The Australasian Society of Clinical Immunology and Allergy

Peanut and tree nut allergy is most common in infants, but may appear for the first time in adults. Peanut allergy causes more problems than other food allergies because it is common (1 in 50 infants), exposure is hard to avoid and in some cases even trace amounts can trigger symptoms. Around 20 % of cases resolve, and while severity may occasionally lessen with age, around 20% of cases can become worse with time.

Food proteins trigger food allergy

The substances triggering allergic reactions to food are proteins. If a person is allergic to one protein present in one food only (eg. peanut, dairy products, hen's egg), then an allergic reaction can only occur if they eat that one food. Some individuals may be allergic to more than one protein in more than one food and so may be allergic to several foods.  

What does allergy to a "nut" or seed really mean?

The meaning of the terms nuts, seeds and legumes is confusing, particularly for allergic patients (or the parents) trying to decide what foods to avoid.  For example, the term "legume" is often used to describe peanut as well as other plants like peas, chickpeas and soy yet this group of plants also includes wattles and the black bean tree of Queensland. 

The term "tree nuts" also has limited meaning, as the foods that we consume from these plants come from a wide variety of different botanical families such as Rosaceae (almonds), Anacardiaceae (cashews), Proteaceae (macadamia nuts) or Lecythidaceae (Brazil nuts). 

When we think of seeds, we often think of small seeds like sesame seed, sunflower seed, poppy seed or pumpkin seed.  In fact, coconut (including the husk and inner white flesh that we eat) is also a seed, albeit a very large one! Many of the foods that we considered to be "nuts" are in fact part of a seed or its food source, often with the outer fruit or coating removed.

So while we often use the terms "tree nuts" or "seeds" may be used as short-hand to describe some foods, these categories are rarely useful for predicting allergy to foods of similar appearance or taste.  With few exceptions (eg. most people allergic to cashew are also allergic to pistachio), it is not possible to reliably predict the likelihood of allergy to seed or nut-like food without allergy testing to that particular food.

Peanut is not a nut!

Peanut is a legume, like peas, lentils and chickpeas (but also diverse other plants like wattles and the black bean tree of Queensland). The proteins in peanut are very different to those in tree nuts (like almonds, brazil nuts, cashews, hazelnut, macadamia nuts, pecans, pistachios or walnuts). So someone allergic to peanut is not automatically going to be allergic to "tree nuts", but may be if they are allergic to proteins present in more than one food.

Peanut allergy is becoming more common

Two studies from the USA and UK have shown that peanut allergy doubled over a 5-year period. Peanut allergy is estimated now affect 1/50 young infants, and tree nut allergy also seem more common. There are many theories as to why this increase has occurred, but at this time, we have few answers.

Allergies may occur to one or many foods

Even though peanut and tree nuts (like almond, brazil nut or cashew) make look and taste much the same, the proteins present in peanuts are generally considered to be very different to those in tree nuts like almonds, cashews and so on. That means that one can be allergic to peanut only, a tree nut only, several tree nuts or any combination of peanut and a few tree nuts, or they can be allergic to anything that looks or tastes like a peanut or tree nut!

Cross-reactivity is difficult to understand and harder to predict

Cross-reactivity means that a similar protein is present in a range of different foods. If the same protein is present in several foods, then that person may have allergic reactions to any food containing that protein. Examples of cross-reactivity include people allergic to similar proteins present in hen's egg and duck eggs; or cow's milk and goat's milk; or cashew nut and pistachio nut. Unfortunately, it is sometimes difficult to predict whether a person will be allergic to one unique protein allergen present in one food only, or several similar "cross-reactive" proteins present in multiple foods, simply based on whether foods have a similar appearance. The bottom line: it is not possible to reliably predict the likelihood of allergy to seed or nut-like food without allergy testing to that particular food.

Allergic reactions to peanut or tree nuts or seeds can sometimes be serious

The majority of allergic reactions to peanut and tree nuts are mild. Contact hives, worse eczema and vomiting are the most common complaints. Some sensitive individuals develop difficulty breathing due to asthma or throat swelling, or a drop in blood pressure. This is known as anaphylaxis, and allergy to peanut/tree nuts is one of the most common triggers.

Other allergies may also be present

Food allergy is more common in people who have other allergies like hay fever, asthma or eczema. As many children have allergies to other foods such as milk, egg or other nuts, your doctor may test for these allergies as well.

Reliable diagnosis of food allergy is important

Your doctor will normally ask a series of questions that may help to narrow down the list of likely causes of allergy such as foods or medicines consumed that day, or exposure to stinging insects. This approach will also help to exclude conditions that can sometimes be confused with food allergy. Skin or blood (RAST, ImmunoCap) allergy testing helps confirm or exclude potential triggers. While the results of allergy testing are a guide to whether the person is allergic, they are NOT a reliable guide to whether the reaction will be mild or severe.

Food allergy does not usually run in families

Most of the time, children with food allergy do not have parents with food allergy. If a family has one child with food allergy, however, their brothers and sisters are at a slightly higher risk of having food allergy themselves, although that risk is still relatively low. Some parents want to have their other children "screened" for food allergy. If the test is negative, that may be reassuring, but does not mean that the other child will never develop an allergy. If their screening test is positive, however, it is not always clear whether it definitely represents allergy, or whether the sensitization detected by the test is clinically irrelevant. The term false positive" is sometimes used to describe this.

A positive allergy test is not the same as being food allergic

It is important to know that a positive skin or blood allergy tests means that the body's immune system has produced a response to a food, but sometimes these are "false positives". In other words, the test may be positive yet the person can actually eat the food without a problem. For that reason, it is important to eventually confirm the significance of a positive allergy test (in some circumstances) with a deliberate supervised challenge. In a child with a positive test of uncertain meaning, this is often done around school age under medical supervision. Interpretation of test results (and whether challenge should be undertaken) will be discussed by your doctor.

Unorthodox so-called "allergy tests" are unproven

There are several methods of unorthodox "tests" for food allergy. Examples include cytotoxic food testing, Vega testing, kinesiology, iridology, pulse testing, Alcat testing and Rinkel's intradermal skin testing. These are unreliable, have no scientific basis and have no useful role in the assessment of allergy. So if you suspect a possible food allergy, have the suspicion confirmed with a reliable test.

Peanuts, tree nuts and seeds are hard to avoid

Peanuts are widely used in processed Western and in Oriental cooking. This poses significant problems for people with severe peanut/tree nut allergy. Laws require that any product, which contains peanut/tree nuts, must be labeled to that effect, so the labels of all foods should be checked before purchase. Some manufacturers will also label their products as possibly containing traces of nuts, or with similar phrases such as manufactured on the same machinery. This labeling is not a legal requirement but may be added by the manufacturer. In such situations, multiple products may be made on the same production line and cross-contamination with traces of nuts cannot be guaranteed. The risk of having a serious allergic  reaction from cross contamination is greatest for those with a history of severe food allergies. Occasionally nut products or oils have been used as unlabelled ingredients in cosmetics such as massage oils. For example, the term Arachnis oil on the label is the scientific name for peanut.

The following list provides some examples of foods that can contain peanut. While peanut may not always be present, foods in this list should be examined carefully to ensure that no traces of peanuts are present.

Foods that may contain peanuts/tree nuts/seeds:  

Food that may contain tree nuts

Types of foods containing tree nuts include mixed nuts, some Asian foods, some commercial pasta sauces, some pestos (which may have other nuts as well as pine nuts), dips and savouries, slices or cakes,  biscuits, some cereals, chocolates, muesli or health bars and almond icing. Many forget that almond icing or marzipan is nut-derived! Occasionally nut products or oils have been used as unlabelled ingredients in cosmetics such as massage oils or even toothpaste and moisturisers.

Eating out with peanut/tree nut allergy

While you can never totally remove the risk of accidental exposure to your food allergic trigger, some simple precautions will dramatically reduce the risk. First, ring ahead! Ring your friends or the restaurant that you plan to visit and let them know of your allergy in advance. When you get there, ask to talk to the manager about any dishes that should be avoided, and ask them to let the chef know so they can take extra care in preparing your meal to reduce the risk of cross contamination. Don't just rely on the menu descriptions of what is in the food! Some people even have a small snack before they go out, not to replace the meal, but so that they are not so hungry as to fall and devour any food put in front of them without thinking first!

There are many traps for the unwary. Remember that pestos and dips may have nuts as an ingredient. And many trendy salads have nuts added for texture. Sometimes nuts can be added to gravies and sauces too. Think of the methods of cooking; are there really several woks that can be used in the kitchen, or only one?

Stop before eating food that you have not prepared yourself. In teenagers or adults who are eating out, cautiously "touch-test" a trace of food on your outer lip before putting it into your mouth. Tell-tale warnings of a burning/chilli-like reaction, tingling or swelling should alert you to the possibility that food allergen is present.

Finally, if you have been prescribed emergency medication, always have it with you!

Other foods may also cause allergic reactions

Despite the name, "peanut", they are actually legumes, coming from the same family as soy, lentils, chickpeas, kidney beans and peas. Fortunately, the majority of peanut allergic people can eat these other foods with safety. There is some evidence that those allergic to peanut may be at increased risk of allergy to lupin, a bean (and another legume) that is sometimes added to baked goods like bread and sometimes confectionary as a source of protein. Its use in Australasia is currently less common than in Europe but at this time, its presence is not indicated on food labels in Australasia. At this time it appears that some patients allergic to both foods are allergic to the same proteins present in both foods (ie. cross-reactive response) whereas others may be allergic to distinctly different proteins in the different foods.

Other nuts and seeds can cause severe allergy

Even though there is little similarity between peanut allergens and those present in tree nuts (like walnut, almond, pecan, pistachio or cashew), there is an increased risk of other food allergies in peanut allergic children. Peanut/tree nut and seed avoidance strategies advised will largely be dictated by choking hazards in infants, the risks of cross-contamination or substitution of one nut for another in commercially-prepared foods, and the potential for confusion in young children (and care givers) trying to differentiate one "nut" product from another. For this reason, it is usually recommended that peanut allergic people avoid all "nuts", and some doctors also recommend that they avoid seeds as well (eg. sunflower seed, sesame seed or poppy seeds).  Unfortunately it is not clear whether such advice will prevent new food allergies from developing.

Highly purified nut oil contains little allergen

Refined peanut oils (not cold-pressed) have been shown to be safe in small studies. Unfortunately, it is difficult to guarantee that the oil is sufficiently refined to remove all traces of peanut protein, which is the trigger for allergic reactions. This particularly applies to restaurants which use peanut oils for cooking, as peanut proteins may leach into the oil during cooking, and the oil may be re-used a number of times. In general therefore, avoidance of peanut oil is frequently advised. When considering the safety of tree nut oils or seeds oils (like sesame), little work has been done in this area to prove safety, so in practical terms, it is often easiest to avoid nut oils too.

The natural history of peanut/tree nut allergy

Over time, around 20% of children with peanut/tree nut allergy grow out of their allergy, around 20% actually get worse (ie. go from having a mild to a potentially dangerous allergic reaction) and the remaining 60% stay much the same in terms of severity. It is not possible to reliably predict who might get better or worse over time. If allergy persists into teenage or adult life, it is very unlikely for that allergy to disappear.

New allergies can develop

Some people with peanut/tree nut allergy will develop a new allergy over time to a similar or related food. While many people with a single peanut/tree nut allergy are often advised to avoid all nut-like foods (and sometimes seeds as well), it is not clear whether this will actually prevent a new allergy from developing.

Avoidance is the only proven treatment for peanut/tree nut allergy

The only proven treatment for peanut/tree nut/seed allergy at this time is avoidance. It is therefore fortunate that omitting peanuts from the diet has no adverse nutritional consequences. Children should take their own nut free food with them to school. They should also be encouraged not to swap/share food. In common eating and food preparation areas, where there are children with severe peanut allergy, nut-containing foods are best avoided. In day care centres and pre-schools with very young children where the risk of food contamination of common eating areas or toys is higher, some centres ban bringing some foods to reduce the risk. This is not a policy that is considered necessary when caring for older children, although the use of nut containing foods in cooking classes and science experiments is discouraged if there are students with peanut/tree nut allergy in that class.

Research into food allergy is ongoing

The increased frequency of peanut/tree nut allergy is driving research into areas trying to find out why it has become more common, and how to treat it. Current areas of research include studies to determine whether early exposure to peanut might actually reduce the risk of allergy developing (http://www.leapstudy.co.uk/index.html ), trying to genetically engineer peanuts to remove the proteins that trigger allergies, or immunotherapy/desensitization ("allergy vaccines") to "switch off" the allergy once it has developed.

Action Plans are essential

The average nut-allergic person will have an accidental exposure every few years, even when they are very careful to avoid their trigger. The difficulties of avoiding peanuts completely make it essential to make back-up Action Plans, and Anaphylaxis Action Plans when EpiPen has been recommended. These are discussed in further details in the article Management of Food Allergy.

Nut allergy can be effectively managed

The good news is that in concert with a trained allergist and a network of supportive contacts, people with allergy to peanuts, tree nuts or seeds can learn to live with their condition. The knowledge that EpiPen (an automatic device for administering adrenaline) is available offers reassurance, but is not a substitute for strategies to minimize the risk of exposure. Research continues to explore new ways of more effectively treating this condition. Anaphylaxis Australia (http://www.allergyfacts.org.au/) offers valuable updates and tips for dealing positively with food allergies.

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Disclaimer

The content of this article has been reviewed by ASCIA members, represents the available published literature at the time of review and is not intended to replace professional medical advice. Any questions regarding a medical diagnosis or treatment should be directed to a medical practitioner.

For further information on allergy, asthma or immune diseases,
visit http://www.allergy.org.au/- the web site of the Australasian Society of Clinical Immunology and Allergy (ASCIA).

ASCIA is the peak professional body of Clinical  Immunologists and Allergists in Australia and New Zealand.

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Content Last Updated 20 June 2008