Allergy or intolerance?

Allergy

A food allergy occurs when the immune system reacts to a food that the body mistakenly believes is harmful. Because the body perceives a threat, it produces immunoglobulin (IgE) antibodies. The IgE antibodies stimulate cells to release histamine and other chemicals, which causes the allergy symptoms.

Severe allergies are usually obvious. Contact with the mouth can cause itching and swelling within minutes, vomiting, cramps, or a life-threatening anaphylactic shock. Milder allergies can aggravate eczema and be harder to identify.

Common allergens

Virtually all allergens are proteins. Peanuts, soybeans, cow's milk, fish, shellfish, egg, wheat, and tree nuts (such as cashews, almonds, or pine nuts) are responsible for 90 percent of food allergies. In some cases, just a tiny amount can trigger a reaction.

Peanuts are the leading cause of severe allergic reactions. Milk, egg, and wheat allergies are the most common in children - but many children tend to grow out of them by the time they're four or five.

Sesame seeds are a fairly new allergen. This may be linked to more frequent use of sesame oil in cooking, and a higher consumption of Asian food and Middle Eastern food – which includes tahini (sesame-seed paste).

Food intolerance

This is a food reaction that doesn't involve an immune response. Food-reaction symptoms have different causes – an enzyme deficiency, a reaction to a toxin in a food, or a side effect from a chemical in a food. Often the cause isn't known and may be psychological.

Symptoms may be similar to an allergic reaction, although they are often slower to occur and more varied. These symptoms tend to be dose-related (unlike those of a food allergy). So a lot of the offending food will make you much worse than a little. It's rare that a food intolerance is life-threatening, however.

So, what are these intolerances? An enzyme-deficiency intolerance occurs when a person doesn't produce a particular digestive enzyme, or produces only a small amount of it. So certain foods won't be digested properly.

Lactose intolerance is an example of this. The gut doesn't produce enough of the enzyme lactase, which is needed to digest lactose (the sugar in milk). Drinking milk or milk products may lead to wind, stomach discomfort, and diarrhoea.

Other people may have allergy-like intolerances. These are reactions to foods that commonly cause allergy but don't involve the immune system. For example wheat gluten can sometimes upset people with sensitive bowels, and they may need to watch how much bread they eat. But this is different from coeliac disease, a condition where the sufferer can't eat gluten because it isn't digested normally and damages the bowel wall. People with coeliac disease must remove all gluten from their diets.

Natural and artificial chemicals in food can bring on food-chemical intolerance, which causes discomfort in some of us.

One example is MSG (monosodium glutamate) – a flavour enhancer that's also found naturally in most foods. Artificial MSG is often added to soups, sauces, and Chinese food.

Additive intolerance may be associated with migraines, and with intestinal or respiratory problems. Most people tolerate the use of sulphites as preservatives in food - but those who have additive intolerance experience a range of different responses, including asthma.

Foods such as caffeine, spices, garlic, and dried fruit may be irritants – they upset the digestive system and cause diarrhoea, abdominal pain, or bloating.