The good news is that very few people with asthma need a special diet. For most people with asthma, healthy eating advice is exactly the same as it is for everyone else: follow a balanced diet that includes plenty of fresh and unprocessed food and is low in sugar, salt and saturated fat.
But a small number of people with asthma are also allergic to certain foods. Coming into contact with a food allergen can lead to an allergic reaction, which may include asthma symptoms such as wheezing, coughing and difficulty breathing.
There's some evidence that if you have both asthma and a food allergy, you may be at greater risk of having an asthma attack that's life-threatening, so it's important to strictly avoid the food. You should also make sure your asthma is well managed, to lower your risk of having an asthma attack.
Why can food increase your risk of asthma symptoms or an asthma attack?
When you come into contact with a food allergen, your immune system overreacts and releases a chemical called histamine. The release of histamine causes the symptoms of an allergic reaction. These can include red, itchy, watery eyes and nose, sneezing, a scratchy or sore throat and itchy skin. For anyone with a food allergy, symptoms can also include wheezing and coughing. In people who have asthma, it can also trigger asthma symptoms such as coughing, tightness in the chest and difficulty breathing. And this is where it can get confusing, because it can be hard to tell the difference between an allergic reaction to a food and an asthma attack.
The signs and symptoms of a food allergy, including asthma symptoms, usually come up seconds or minutes after you've come into contact with the allergen. But it's also possible that you may not start getting symptoms until as much as 48 hours later.
Food allergy shouldn't be confused with food intolerance, which isn't connected with asthma symptoms. Food intolerance is common, causing symptoms such as stomach ache and bloating hours to days after eating a food. An example is lactose intolerance, where someone lacks the enzyme needed to produce the sugars found in milk products, leading to bloating and diarrhoea. If you have a food intolerance, rather than a true allergy, there's no evidence to suggest it will trigger asthma symptoms.
How do you know it's a trigger?
Check with your GP or asthma nurse if you think a food is affecting your asthma. Your doctor can carry out or refer you for skin prick testing to work out whether you're allergic to the food you suspect. They may ask you to try to remember what you'd eaten in the days before you had a reaction.
What's the best way to reduce the risk of the trigger affecting you?
If you know you have a food allergy, you will need to avoid the food completely, recognise the symptoms of a reaction and know what to do if it happens. Your GP should refer you to an allergy specialist to help you achieve this.
If you're concerned you may have an allergy but haven't been diagnosed, speak to your GP or asthma nurse. Don't start cutting out foods as you may then miss out on important nutrients.
It's impossible to avoid all triggers so it's important to manage your asthma. If you've been prescribed a preventer inhaler, take it every day, as prescribed. It helps to control inflammation in your lungs, meaning you're less likely to have an asthma attack even if you do come into contact with a trigger such as a food allergen.
Other tips that could help
You can also try some of the tips shared below by people with asthma who have food allergies:
Obtain "free from" lists - Supermarkets can provide these free of charge for most allergens, showing all the products free from the allergen you have to avoid.
Eat fresh, whole foods - A diet based mainly on fresh foods which you prepare yourself can help you avoid allergens more easily, as you'll be able to control exactly what goes into each meal.
Plan ahead for eating out - When you book a table, make sure the staff know about your allergy. As well as telling the chef and kitchen staff, you also need to make sure waiting staff know about the importance of avoiding cross-contamination.
Read labels - By law, the 14 major allergens (including wheat, milk, nuts and egg) have to be clearly listed on pre-packed manufactured foods throughout the EU. Get into the habit of reading the full ingredients list on food, particularly if your allergy is to an ingredient that doesn't have to be clearly listed. Avoid unlabelled foods. For more detailed information on food labelling, visit the Food Standards Agency website.
Which foods and ingredients can cause allergies?
The most common are:
Milk and milk products - You may have heard there's a link between dairy foods and asthma, but only a very small percentage of people are allergic to milk products. For them, eating these foods may result in wheezing. Dairy allergy is more common in children but they often grow out of it as their digestive system matures. Calcium-rich dairy products are essential for healthy bones, especially for children and adolescents. And people with asthma can be at higher risk of the bone disease osteoporosis (which causes thinning, brittle bones and increased risk of fractures) because of the use of steroid medication. So you should only avoid dairy products if necessary, ensuring you replace them with other sources of calcium under the guidance of your GP, nurse or a dietitian.
Eggs - This allergy is more common in children, who often grow out of it. Many of those with egg allergy are able to eat well-cooked eggs and foods containing them, and are more likely to react to raw or under-cooked eggs. Remember foods that contain eggs may also cause reactions.
Nuts, including peanuts - This is the most common food allergy in adults and children. Reactions can vary from mild (such as rashes) to severe, including anaphylactic shock, which can be life-threatening.
Seeds - Most people with an allergy to seeds are allergic to sesame. It can cause strong reactions in people who are susceptible, and the allergy is becoming more common in the UK because we're eating more sesame, for example, in bread and muesli. It's also possible to be allergic to other seeds, such as sunflower and poppy seeds, but this is far less common.
Fish and shellfish - Allergy to fish affects about one in 200 people, while shellfish allergy affects one or two in 100. Reactions can be severe and some people can even be sensitive to the vapours produced when seafood is cooked.
Wheat - People can have different reactions to wheat, but the form that can be associated with asthma symptoms is usually true, immediate wheat allergy. This causes instant symptoms, which may include rashes, wheezing, conjunctivitis, diarrhoea and a worsening of eczema. There have also been reports of people working in bakeries developing asthma-like symptoms as a result of inhaling flour.
Soya - Soya is found in lots of processed foods, from the obvious such as soy sauce to the more surprising, like margarine and soups.
Food additives - Sulphites and tartrazine are the most common allergy-triggering additives. Sulphites are salts used for preserving processed foods and are often found in pickled products, wine and dried fruit. Tartrazine is a yellow food colouring. Other food preservatives includings benzoates, butylhydroxyanisol (BHA), butylhydroxytoluen (BHT) and nitrites may also cause reactions.
Salicylates - Salicylates are chemicals found naturally in plant foods including tea, coffee and dried herbs and spices, and are also a main ingredient of anti-inflammatory painkillers such as aspirin. Some people with asthma are sensitive to these substances in painkillers but it's rarer to have a reaction to salicylates in foods.
Last reviewed March 2015