Dr Wynnie Chan clarifies the difference between food allergies and intolerances, and explains how to tell if you have one

You've read about them in magazines and probably know someone who suffers from one, but what are food allergies and intolerances, and how can they affect you?

Food intolerance is an umbrella term, which encompasses food allergies, enzyme defects like lactose intolerance, pharmacological reactions to foods and substances (such as headaches or facial flushing that result from histamine, serotonin and tyramine contained in cheese, coffee and pickled fish), and idiosyncratic responses to foods due to toxins they contain. Responses vary according to the individual, and how severe their reaction is to a particular food, but will generally include some form of rash. A food allergy is a specific kind of food intolerance, where the immune system reacts in an abnormal way to a food or food ingredient.

The way our immune system protects us against harmful, 'foreign' particles (called antigens) is by eliminating them from our body. This process of elimination can result in an immediate or a delayed allergic reaction. An immediate reaction is characterised by symptoms such as a runny nose, asthma, hot flushes, difficulty breathing, and swelling of the lips. This reaction is then mediated by the production of antibodies, which bind with the antigens and eliminate them from the body. Reactions vary in severity, and depend on how much of the offending food you've eaten. Unfortunately, it is difficult to know if you have a food allergy, until you actually have an allergic reaction, at which point you should consult a physician. Most likely, he or she will tell you to avoid the food and any related or trace elements of it. Delayed allergic reactions to food are more complex, and can take several hours to develop.

Four of the most common food intolerances include lactose, wheat, egg and peanut intolerance. Many people also have intolerant reactions to food additives, such as MSG (monosodium glutamate) or food dyes. However, these substances have not yet been scientifically proven as a cause of intolerance and, therefore, not officially classified as such. The following is an explanation of the above listed food intolerances:

• Lactose intolerance: This is the most common form of adverse reaction to cow's milk. Lactose, which is a milk sugar, must be broken down by an enzyme called lactase before it can be absorbed by the body. In affected adults and children, lactase is not produced in sufficient quantities for the milk sugar to be broken down. The result is that it passes through the body undigested, causing stomach pain, bloating and diarrhoea. However, people with lactose intolerance can often eat small quantities of dairy products without experiencing any symptoms. Additionally, dairy products such as yoghurt, hard cheeses and lactose-reduced milk can be tolerated in most cases, because the form of lactose in these products does not seem to affect people as severely.
• Wheat intolerance: The bowel affliction coeliac disease is the main form of wheat intolerance. It affects around one in 300 people in the UK, who are susceptible to gluten, a protein present in wheat, rye, barley, and possibly oats. Gluten can damage the lining of the gut, which leads to a reduced capacity to absorb nutrients from foods and results in diarrhoea and malnutrition. In the general population, coeliac disease affects two women to every man. This discrepancy may be affected by nutritional stresses such as pregnancy or blood loss. It is also believed that coeliac disease may be caused by introducing foods containing gluten to children, who are weaning. Generally, young babies' digestive systems are not mature enough to process large proteins. These are, therefore, treated as foreign particles and removed from the body. People with coeliac disease need to follow a strict, lifelong, gluten-free diet. The UK Coeliac Society (01494 437 278) publishes an updated list of gluten free foods, as do most major supermarkets.
• Egg intolerance: This type of reaction occurs within minutes of eating an egg. Symptoms include, rashes and swelling on the face and around the mouth. People with an egg intolerance need to avoid all foods containing egg white or egg yolk, from all sources, including chicken, goose and duck. Many food products, like cakes, bread, mayonnaise and custard, contain egg or egg proteins, so be vigilant about checking ingredients labels before you buy.

• Peanut intolerance: The prevalence of peanut intolerance in the UK is unknown, although some studies have suggested that, one in 200 may be affected in some way. Adults and children, who suffer from a peanut intolerance, may experience a tingling of the mouth and lips, swelling of the face, a skin rash called urticaria, nausea, colicky pain (a gripping stomach pain) and feelings of tightness in the throat. A small number of people suffer very severe reactions within a few minutes of eating peanuts including swelling of the airways and anaphylactic shock (obstruction of breathing), a sudden drop in blood pressure, collapse and unconsciousness. People with a peanut allergy must strictly avoid all traces of peanuts in their food.

Food intolerances and allergies are actually much less common than you might think. Current estimates suggest that food intolerance exists in about 1 to 2% of the adult population in the UK, whilst only a small proportion of these people are truly allergic to food. The incidence of food intolerance is higher for children, at 5 to 7%, but it is believed that, most children outgrow their intolerances by age 4 or 5 years, with the exception of peanut allergies, which remain a lifelong problem.

Although the general consensus is that food allergies are on the rise, it is unclear as to why. Many people who think they have a food allergy or intolerance have not been tested by a doctor. If you suspect you have a food allergy or intolerance, you should seek the advice of your GP who will conduct specific medical tests to confirm diagnosis. Once diagnosis is confirmed, a State Registered Dietitian can suggest dietary modifications, to ensure that you don't unnecessarily exclude important foods from your diet.

For more information on the subject, visit the British Nutrition Foundation (020 7404 6504). Additionally, many supermarkets have free leaflets about food allergies and intolerances.