However, a large hospital asthma clinic looked at the results of all the allergy tests done in their clinic over many years (a survey of 25,000 asthma sufferers!) and found rates of food allergy that are not much different from that seen in the general population. 

Whatever the true situation it is certainly clear that a far greater proportion of asthma sufferers, over two-thirds in one UK survey, perceive that various foods are causing asthma exacerbations. When such cases are assessed, the standard allergy tests are usually negative. Doctors have tended to dismiss such associations as psychological. However it appears likely that a delayed form of food allergy or food intolerance may explain some of these cases. Unfortunately, because there are no accurate food intolerance tests, and because population studies requiring food elimination and blinded food challenge are very expensive to undertake, we do not know how common such cases are.

Asthma and food allergy

In most cases of true food allergy, a small number of individual foods causes the majority of the reactions. The foods most likely to cause allergy are egg, milk, soya, wheat, peanuts, tree nuts (almond, Brazil, cashew, hazel, walnut etc.) fish and shellfish. Different food allergies tend to arise at different ages. For example, milk, egg, soya and wheat allergies most commonly arise in the first year of life whilst fish, shellfish and nut allergies tend to start later. Milk, egg, soya and wheat allergies (those that start in the first year) tend to disappear by five years of age whilst fish, shellfish and nut allergies (those that tend to start later) tend to persist long-term.

Adults who suffer from both asthma and food allergy are far less likely to have the asthma made worse by food allergy (a rare event in adult asthmatics) than to find that the food allergy is causing quite different symptoms. The most likely are symptoms in and around the mouth (itchy mouth, itchy lips, itchy throat, lip swellings) caused by a mild form of true food allergy to certain fresh fruits, certain nuts (almond, hazel, walnut) and certain fresh vegetables, a condition linked to spring hay fever and called the pollen food syndrome. 

When asthmatic children have food allergies, lower respiratory (cough or wheeze) symptoms are caused far more commonly than they are in adults. Even so, in many cases the food allergy symptoms may be confined to the skin (itchy rash or swellings, eczema) or they may be confined to the gastrointestinal tract (nausea, vomiting, abdominal pain, diarrhoea).

Several studies have tried to answer the question "in what percentage of asthmatic children is a food allergy causing the asthma to get worse?". These studies have come with widely different answers; from 2% in one study to 24% in another with other studies coming up with a range of answers in between! Differences in the methods and in the types of patients studied probably account for these differences.

Asthma and food intolerance

When food allergy tests are negative, a properly conducted food elimination and challenge procedure may be used as the means to identify food-induced asthma. Cases that are identified in this way are called food intolerance and appear as a kind of delayed (hidden) allergy and doctors do not fully understand the underlying mechanism. This form of food hypersensitivity may be more common than is realised because the symptoms may be delayed by several hours after the culprit food has been eaten. It is also likely that the frequency of consumption of the culprit food actually influences the frequency and severity of the symptoms caused.  A diet diary may be used to in an attempt to spot any association although this approach is often less helpful than might expected. The only valid way to establish if reactions to food are causing asthma exacerbations is to undertake an a properly conducted food elimination and challenge procedure under the supervision of a dietitian.

Further studies are needed both to investigate how commonly food intolerance affects asthma and to elucidate the responsible mechanisms. 

Asthma and nutrition

The role of diet in asthma has been the subject of many studies in recent years.

The effect of individual nutrients such as potassium, magnesium, and the anti-oxidants nutrients vitamins C, vitamin E, and essential blood fats on asthma have been separately studied. Effects for individual nutrients are seen although these are quite small.  However, the beneficial effect of individual nutrients on the metabolic processes that affect asthma will be limited if less than ideal levels of other nutrients co-exist. So the full effect of nutritional supplementation on asthma will not be fully known until further studies of the combined effects of various nutrients have been undertaken.

More convincing are epidemiological studies of the relationship between people's dietary intake of essential nutrients and the severity of asthma.  These have shown that there is a relationship between the degree of asthma and the dietary intakes of magnesium, potassium, vitamin C the essential fats.