Lactose Intolerance

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Lactose intolerance is a condition in which the body has difficulty handling lactose. People with this condition may get diarrhoea, stomach pains and bloating if they drink milk or eat dairy products. Some people are born with a tendency to develop lactose intolerance; others get it as a result of gastroenteritis or chemotherapy. The treatment is mainly to avoid lactose.


Lactose is a sugar found in milk. It cannot become absorbed by the body unless is gets changed into more simple sugars called glucose and galactose. This change happens when the lactose passes through the stomach into the upper part of the gut (small intestine) and comes into contact with a chemical called lactase.

Lactase is made by cells that line the upper part of the small intestine.

If there is not enough lactase in the small intestine, lactose cannot be broken down and cannot get absorbed. This leads to lactose intolerance.

Some people confuse lactose intolerance with allergy to cow's milk. With milk allergy, your immune system reacts to proteins found in milk which can cause symptoms.

Lactose intolerance is not an allergy. Symptoms are caused by the undigested lactose in the gut.

There can be a number of causes:

Inherited forms
These are passed on through families:

  • Primary lactase deficiency: this causes low levels of lactase. Symptoms can develop at any age but rarely before the age of 6.
  • Congenital lactase deficiency: this causes a complete lack of lactase from birth. Symptoms develop as soon as the baby is given milk or lactose formula.

Secondary lactase deficiency
This happens when something damages the cells in the lining of the upper gut (small intestine) which produce lactase. It is common in children and often occurs after a stomach infection (such as viral or bacterial gastroenteritis). It can also be a complication of other bowel diseases or chemotherapy.

Developmental lactase deficiency
When a baby is born, it takes time for a reasonable amount of lactase to build up in the digestive system. Babies more than six weeks premature may be born with very low levels of lactase, leading to temporary lactose intolerance. The condition disappears as the baby gets older.

It varies according to ethnic group and how much dairy produce is eaten within the country. In communities where a lot of dairy food is eaten, lactose intolerance is less common. Two in 100 people in Northern Europe have the inherited form, whereas this figure can be up to 80 in 100 people in Hispanic communities.

Lactose intolerance is very common in adults worldwide.

You may get bloating, or stomach pains. Excessive burping or passing a lot of wind can occur. You may also get watery diarrhoea, and itching around your bottom (anus). These symptoms tend to develop from one to several hours after milk, dairy products or any food containing lactose.

How bad the symptoms are depends on how much lactose you take into the bowel. Many people who have lactose intolerance can eat some lactose without developing symptoms. In general, the more lactose you eat, the more likely that symptoms will develop. The inherited condition tends to cause less severe symptoms than the intolerance which develops after gastroenteritis or chemotherapy.

Babies and toddlers may have signs of malnutrition and poor growth (failure to thrive) but this is unusual.

If you get symptoms after drinking milk or eating dairy products or other lactose-containing foods, it is fairly clear that you have lactose intolerance. Tests are not usually needed.

If there is any doubt, special tests can be performed on your breath or blood. The breath test involves measuring the amount of a gas called hydrogen in your breath after taking a test dose of lactose. Rarely, you may need an intestinal biopsy (a procedure which takes a sample of lining of the gut (small intestine)) to make the diagnosis.

Most people do not have any long-term problems. Babies with severe deficiency of lactase may develop lack of fluid in the body (dehydration) and malnutrition if the condition is not diagnosed early enough.

Avoiding all dairy products can mean you don't get enough calcium. Calcium is a mineral needed for bones to grow normally and be strong. Lack of it could mean children don't grow as well as they should, or adults could have weak bones that break easily.

People with primary lactose intolerance (the inherited form) should be tested to find out how much lactose they can handle. People do have varying levels of intolerance. Sometimes the amount they can tolerate can be increased by giving milk or dairy products little and often. Having milk with meals may help. Full-fat or chocolate milk may be better than skimmed milk. Thicker foods such as yoghurts and curds are likely to be better tolerated because they move through the bowel at a slower rate. Live yoghurts and hard cheese (such as Cheddar, Edam, Emmental or Parmesan) may not cause problems. Lactose-free milks are available but may be less nutritious than cow's milk. Check they are calcium-enriched. It is possible to buy lactase supplements at health food shops, to be taken with dairy products. However, these may be expensive and cannot be prescribed.

Secondary lactose intolerance, due to damage to the lining of the gut (small intestine), may need treatment by fluid through a drip if the diarrhoea is very severe. Most doctors advise parents of babies and children with gastroenteritis to carry on with breast milk, formula milk or cow's milk. In some cases if the diarrhoea is very prolonged, or in very young babies, some doctors recommend withdrawing lactose for three weeks after the infection.

The chance of premature babies getting lactose intolerance due to developmental lactase deficiency can be reduced by feeding them half-strength lactose formula or breast milk.

If you have lactose intolerance you should read the labels of foods and drinks very carefully. Not all foods with milk in them will cause problems. For example, lactose is broken down by fermentation processes and is not found in hard cheese such as Cheddar or Emmenthal. Foods that may cause problems include milk, cream, cottage cheese, yoghurts, ice cream and milk chocolate. Lactose is not present in dark chocolate.

Foods containing 'hidden' lactose may include some types of the following:

  • Bread
  • Cakes
  • Cereals
  • Margarine
  • Dressings
  • Sweets
  • Snacks

To access the best possible advice, ask your GP about seeing a dietician. Also remember that many tablets contain lactose so you should check the leaflet that comes with any medication you are taking.

Further help & information

Original Author:
Dr Laurence Knott
Current Version:
Peer Reviewer:
Dr Adrian Bonsall
Document ID:
13592 (v2)
Last Checked:
Next Review:
The Information Standard - certified member
See also: Lactose Intolerance written for professionals

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