Peanuts, pulses and tree nuts

Peanuts cause more life-threatening allergic reactions than any other food. They're a pulse (or legume), like peas, rather than a true nut. Foods and ingredients to avoid include, arachis oil and some hydrolysed vegetable proteins. Baked foods, sauces, cereal bars can contain hidden peanut ingredients. Many people with a peanut allergy will also react to tree nuts, such as Brazil, walnuts or hazelnuts.

Milk and dairy foods

A milk allergy is the most common food allergy in children, but most outgrow it by the age of three. Infants may need hypoallergenic (extensively or fully hydrolysed formula available on prescription), Milk proteins, including casein, are widely found in processed foods, even found in tinned tuna. If you're allergic to cow's milk, goat's milk may not be safe, either. People who are allergic to cow’s milk react to the milk protein. This is not the same as an inability to digest the milk sugar (lactose) as in lactose intolerance, which is not an allergy, but a digestive problem.

Eggs

Eggs are the second most common cause of food allergy in children, although they usually outgrow this allergy, as well. Read the labels carefully for noodles, mayonnaise and baked foods. Eggs can also be found in some unlikely products: the foam topping in drinks or the fondant icing inside chocolate eggs. Eggs are used to produce the flu vaccine, so seek medical advice before having a flu jab.

Shellfish

An allergy to shellfish most often develops in adulthood, and it is a lifelong allergy. Prawn, crab and lobster - crustaceans - produce the most severe allergic reactions. Molluscs can trigger reactions, too: clams, mussels, scallops, escargot, octopus and squid. People allergic to shellfish should avoid areas where shellfish is being cooked because even the vapours can trigger a reaction.

Tree nuts

Walnuts, almonds, pecans, hazelnuts, cashews, pistachios, Brazil nuts and pine nuts may all be off limits if you have a tree nut allergy. These must be clearly labelled in packaged foods, but nuts are more difficult to avoid in restaurants and bakeries. Nutmeg, water chestnuts, sunflower seeds are not nuts and can be eaten safely. Be aware that tree nut oils, such as shea oil, may be used in skin lotions.

Fish

The protein in fish can cause severe allergies, most commonly from eating salmon, tuna or halibut. If you’re allergic to one type, you may react to others, too. Many Thai and Chinese restaurants flavour dishes with fish sauces. Beware of Caesar dressing and Worcestershire sauce, made from anchovies. Some people can safely eat tinned tuna and salmon, but not fresh fish.

Soya

If you’re allergic to soya, you need to read the fine print very carefully on food labels. Soya protein is widely used in breads, biscuits, tinned soups, processed meats and snack foods. Foods to avoid include edamame, (young green soybeans), tofu, soya milk, miso and soy sauce. Most people with soya allergy can still eat soy oil and soya lecithin. Soya allergy is more common among babies and children.

Wheat

Wheat allergy is an allergic reaction to one of four proteins in wheat. You do not necessarily react to the gluten. People allergic to wheat can usually eat other grains, including barley, oats, rye, corn and rice. Wheat allergy is more common among children and is often outgrown. Bulgur, couscous and farina contain wheat protein, and many products, including beer, salad dressing and processed meats, may contain wheat.

Gluten intolerance - Coeliac disease

People with coeliac disease have an autoimmune disorder (not an allergy) that is triggered by gluten, a protein in wheat, rye and barley. Some people are also sensitive to oats. Eating gluten actually damages the small intestine. Symptoms include diarrhoea, bloating, weight loss, chronic fatigue and weakness, but not the skin rashes or wheezing often seen with a food allergy. Coeliac disease can be diagnosed with a blood test and an intestinal biopsy.

How a food allergy begins

With the first exposure to a trigger food, your body treats it as something harmful and creates immunoglobulin E antibodies to target the threat.. You won’t notice symptoms at the first exposure, but your body is primed to release histamine the next time. Although some food allergies are more common among young children, food allergies can develop at any time in life.

Food allergy symptoms

Symptoms usually occur within a few minutes to two hours after eating the food. Reactions range from mild to severe and can include:

• Hives or other skin rash
• Tingling or itching in the mouth
• Swelling of face, tongue or lips
• Coughing or wheezing
• Vomiting, diarrhoea or abdominal cramps
• Swelling of throat and vocal cords
• Difficulty breathing

Anaphylaxis: Severe reaction

Mild symptoms can sometimes progress to a dangerous condition known as anaphylaxis, so it’s important to act quickly. This life-threatening reaction involves constricted airways, throat swelling that may cause suffocation and a large drop in blood pressure. You will be recommended to have an adrenaline injection kit that you should carry with you at all times. Always make sure the one you have is within its use-by date. It is also a good idea to wear a MedicAlert bracelet or pendant, or carry a card that identifies your allergy.

Myth: Food allergy is predictable

If one bite of seafood went down OK last time, will you always be safe with one bite? It’s possible, but in general, your reaction will depend on the extent of your allergy and the amount of the food you eat. Reactions can be unpredictable, so you may have a rash once but vomiting or breathing problems on a different occasion.

Contrast: Food intolerance

Trouble digesting a food isn’t the same thing as a food allergy. An allergy occurs as an immune reaction. Food intolerance, such as lactose intolerance, can cause bloating, cramps and diarrhoea but it doesn’t cause an immune system reaction. Lactose intolerance occurs when the body doesn’t produce enough lactase, an enzyme that breaks down lactose, the sugar found in milk and dairy products.

Contrast: Food additives

A reaction to food additives also can be confused with food allergy. Monosodium glutamate (MSG), a flavour enhancer, can cause sweating, warmth, headache and chest discomfort. Sulphites, sometimes used to prevent mould growth, can cause breathing problems for people who have asthma. Sulphites also are found in wine and other products. EU rules require pre-packed foods in the UK to show clearly on the label if it contains sulphur dioxide or sulphites over a certain level.

Variant: Oral allergy syndrome

An allergy to certain raw (not cooked) fruits and vegetables is known as oral allergy syndrome. It occurs most often in people who have hayfever, especially hayfever triggered by grasses, weeds and trees (especially birch). Raw produce such as apples, cherries, kiwis, celery, tomatoes and green peppers may cause tingling, itching or swelling of the lips, tongue or throat, watery or itchy eyes, a runny nose and sneezing.

Exercise-induced food allergy

This food allergy only occurs when the food is eaten just before exercising. The rise in body temperature and the food trigger the reaction, which can range from itching, rash or lightheadedness to anaphylaxis. The foods most commonly associated with exercise-induced allergy are shellfish, alcohol, tomatoes, cheese and celery.

Elimination diet

Sometimes a reaction is immediate after eating a particular food, but if you’re not sure what caused your reaction, you may begin by keeping a diet diary. In an elimination diet, you eliminate one food at a time from your diet. This may help you work out which food is causing your problem. An elimination, or exclusion, diet is best performed under medical supervision.

Allergy testing

In a prick skin test, an allergy specialist places a drop of solution on your skin, then pricks the skin to allow it to penetrate. A negative result means you are probably not allergic, but sometimes there are false-positive results. A blood test measures antibodies to a particular food and also can produce false positives. Your doctor may also refer you for a medically supervised food challenge test to see if you react to a food.

Outgrowing allergies

Children are likely to outgrow allergies to milk, egg, wheat and soya but to have lifelong allergies to peanuts, tree nuts, fish and shellfish. Blood tests for food-specific antibodies may help determine whether the child has outgrown an allergy. A doctor may supervise a "food challenge test" to see if the child has outgrown the allergy. Do not try a food challenge on your own. Even a small amount of a food may produce a life-threatening reaction in some people.

Living with food allergies

There is no cure for food allergies, so you need to avoid the food that causes a reaction. EU directives on allergen labelling require the declaration of specified allergenic foods and ingredients. Call 999 at the first signs of anaphylaxis (wheezing, trouble breathing, dizziness) and use an adrenaline injection. Maintain a food allergy emergency care plan for yourself or your allergic child. Consider wearing a medical ID bracelet indicating the allergy.

Related reading

• Best diet for allergies
• Food symptom diary
• Is it allergy or intolerance?
• Feeding baby: Avoiding allergies
• Living with nut allergy
• Food allergies and asthma