Lactose intolerance

Health and Nutrition > Diseases > L Reviewed by Dr Alan Ogilvie, gastroenterologist

What is lactose intolerance?

Lactose intolerance is the inability to absorb lactose - the predominant sugar in milk - into the digestive system. If lactose is not absorbed properly, it ferments and this results in abdominal pain, a bloated stomach and diarrhoea.

What causes lactose intolerance?

Lactose is a disaccharide, which means that it is composed of two other sugars bound together. In order for lactose to be absorbed, it must be split into those two smaller sugars. This split is performed by an enzyme called lactase, which is present in the lining of the small intestine.

If the levels of the lactase enzyme are low or absent, then that splitting does not occur. The lactose is fermented by the bacteria in the large intestine and this produces excess gas and stomach rumbling and leads to diarrhoea.

Lactase activity is high in babies and declines as the amount of milk in the diet decreases. Some people may have very low lactase levels but not have any symptoms. The reason for this is not known.

Who is at risk of developing lactose intolerance?

Most adults in northern Europe and North America are able to absorb lactose. However, the majority of the world's population is lactose intolerant.

- People who have had major bowel or stomach surgery, or suffer from coeliac disease are often lactose intolerant until the underlying condition is treated.
- Many people become lactose intolerant for a few days after they have had diarrhoea. This is because the lining of the small intestine has been slightly damaged by the disease, thereby reducing the production of lactase.

What are the symptoms of lactose intolerance?

- Stomach rumbling, abdominal distension and increased wind.
- Diarrhoea.
- Abdominal colic.
- Nausea.

What can I do to help myself?

- If you are suffering from diarrhoea, or have recently recovered, cut down on dairy products.

How is lactose intolerance diagnosed?

'The domestic method' - avoid eating foods that contain lactose for a couple of days and then drink two to three glasses of milk. If you get a stomachache or diarrhoea within half an hour, you are lactose intolerant.

If you wish to have your suspicions confirmed, you will need one of the following more formal tests.

- The lactose tolerance test - your doctor measures your blood sugar levels before and after you drink a liquid containing lactose. If the blood sugar goes up with this test, then you are not lactose intolerant.
- The breath test - you are given a lactose solution to drink and the doctor analyses your breath for hydrogen gas, which is only present if the lactose is fermented.
- An analysis of a sample from the lining of the small intestine is obtained during an endoscopy, which involves inserting a small tube into the stomach.

How is lactose intolerance treated?

. It depends on the severity of the symptoms.

People with mild symptoms may feel better simply after reducing the amount of dairy products in their diet.

For those with severe symptoms, a formal lactose-free diet should be adopted. This is best achieved by a consultation with a dietitian, which can be arranged through a GP.

The symptoms of lactose intolerance vary greatly. Some sufferers can easily manage cream in their coffee whereas others get diarrhoea from even a small amount of lactose. Yoghurt may be tolerated by some while other lactose-containing foods may not be.

Lactose intolerance is a harmless condition. If for any reason, you are unable to follow your diet, you are not in any danger, and you will not damage yourself in any way. However, the symptoms may well recur.

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