Latin name: Allium cepa
Source material: Freeze-dried onion
Family: Alliaceae (Liliaceae)
Common names: Onion, Garden Onion, Shallot
Only a few members of this large family are important as food plants, notably Onion, Garlic, chive, leek, and rakkyo. Onion is known only in cultivation, although related wild species occur in Central Asia. References to Onions date back 3,000 years, and they were probably among the earliest cultivated vegetables. They are the most important of the bulb crops. They are in use worldwide, but are best grown in Mediterranean climates.
Most Onions are grown only for the mature edible bulbs, but there are some cultivars, the Spring or Green onions, or Scallions, that are eaten immature, along with the leaves. The Dry onions are mature Onions with a juicy flesh covered with dry, papery skin. There are 2 categories of Dry onions: Storage onions and Sweet onions. Storage onions are low in water and high in sulphur, so they store well and are available year-round. Sweet onions are usually available only in spring and summer. Storage onions are more pungent and flavourful than Sweet onions.
Edible Onions are not known in the wild, but are grown only in cultivated beds. The bulb can be eaten raw in salads, sandwich fillings, etc., or cooked or preserved in a variety of ways. Fresh, dried, pickled, canned or even frozen, it is the most common flavouring in many cuisines. The leaves, flowers and sprouted seeds are also eaten.
Onion is used as a homeopathic remedy for a variety of conditions. Onion juice rubbed into the skin is said to be a remedy for baldness. The growing plant is said to repel insects and moles and can therefor be rubbed onto the skin.
The plant juice can be used as a rust preventative on metals and as a polish for copper and glass. A yellow-brown dye is obtained from the skins.
A 12 kDa protein band to young Garlic, mature Garlic, Onion, and leek extracts has been detected in a Garlic-allergic individual. Similar bands could also be detected with Mugwort pollen and Hazelnut extract (1).
A heat-labile allergen has been detected (2).
The following allergens have been characterised:
- All c 3, a 12 kDa lipid transfer protein (3).
- All c 4, a profilin (4).
- All c alliin lyase (5).
Diallyl disulphide is a major allergen in Garlic and Onion, causing contact dermatitis. Diallyl disulphide penetrates most commercially available glove types, and therefore patients with Onion contact dermatitis may not be protected by most commercially available gloves (6).
An extensive cross-reactivity among the different individual species of the genus could be expected, as well as to a certain degree among members of the family Alliaceae (previously categorised as Liliaceae), including Onion, Leek, Garlic, Asparagus, and Chives, but the level of cross-reactivity varies among individuals (7-8).
Onion contains a lipid transfer protein (LTP), All c 3, which may result in cross-reactivity with other lipid transfer protein-containing foods. The LTP has been detected to young Garlic, mature Garlic, Onion, and Leek extracts, and was similar to that detected in Mugwort pollen and Hazelnut extract (1). In a study examining the relationship between Peach LTP-specific IgE levels and cross-reactivity to several non-Rosaceae plant-derived foods, increasing levels of IgE to Peach LTP were associated with skin reactivity to nuts (72%), Peanut (67%), Maize (41%), Rice (36%), Onion (35%), Orange (28%), Celery (27%), and Tomato (20%) (9).
Onion contains a profilin, All c 4, which may result in cross-reactivity with other profilin-containing plants (4). However, the clinical significance of this has not been elucidated to date.
Garlic alliin lyase showed strong cross-reactivity with alliin lyases from other Allium species, namely Leek and Onion (5).
The presence of structurally similar allergens in Garlic, Onion, and certain pollens of Phleum and Chenopodium has been described. There was partial abolishment of the IgE binding to several of these allergens (10). The clinical significance of this is not yet known.
Onion can induce symptoms of food allergy, asthma, rhinoconjunctivitis, and contact dermatitis in sensitised individuals (2-3,11-16). Onion is one of the commonest causes of contact dermatitis of the hands (17-18).
In a European study of 589 individuals with a history of food allergy, skin reactivity to Alliaceae (Garlic, Onion, Chive) was documented in 4.6% of children and 7.7% of adults (19). A study was conducted at 17 clinics in 15 European cities to describe the differences among some Northern countries regarding what foods, according to the patients, elicit hypersensitivity symptoms. It was found that Onion was the 56th most frequently reported food, and responsible for symptoms in 8.2% of 1,139 participants (20). In an Indian study of 24 children aged 3 to 15 years with documented deterioration in control of their perennial asthma, IgE antibodies for Onion were detected in 6 (25%) (21).
A number of case reports illustrate the range of clinical effects experienced with Onion.
A 19-year-old atopic woman who experienced urticaria after ingestion of raw Onions was described. She had suffered with rhinoconjunctivitis and contact urticaria to Mugwort for the previous 2 years, and she described the subsequent onset of oral pruritus after ingestion of raw Onion and Peach peel. Skin prick tests were positive for, among other allergens, Mugwort, Onion, and Peach. IgE antibody tests were positive to Onion (5.41 kUA/l), Peach (18 kUA/l), Mugwort (0.86 kUA/l), and Pru p 3 (23.07 kUA/l). IgE immunoblotting of Onion extract revealed only a 12-kDa IgE-binding protein band, identified as a lipid transfer protein. The authors pointed out that, while the clinical history suggested Mugwort LTP as the primary sensitising agent, laboratory results indicated that Mugwort pollen allergy was independent of Onion and Peach food allergy (3).
A 45-year-old man reported at least 5 episodes, over several years, of severe, systemic urticaria/angioedema some minutes after eating raw Onion. He tolerated cooked Onion as well as other fruits and vegetables, except for Peach, which he had avoided since childhood because of the oral allergy syndrome and gastric pain it induced. Skin prick tests were positive to, among other allergens, commercial extracts of Garlic and Onion, and Peach peel extract. The patient’s serum strongly reacted against both Peach LTP and Onion extract. Analysis showed the patient’s IgE was reactive to both a single band at approximately 15 kDa and to a double band at approximately 43 kDa in Onion extract. The former was shown to be an LTP. However, IgE reactivity to Onion proteins at other molecular weights was also demonstrated. As none of more than 50 patients hypersensitive to LTP reported Onion allergy, it was suggested that the 43 kDa band might have been the relevant allergen (13,22).
A study describes a number of patients with asthma, rhinoconjunctivitis, and/or contact dermatitis caused by Onion, illustrating the range of symptoms and clinical patterns of Onion allergy. Patient 1, a 45-year-old homemaker who for years had experienced episodes of rhinoconjunctivitis, dyspnoea, and coughing when chopping Onion went on to develop eczematous dermatitis, occurring mainly on the fingertips and exacerbated when she was chopping Onion. Patient 2, a 37-year-old female with rhinoconjunctivitis and asthma induced by pollen allergy, had for 15 years noticed intense rhinoconjunctival symptoms and slight dyspnoea when chopping Onion. Patients 3 and 4 were revealed when skin prick tests were performed on 106 randomly selected clinic patients, of whom 39 were atopic. Skin prick tests for Onion were positive in 8 (7.5%), of which all were sensitive to grass pollen. Patient 3, a 31-year-old man, and Patient 4, a 38-year-old female, had intense rhinoconjunctivitis, and 1 of the 2 also experienced chest tightness, wheezing, and dyspnoea after being exposed to the aerosols from Onion. All 4 patients were positive on skin prick testing with fresh Onion, Garlic, fried Onion, and leek. All were also positive to heated and non-heated Onion extract. Patients 2, 3, and 4 also had a positive skin test reaction to grass pollen. None of the 4 patients experienced a delayed skin reaction. Bronchial provocation tests with Onion extract in Patient 1 resulted in an immediate asthmatic response, and in Patient 3 at 10 minutes after challenge. Patients 2 and 4 were not challenged. A nasal provocation test resulted in an immediate nasal reaction in Patient 3. Double-blind oral provocation tests carried out in patients 1 and 3 were negative (with up to 2 gm of Onion powder).
Patch tests done in patient 1 were positive. IgE antibody levels for Onion were positive in 3 patients (Patients 1 [1.4 PRU/ml], 3 [0.7 PRU/ml], and 4 [2.2 PRU/ml]). The authors concluded that the respiratory reactions occurring in Patients 1, 3, and 4 were IgE-mediated and that cell-mediated (type IV) mechanisms were involved in the contact dermatitis in Patient 1. The authors suggested that, since among 106 persons randomly selected for the study, 2 individuals had been found to have clinical Onion allergy, and 6 more were sensitised, Onion allergy was not as uncommon a condition as had been previously thought (11).
A 44-year-old woman experienced, over the last four years, immediate reactions (at least 5-6 episodes) occurring within minutes after eating raw or lightly cooked Onion. The reactions had become increasingly severe with intense itching in the mouth followed by a state of confusion, blurred vision, transient loss of consciousness, profuse sweating, tachycardia and subsequent transient prostration. The last episode had been the most serious, lasting several hours, and with the development of urticaria on the face and the neck. During the last few months she reported intolerance even simply to contact with raw Onion and to inhalation of the steam of Onion during cooking. She tolerated well-cooked Onion. She was shown to be monosensitised to Onion, unlike other cases in the literature. IgE antibody level for Onion was 3.7 kUA/l. Her IgE antibodies recognised only thermolabile Onion fractions. She was not cross-reactive to other foods from the Liliaceae (Alliaceae) family (2).
Food-dependent exercise-induced anaphylaxis following the ingestion of Onion has been reported (23-24). A report describes a 26-year-old woman who, in a single year, experienced at least 10 episodes of generalised erythema with itching, urticaria, angioedema of the face, profuse sweating, malaise, chest tightness, wheezing and dyspnoea. Running precipitated the episodes (19).
Onion has also been shown to be a common cause of eosinophilic oesophagitis in adults. In a study of men and women aged 18-57 years, 17 of 21 were polysensitised to several different environmental allergens, and 19 of 23 (82%) had IgE antibodies specific for 1 or more food-associated allergens, with Wheat, Tomato, Carrot, and Onion identified most commonly (25).
The seeds of Onion have been reported as a new occupational allergen. A 27-year-old female developed episodes of rhinoconjunctivitis followed asthma attacks, which she attributed to contact with Red onion seeds (A. cepa, Brunswick variety). She had worked for 3 years in packaging of the seeds. Mild symptoms were experienced on contact with Onion seed other than Red onion seed. Handling of other seeds, including those of other members of the Liliaceae family (Asparagus, Leek) did not elicit symptoms. She was able to touch and eat raw and cooked Onion without experiencing symptoms. IgE antibody test to Onion was negative. Three different forms of Red onion extracts were created; skin prick tests with Red onion extract 3 and Italian and White Onion seed extracts were positive, and greater with the violet variety. Skin prick test with Red onion extract 1, Leek, Asparagus, and other non-Liliaceae seed extracts, and common aeroallergens, were negative. A bronchoprovocation test was positive (26).
When Onion tissue is damaged, an enzyme reaction releases sulphur-containing volatile compounds, which give Onion its characteristic flavour and lacrimatory properties. This special characteristic of the Onion, causing tearing and rhinorrhea in persons who slice it, is due to thiopropanal-s-oxide, a sulfur compound with irritative characteristics, which is released when the Onion is cut (27).
Onions are high in indigestible carbo-hydrates that may contribute to flatus. Onions are also rich in etheric oils and other irritants. There have been cases of poisoning caused by the consumption, in large quantities and by some mammals, of this plant. Dogs seem to be particularly susceptible. Onions can prevent blood clotting, but can trigger migraine in susceptible people.
Contact dermatitis and eczema have occurred in housewives, and in greengrocers, cooks and other food industry workers (28-31). A worker in the spice industry, exposed to Onion and Garlic dust, developed rhinitis. Skin-prick tests were positive for Onion, Garlic powder and other fresh Liliaceae (not specified). Presence of IgE antibodies for Garlic and Onion was demonstrated. Nasal challenge resulted in an increase in inspiratory nasal resistance for both Onion extract and Garlic (32).
The triggering of asthma after eating pickled Onions due to the preservative sulphur dioxide has been reported (33). A female Onion and Potato sorter developed hypersensitivity pneumonitis as a result of exposure to Penicillium species and Fusarium solani found in the workplace (34).
Onions can be a potent and long-lasting refluxogenic agent in heartburn patients (35).
Maternal intake of Cabbage, Cauliflower, Broccoli, Cow’s milk, Onion, and Chocolate were significantly related to colic symptoms in exclusively breast-fed infants (36).
Compiled by Dr Harris Steinman, firstname.lastname@example.org
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