Lactose intolerance is the inability to absorb lactose into the digestive system. Find out what causes it.
Medically reviewed by Dr Louise Wiseman MBBS, BSc (Hons), DRCOG, MRCGP and Based on a text by Dr Torben Nathan, Dr Ove Schaffalitzky de Muckadell
Lactose intolerance is a digestive disorder where your body does not produce enough of the enzyme lactase, which breaks down lactose in your gut. As a result the body struggles to digest lactose, the main carbohydrate in dairy products.
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 in the colon and this can result in abdominal pain, a bloated stomach and diarrhoea.
It’s important to remember that lactose intolerance is not an allergy. An allergy refers to a specific immune mediated response often resulting in severe abdominal pain, wheezing, skin rash and lip/tongue swelling.
Lactose intolerance leads to food containing lactose being fermented in the colon rather than being broken down by enzymes. This leads to flatulence and diarrhoea.
Lactose intolerance causes
Lactose is a disaccharide, which means that it is composed of two other sugars bound together (glucose and galactose).
In order for lactose to be absorbed, it must be split into those two smaller sugars.
This breakdown 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 breakdown does not occur. The lactose reaches the colon and is fermented by bacteria in the large intestine and this can produce excess gas and stomach rumbling and leads to diarrhoea.
Lactase activity is high in babies who require mother’s milk. Lactase activity 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?
All humans can absorb lactose as babies. Most adults of Caucasian (white) origin can absorb lactose into adult life.
However, many people have a reduced ability to digest lactose after infancy. About 65 per cent of the human population are affected. It is most prevalent in East Asian individuals and also very common in those of West African, Arab, Jewish, Greek and Italian descent.
It is less frequent in communities that use unfermented milk products as a main food source such as some areas of Northern Europe.
Other risks include:
- People who have had major bowel or stomach surgery, or suffer from coeliac diseaseare often lactose intolerant.
- 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.
Lactose intolerance symptoms
Some people may be able to tolerate a small glass of milk with no symptoms, others may not even tolerate a dash of milk in their coffee.
- Stomach rumbling, abdominal distension and increased wind
- Stomach cramps and pains
- Abdominal colic
It is of course possible that these symptoms could be related to several other conditions, so it is important to seek professional help before entirely removing milk and dairy products from your diet. Irritable bowel syndrome can present similarly and milk protein intolerance (adverse reaction to the protein in cow’s milk – not the same as an allergy or lactose intolerance).
Lactose intolerance diagnosis
‘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 stomach ache or diarrhoea within half an hour, you may be lactose intolerant.
Sometimes GPs advise patients to avoid food and drinks containing lactose for 2 weeks to see if symptoms improve.
If you wish to have your suspicions confirmed, you will need one of the following more formal tests.
- The most common test for lactose intolerance is the lactose hydrogen breath test. This involves taking a lactose solution by mouth and measuring the hydrogen produced in the breath over the next two hours or so. If excess hydrogen is produced then lactose has not been absorbed but fermented in the colon. This test is painless but may produce symptoms.
Other methods of diagnosis include:
- Lactose tolerance test– measuring blood sugar levels after drinking lactose. If sugar levels rise the patient is not lactose intolerant.
- Directly measuring lactase activity on a sample of small intestinal lining obtained during endoscopy. This test requires inserting a tube into the stomach.
Lactose intolerance treatments
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.
Which foods contain lactose?
- Milk, butter, margarine, fermented milk products, yoghurt and cheese
- Milk powder
- Bread and other baked goods – read the label to check
- Prepared foods – again, read the label
- Many types of tablets
Alternative foods for those with lactose intolerance
- Lactose-free cows’ milk
- Alternative milks such as almond, coconut, rice, hazelnut, quinoa
In some patients lactase substitutes are prescribed and in certain situations doctors may advise calcium and vitamin D supplements to prevent risk of osteopenia and osteoporosis (loss of bone mineral density) and malnutrition but most patients can be managed by being careful with their diet.