Everything you need to know about food allergies

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Concerned you might have a food allergy? We look at the causes, symptoms and treatment options for food allergies.

Medically reviewed by Dr Roger Henderson and words by Annie Hayes

Tingling mouth? Red, itchy rash? Nausea? If you experience any of these symptoms after eating a specific food, your first thought might be ‘food allergy’. A food allergy occurs when your immune system overreacts to certain proteins in food, identifying them as threatening and triggering a protective response.

While food allergies can be fatal and should always be taken extremely seriously, diagnosed allergies are rarer than you might think. Around 20 per cent of Brits believe they are allergic to a certain food, but only between one and 10 per cent actually are. The remainder are likely to be intolerant, rather than allergic – and there’s a big difference between the two.

Dr Carrie Ruxton, dietician for the Health and Food Supplements Information Service and Shaeeb Ali, advanced clinical practitioner and pharmacist prescriber at MedsOnline247 explain what a food allergy is, what causes it and how food allergy differs from food intolerance:

What is a food allergy?

A food allergy occurs when your immune system reacts unusually to specific foods. This causes inflammation of the body’s tissues, which in some cases can be life-threatening. You develop an allergy when your immune system overreacts to certain proteins in a given food and classes them as potential pathogens. These proteins are then known as allergens.

  • Immunoglobulin E (IgE) food allergy

In response to the offending foods, your body produces an antibody called immunoglobulin E (IgE), which attaches to mast cells in your body. They exist in all bodily tissues, but especially in your nose, throat, lungs, skin, and digestive tract. The next time you eat the food, the allergens interact with the antibody, triggering the mast cells to release chemicals like histamine. These chemicals cause different symptoms, depending on which parts of the body the mast cells are in.

  • IgE-mediated food allergy

Some food allergens aren’t broken down by cooking or digestive enzymes, which means they can enter your bloodstream and trigger a cascade of reactions throughout the body. This is what’s known as an IgE-mediated food allergy, and symptoms can appear ‘from a few minutes after ingesting the allergen to a few hours after, followed by a second wave that can last for several hours and sometimes even longer,’ says Ali.

  • Non-IgE-mediated food allergy

A less-common type of reaction is called non-IgE-mediated food allergy. This means the allergic reaction isn’t caused by immunoglobulin E, but by other cells in the immune system. It’s often difficult to diagnose, as symptoms can take hours to develop. In some cases, people may experience symptoms caused by both IgE-mediated and non-IgE-mediated food allergies.

What is a food intolerance?

When food intolerance occurs, your immune system isn’t responsible. Often it’s your gut. Not being able to digest milk effectively – known as lactose intolerance – differs considerably from a full-blown dairy food allergy.

Often, an intolerance will be limited to the digestive system, perhaps involving pain, indigestion and diarrhoea. You might also experience additional symptoms, such as joint pain and headache, but none are life-threatening.

What causes a food allergy?

While food allergies are most common among babies and children, they can appear at any age. You can even develop an allergy to foods you have eaten for years with no prior issue. The causes aren’t fully understood, but experts believe it involves a combination of genetics and a trigger event, such as a virus, says Dr Ruxton.

‘Studies show that the risks of developing allergies and intolerances are greater when you have unbalanced gut bacteria, low levels of outdoor exercise, are obese, and have used antibiotics a lot,’ she says. ‘In children, C-section births, formula feeding and late weaning onto narrow diets are all linked with a higher risk.’

The risks of developing allergies and intolerances are greater when you have unbalanced gut bacteria.

Food allergies tend to run in families. You are more at risk if you or any family members have other allergic diseases, such as eczemaasthma and hay fever. ‘If you have any concerns about food allergies in your family, speak to your GP and ask for referral to a dietitian,’ says Dr Ruxton.

Although it’s possible for any food to cause an allergy, most food allergies are caused by just eight foods:

  1. Shellfish
  2. Milk
  3. Fish
  4. Soya beans
  5. Wheat
  6. Eggs
  7. Peanuts
  8. Tree nuts

If you have a reaction to one food, it’s possible you might also react to a related group – called cross-reaction. For example, someone with an allergy to shrimp may also find they’re allergic to lobster – or someone allergic to peanuts (a legume) may also react to tree nuts like cashews or almonds. Determining if you are cross-reactive is not straightforward, which is why it’s so important to manage the allergy with the help of a doctor or allergy specialist.

Additionally, people with hay fever sometimes develop an allergic reaction to fresh fruits or vegetables, because their allergy antibodies mistake the proteins for pollen. When this happens, it’s called oral allergy syndrome, pollen food syndrome, or pollen food allergy syndrome. Generally, it causes itchiness in their mouth and throat, with mild swelling. It’s possible to ‘deactivate’ the allergens by cooking the fruits or vegetables.

Food allergy symptoms

Common food allergy symptoms can occur anywhere from a few minutes after exposure to a few hours later. Although allergic reactions are often mild in nature, they can also be very serious. Rashes are common, as are hives – also called urticaria – which present as small, red, elevated areas of swelling with a pale centre. ‘In more serious reactions, angioedema can develop leading to lip, facial and throat swelling, which requires emergency treatment,’ says Ali.

‘In severe cases, food allergies can lead to anaphylactic shock and death,’ says Dr Ruxton. ‘For less severe allergies and intolerances, the effects are fairly short acting and cause inconvenience and discomfort rather than complications.’

Food allergy symptoms can affect the skin, respiratory tract, gastrointestinal tract and cardiovascular system. Symptoms may include one or more of the following:

  • Swollen lips, face or throat
  • Tingling or itching in the mouth and lips
  • Vomiting or nausea
  • Abdominal pain or diarrhoea
  • Low blood pressure
  • A raised, itchy red rash (hives)
  • Difficulty breathing or wheezing
  • Hay fever-like symptoms, like sneezing or itchy eyes

Once you have become sensitised to a food, your immune system produces antibodies every time you eat it. It’s impossible to predict the severity of a future reaction – symptoms that are mild on the initial reaction may become serious on the second or third reaction. The only way to avoid the reaction is to avoid the trigger food.

What is anaphylaxis?

Anaphylaxis – the most severe allergic reaction – can come on within minutes of eating a trigger food. It happens when the body is flooded with chemicals that put the person into shock. Symptoms appear quickly, and worsen rapidly. They may include:

  • A rapid drop in blood pressure
  • Rash that spreads rapidly and covers the body
  • Worsening breathing problems
  • Sneezing, streaming nose and eyes
  • Itchy throat
  • Accelerated heartbeat (tachycardia)
  • Throat, face and mouth swell rapidly
  • Loss of consciousness

Anaphylaxis must be treated with an injection of epinephrine (adrenaline). If you or someone close to you experiences difficulty breathing, call 999 for help immediately.

⚠️ If you experience any difficulty breathing and think you might be having a severe allergic reaction, call 999 for emergency medical treatment right away.

How is a food allergy diagnosed?

If you suspect you may be allergic to a certain food (or foods), make an appointment with your doctor and keep a symptom diary in the interim. You doctor will also want to know details about your reaction – including the symptoms, how fast the reaction occurred, which foods appeared to cause it, whether the food was cooked and where you ate it.

The most accurate way to diagnose a person’s allergy triggers is medical testing via your doctor. This might involve:

  • Skin prick testing:this involves putting a drop of liquid that contains a substance you may be allergic to on your forearm. The skin under the drop is then pricked.
  • Blood test: a blood sample is taken and analysed for antibodies in a lab.
  • Patch test: a small amount of the suspected allergen is added to specially-designed discs and taped to your skin for 48 hours.
  • Elimination diet: this involves avoiding eating a particular food to see if your symptoms improve.
  • Food challenge test: you consume a certain food in gradually increasing amounts under close supervision.

Do food allergy home test kits work?

Test kits that are available in health food shops, over-the-counter in pharmacies or via the internet are often unscientific and inaccurate. Complementary therapists also often offer food allergy testing using methods such as kinesiology and hair analysis, but they are not scientifically-proven and should be avoided to prevent an incorrect diagnosis.

Aside from a lactose intolerance test obtainable via your GP, there are no reliable ways to test for intolerances so home test kits should be avoided.

How is food allergy treated?

The most important treatment for a food allergy is to stop eating the foods that trigger your allergic reaction. ‘People with severe allergies will be offered medications – oral or injectable – by their GPs and will need to remain especially vigilant,’ says Dr Ruxton. These medications may include antihistamines – histamines are the chemicals that cause most allergy symptoms, and antihistamines block their effects – as well as epinephrine, which is used to treat anaphylaxis.

People will often see a dietitian after being diagnosed with a food allergy. It’s important that the trigger food (or food group) is eliminated in a way that doesn’t harm their general health – for example, if they have to cut out dairy, they will need to consume other sources of calcium and protein. However, most people with food allergy are able to live a normal life with only a few restrictions in their diet.

In rare cases of multiple allergies, it becomes harder to get all the nutrients you need, which can lead to malnutrition and associated conditions, like anaemia. ‘It’s worth taking a vitamin and mineral supplement to ensure you don’t miss out on key nutrients,’ says Dr Ruxton. ‘Also, people with fish allergies could benefit from additional omega-3 fats from an algae supplement.’

Food allergy prevention

Ultimately, the best way to prevent an allergic reaction is to identify the food that causes the allergy and avoid it. You can also try the following:

  • Note any foods you are allergic to as well as any cross-reactions.
  • Always check food and drink labels carefully before consuming them.
  • Keep cutlery, cooking surfaces and chopping boards free of the allergen.
  • Be extra careful when ordering from or eating in restaurants.

Be aware of the signs of an allergic reaction and consult your doctor if you are in any doubt.

Net Doctor

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