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Dr Wynnie Chan looks at the difference between food allergies and intolerances, and how you can 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 do they affect you?

A food allergy occurs when the immune system reacts in an abnormal way to a food or food ingredient. The body's immune system will respond to foods that non-sufferers find completely harmless, immediately and fairly aggressively. Reactions can manifest themselves in immediate symptoms such as a runny nose, a rash, swelling, wheezing and asthma symptoms or headaches. Your GP will be able to refer you for allergy tests. If a food allergy is diagnosed you will most likely be asked to avoid the food and any related or trace elements of it.

Food intolerances, on the other hand, are a range of symptoms that occur in reaction to food. They do not always appear immediately and are difficult to pin done through medical tests. Reactions may include joint pain, bloating, fatigue and can even lead to learning difficulties in children. Common food intolerances include lactose intolerance, reactions to histamine, serotonin and tyramine contained in cheese, coffee and pickled fish, and responses to toxins in food. The severity will vary according to the individual, and how severe their reaction is to a particular food. These delayed 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 colouring.

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. This means 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.

Wheat intolerance: The bowel condition, coeliac disease, is the main form of wheat intolerance, a reaction to gluten, a protein present in wheat, rye, barley, and possibly oats. It affects approximately one in 300 people in the UK, according to Coeliac UK. 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 more women than men, at a ration of 1.3:1. 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.

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