Peanut is a legume, native to South America. The peanut, Arachis hypogea, belongs to the Papilionacea family of the Fabales order, along with lentils, chickpeas, peas, beans and soy. Other common names for peanut are groundnut and earthnut due to the characteristic feature of the pods maturing underground. Peanut is a common snack food in westernised society, although it is a common weaning food in other societies. The world export market for peanut is 1.3 million metric tons per year (shelled basis), with the US, China and Argentina being the dominant exporters. Sixty percent of U.S. raw peanut exports are to the EU. There are three broad types of peanuts which account for more than 99% of US production: Flo-Runner, Virginia and Spanish. There are no obvious differences between them with respect to their potency to cause allergy.
Peanut allergy is a so-called IgE-mediated food allergy. IgE (Immunoglobulin E) is the allergy antibody. Allergy to peanut is caused by proteins in the kernel. As all seeds, peanuts are rich in protein; in particular the so-called storage proteins that serve as “source material” during the growth of a new plant. Some of these storage proteins have been shown to be the most important allergens: a vicilin and an albumin. In professional literature, these allergens are usually called Ara h 1 and Ara h 2 from the Latin name of peanut, Arachis hypogea. Both allergens are very stable during processing steps that involve heating, but significant differences have been reported depending on the method. Roasted peanuts appear to be more allergenic than boiled and fried peanuts.
Peanut is the commonest cause of fatal food-related allergic reactions. Allergic reactions to peanut show the typical range from mild local swelling and itching in the mouth and throat to severe life-threatening reactions including shortness of breath and drop of blood pressure (anaphylactic shock). In contrast to some other food allergies however, severe reactions seem to play a dominating role, even on first exposure. Symptoms like tingling of the lips are a clear warning for contact with peanut that might develop rapidly into a severe reaction. Proximity to peanut and even skin contact appear safe for most peanut allergic individuals, although there have been reports of reactions induced by inhalation and after minimal cutaneous contact and even kissing.
Related foods (cross-reactions)
As peanuts are legumes, there has been real concern that reactivity to other legumes would be common in peanut allergic subjects. This concern is based on the expectation that similar proteins in related foods will also cause reactions. Such reactions are referred to as cross-reactions. Indeed, storage proteins in e.g. soy and pea are very similar as their counterparts in peanut. In contrast to some other well known food allergies, cross-reactive allergy appears to be rare for legumes. Most peanut allergic individuals are tolerant of soy. This does not mean that absolutely no patients exist that demonstrate cross-reactive allergy to multiple legumes, but it is rare. Allergy to nuts from unrelated botanical families of trees, such as almond, walnut, hazelnut and Brazil nuts is more frequent among peanut allergic patients. Approximately thirty to fifty percent of individuals with peanut allergy will make IgE antibodies to one or more of these tree nuts. They are at risk of developing allergy to these nuts and should therefore be screened for that.
Who, when, how long and how often?
Up to 80% of peanut allergic children react to the first known exposure to peanut. Until now, it is unknown how these children developed allergy without direct contact with the food. One of the possibilities is that consumption by the pregnant and weaning mother brings the child in contact with peanut protein. Some investigations support this explanation but others do not find the same evidence. Whether avoidance by the mother is effective remains to be established by further studies. There is also some concern that exposure through the skin by the use eczema creams containing peanut oil in the early 1990’s might have contributed to the occurrence of peanut allergy. The UK Government have recently advised that soy allergic individuals should avoid peanut oil containing medications and visa versa, but the evidence for this recommendation is not secure at present.
In Westernized societies, the prevalence of peanut allergy among children is estimated to be around 0.5 to 1%. In contrast to what is observed for milk and egg allergy, most children do not outgrow their peanut allergy. It is estimated that around 75% of the cases are persistent. It is important to evaluate young children by challenges at regular intervals for outgrowth of their peanut allergy, because avoidance is a heavy burden.
The occurrence of peanut allergy appears to be increasing, especially in children. It is equally common across Northern Europe as in the USA, according to the increasing Americanisation of the European diet. Allergy to other legumes such as lentils and chick peas is more common in the Mediterranean region.
How much is too much?
Peanut was the first food to be formally examined to establish threshold doses. The lowest reported threshold doses for peanut appear to be below or near 1 milligram. The differences in the levels of threshold dose in these studies may relate to the preparation method of the peanut source used and standardised, multi-centred studies are underway. Nevertheless, one thing is clear; a really tiny piece of a peanut can be a threat to a peanut allergic patient. An average scaled peanut weighs between 500 and 1000 mg. This means that 1/1000 of a peanut is enough to trigger a reaction in some patients.
Diagnosis of peanut allergy starts with recording a clear clinical history to establish a link between allergic reactions and peanut. Skin prick tests and measurement of specific IgE levels are used to support a history-based suspicion of peanut allergy. The improved quality of these tests has considerably decreased the need for challenges with peanut to confirm allergy. In a hospital-based clinic population, positive skin prick tests appear to be very sensitive with sensitivity and positive predictive values above 90%. Nevertheless, the gold standard of diagnosis still is the double-blind placebo controlled food challenge with peanut. In this challenge increasing doses of peanut are administered to the patient as well as placebo meals not containing peanut. Both patient and doctor are unaware of the meals with and without peanut. Effective blinding of the taste of peanut is essential for such challenge procedures. Atopic subjects who are skin prick positive to peanut have been shown to have approximately a fifty percent chance of having clinical reactivity confirmed during a double blind food challenge. Young children with a positive skin prick test but with no history of reactivity to peanut need to be fully assessed, including a food challenge if necessary, before they enter the school environment.
Where do I find peanut?
Peanut has been used in many different forms as an individual food but also as a cheap filler or substitute for more expensive or exotic foods. Peanut flour is a very versatile product with relatively low carbohydrate and high protein content. It can constitute up to 4-8% of high protein bars and up to 15% of baked goods like (low-carbohydrate) cookies. It is widely applied in seasoning blends, sauces, and dressings. Although this latter practice has probably decreased recently with the increased awareness of peanut allergy, such less obvious use of peanut is still a significant risk for peanut allergic patients. Other products that might contain peanut are breakfast cereals, candies, chilli, egg rolls, ice cream, margarine, marzipan, milk formula, pesto sauce and soups. Peanut is often found in non-European dishes such as Chinese or Thai food prepared in restaurants for on-site consumption or home consumption. Peanut oil is a commonly used form of vegetable oil, but in the vast majority of cases this is highly refined peanut oil. Refined peanut oil appears to have a good safety profile in most countries, whereas crude peanut oil is likely to be allergenic for a substantial proportion of individuals.
Peanut-derived ingredients can be found in detergents for dishwashing and laundry, in hand soaps, shampoos, lotions, salves and crèmes, shaving cream, cosmetics, and even in metal polish, bleach, ink and axle grease. Contact with such products can cause allergic reactions in peanut allergic patients.
In the absence of any definitive treatment at present the mainstays of treatment for peanut allergy are vigilance and avoidance. The use of epinephrine, i.e. carrying an epi-pen, is essential to be able to act in case of an adverse reaction. Peanut is very difficult to avoid and it can be predicted that accidental exposures will happen. In many cases reactions are less severe after follow up and institution of a management plan, but that cannot be guaranteed as other circumstances may supervene such as dose and route of exposure, asthma status, exercise prior to ingestion, use of alcohol or pollen season.
Peanut allergic patients should in all cases carefully read labels on food products. According to recent EU legislation (2003/89/EC) and the list of the Codex Alimentarius Commission, the presence of peanut or peanut-derived ingredients should always be on the label. Despite this obligation, it is still advised to carefully study food labels. Other indications that peanut is a possible ingredient are ground nut, arachis oil, hydrolyzed plant protein, vegetable oil, vegetable protein, natural flavouring and oriental sauce.