vegetables to one of the most vicious allergies, which has, among other depredations, forced some surgeons and dentists out of practice. The Mango component in an allergy complex may be trivial; but the Latex component could be devastating.

Occupational fruit and/or vegetable allergy are also important and increasing. Those employed in fruit and vegetable growing, handling and processing – not an inconsiderable group of people – are at risk from topical exposure to produce. Topical allergy occurs, and non-allergy topical reactions are factors as well, as fruits contain a variety of volatile chemicals and other substances that can be irritating to the skin. Even mechanical irritation is a problem in some fruit workers. Finally, sulphite as a preservative of fruits and vegetables would logically affect producers and handlers much more than it affects consumers.

A thorough interview and specific testing are necessary to determine the exact aetiology of occupational reactions to fruit, as well as of reactions to ingestion, in which such factors as histamine and the toxicity of pits and seeds may need to be taken into account.

Cross-reactivity
The analysis of cross-reactivity was initially fruit to fruit and vegetable to vegetable. It then moved from the botanical family level to the level of panallergens, which allow cross-reactivity among much more distantly related entities. It was panallergens that explained pollen-fruit cross-reactivity, pollen-vegetable allergy, and fruit-vegetable allergy. A greater number of culprit pollens can now be recognised, and a substance as unexpected as Natural rubber latex can be included in allergy equations.

Therefore, though fruits remain central to considerations of cross-reactivity, this phenomenon appears increasingly complex. To begin with, all fruits and vegetables contain a number of allergens, some of which are panallergens. It is possible simply to be allergic to Apple, because of one or more unique Apple allergens. It is more likely, however, that a patient will have a cluster of allergies, and it is conceivable, because of multiple panallergens, to be vulnerable to overlapping patterns of cross-reactivity. (What actually manifests itself clinically depends, of course, not only on the array of allergens but also on the particular vulnerabilities and experiences of subjects. A heat-labile allergen in Apple, for example, would never affect someone who ate Apples only in pies. A heat-stable allergen in Apple, which occurs, would).

Certain genera such as Citrus (Grapefruit, Lemon, Lime, Mandarin, Orange) display cross-reactivity that is demonstrable at more or less the expected degree, but other genera simply do not show the expected cross-reactivity. The Rosaceae fruits (Apple, Apricot, Blackberry, Blueberry, Cherry, Peach, Pear, Plum, Raspberry, Rose hip, Strawberry) show cross-reactivity at a family level. These are the most important botanical relationships as far as demonstrated cross-reactivity is concerned, but the necessarily very incomplete nature of such an account should be kept in mind. The large number of fruit species, and the exotic history of many fruits, has meant that many species have not yet been adequately examined for their allergenic characteristics (6).

On the other hand, because of panallergens, there are strong patterns of cross-reactivity spanning distant, non-fruit relationships. These can be summarised under the headings of Latex-fruit cross-reactivity and pollen-fruit cross-reactivity.

Regarding the former, approximately 30-50% of individuals who are allergic to Natural rubber latex (NRL) show an associated hypersensitivity to some plant-derived foods, especially fresh fruits (7-9). An increasing number of plant sources, such as Avocado, Banana, Chestnut, Kiwi, Tomato, Potato and Bell pepper, have been associated with this syndrome (10-13). Chitinase appears to be the main panallergenic culprit in Latex-fruit cross-reactivity, but other panallergens play a role (8,14).

Regarding pollen-fruit syndrome, studies have reported cross-reactivity between Birch pollen and a number of foods, e.g., Apple, Pear, Melon, Hazelnut, Peach, Cherry, Plum, Celery, Carrot and Potato, with oral allergy syndrome and allergic rhinitis being the pre-dominant features, and profilin being the panallergen most frequently implicated (15-21). Subsequently, a number of allergens or panallergens have been identified, and this has shed light on causes and patterns. Profilin was originally considered to be unquestionably the most important factor, but LTP is now receiving significant attention (22-23).

Cross-reactivity may occur between fruits and pollens other than Birch. In a laboratory study, cross-allergenicity between Apple pulp and 5 pollen species, investigated by RAST inhibition, demonstrated that Apple pulp extract effectively inhibited RASTs to all the pollens except one, Japanese Cedar pollen (24). Similarly, a study reported on an association between grass pollen allergy and sensitisation to Tomato, Potato, Green pea, Peanut, Watermelon, Melon, Apple, Orange and Kiwi (25).

Pollen-fruit cross-reactivity is strongly (though not exclusively) characterised by oral allergy syndrome, which creates a fairly clear diagnostic guide. The particular symptoms of oral allergy syndrome (see above) should suggest to the clinician that he consider the involvement of a number of other fruits, and the probability of a co-existing allergic rhinitis to specific pollens. This is particularly relevant in the Northern Hemisphere, with its abundance of Birch, Mugwort and other implicated pollens.

Some panallergens, such as profilin, may result mostly in mild symptoms. However, others, and in particular lipid transfer proteins, are heat-stable and may result in severe reactions, including anaphylaxis; and importantly, they may be more prevalent in certain population groups than others (26). For example, Peach allergy has two different patterns: that of central Europe, with oral allergy syndrome (OAS), related to a primary sensitisation to Birch pollen Bet v 1 and profilins; and that of southern Europe, with mostly systemic symptoms, in many cases due to sensitisation to lipid transfer proteins (27-28).

References:

    1. Wikipedia contributors, ”Fruit,” Wikipedia, The Free Encyclopedia, http://en.wikipedia.org/w/index.php?title=Fruit&oldid=274065922 (accessed March 2, 2009)
    2. Wikipedia contributors, ”Vegetable,” Wikipedia, The Free Encyclopedia, http://en.wikipedia.org/w/index.php?title=Vegetable&oldid=274209697 (accessed March 2, 2009)
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