Dr Trisha Macnair

Not to be confused with food allergies, a food intolerance can cause various symptoms including bloating, abdominal pain and diarrhoea. We look at the most common causes and how to get a diagnosis.

Food intolerance is an adverse reaction to some sort of food or ingredient that occurs every time the food is eaten, but particularly if larger quantities are consumed.

This isn't the same as a food alergy because the immune system isn't activated. Neither is it the same as food poisoning, which is caused by toxic substances that would cause symptoms in anyone who ate the food.

Food intolerance doesn't include psychological reactions to food either.

What causes it?

Food intolerance occurs when the body is unable to deal with a certain type of foodstuff. This is usually because the body doesn't produce enough of the particular chemical or enzyme that's needed for digestion of that food.

For example, one of the most common types is intolerance of cow's milk, which contains a type of sugar called lactose. Many people have a shortage of the enzyme lactase, which is normally made by cells lining the small intestine. Without this enzyme they can't break down milk sugar into simpler forms that can be absorbed into the bloodstream.

Lactose intolerance can cause symptoms very similar to irritable bowel syndrome.

Another common example is a deficiency of an enzyme called aldehyde dehydrogenase, which is needed to break down alcohol. Drinking even small amounts can make affected people feel unwell.

Some people have adverse reactions to chemical preservatives and additives in food such as seen with sulphites, benzoates, salicylates, monosodium glutamate, caffeine, aspartame and tartrazine.

The lack of a specific enzyme in the body may lead to the build up of toxic byproducts and histamine, which then mimic the symptoms of an allergy. This is called a 'pseudo-allergic' reaction.

Who's affected?

There's a strong genetic pattern to food intolerances. Lactose intolerance is less common among northern and western Europeans (10 to 15 per cent are affected) than in Asian, African, native American and Mediterranean populations (70 to 90 per cent are affected).

Babies are usually born with higher levels of lactase, so lactose intolerance usually only begins after the age of about two, as the body begins to produce less of the enzyme. However, many people don't experience symptoms until they're much older. A temporary lactase deficiency may follow gastroenteritis, especially in children.

Alcohol intolerance is common among Asian people - 50 per cent are affected.

What are the symptoms?

Food intolerances are rarely harmful but may cause unpleasant symptoms, including nausea, bloating, abdominal pain and diarrhoea, which begin about half an hour after eating or drinking the food in question.

The severity of symptoms varies depending on the amount of enzyme the person makes and how much of the food has been consumed. In alcohol intolerance, there may be intense flushing of the skin, nausea, palpitations, headache and feeling faint.

How is it diagnosised?

The easiest test for a food intolerance is to remove the food from your diet, see if symptoms improve and then try reintroducing the food. If symptoms return, an intolerance is likely.

Lactose intolerance can be tested for more thoroughly using a lactose tolerance test, a hydrogen breath test and a stool acidity test. Your doctor can arrange these and other food intolerance tests if necessary.

What's the treatment?

Food intolerance can be managed simply by cutting the food out of your diet. Babies or younger children with a lactose intolerance can be given soya milk or hypoallergenic milk formula instead of cow's milk. Adults may be able to tolerate small amounts of troublesome foods, so may need to experiment to work out what they can eat.

Food intolerances are rarely more than an inconvenience - talk to your doctor if you're worried.

This article was last medically reviewed by Dr Rob Hicks in October 2006.

Disclaimer

All content within BBC Health is provided for general information only, and should not be treated as a substitute for the medical advice of your own doctor or any other health care professional. The BBC is not responsible or liable for any diagnosis made by a user based on the content of the BBC Health website. The BBC is not liable for the contents of any external internet sites listed, nor does it endorse any commercial product or service mentioned or advised on any of the sites. See our Links Policy for more information. Always consult your own GP if you're in any way concerned about your health.