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(educational arm of the Gastroenterological Society of Australia)
Website: www.gesa.org.au ... Copies reviewed January 2006


The small intestine is a flexible 1cm diameter tube which is several metres in length. It starts at the stomach outlet and finishes where it joins the large bowel at the caecum (near the appendix).

It is an important tube, because all food is digested here and then absorbed through its wall into the blood stream. The cells which line the inside of the tube have specialised functions. Many contain enzymes (chemicals), which are necessary for digestion of sugars in the diet. These sugars include milk sugar (lactose), which is in breast milk and cow's milk, and table sugar (sucrose), which is present in many fruits and vegetables as well as in cakes, biscuits, sweets and cordials.

The enzymes are present in a series of fine projections on the cell surface called the brush border.

Lactose (milk sugar), and sucrose, (table sugar), are double sugars: that is they are each made up of two single sugars joined together. The body is unable to absorb them or use them for energy unless they are first split into single sugars. It is this step of splitting the double sugars for which the enzymes are necessary.

Each enzyme is named after the sugar which it splits, by changing the end of the sugar name to "ase". Thus the enzyme which splits lactose is called lactase and the enzyme which splits sucrose is called sucrase.

Most children who have sugar malabsorption will be intolerant of either lactose or sucrose but not both. If for some reason one of the enzymes, (lactase or sucrase) is absent or low, the sugars in the diet will not be split and so will not be absorbed. This has two consequences:

  1. The bowel motion will contain undigested sugar. Sugar attracts extra water so the motions become watery and are passed frequently. Bacteria (germs), in the large bowel use the sugar, but in the process produce acids which are also passed in the motions. These acids burn the skin and cause a rash on the bottom which may even bleed. The bacteria also produce gas when they ferment sugar so the motions may be frothy. Gas also contributes to abdominal swelling, crampy pains and irritability.
  2. The undigested sugar represents a loss of calories and in severe cases may lead to weight loss, muscle wasting and pot belly.


Lactose malabsorption

This is often due to low or absent lactase enzyme and is quite common after gastroenteritis. Lactose malabsorption is temporary and will eventually subside. The lactase enzyme is situated on the surface of the lining cells of the intestine and so can be easily damaged by bacteria and viruses. Gastroenteritis is caused by a germ damaging these lining cells, and not surprisingly, the enzyme is also damaged. Fortunately the lactase recovers when the damaged cells have been replaced by new cells. This may be in a few day's time or it may take several weeks. During this time lactose cannot be digested.

For infants, milk is the major and sometimes the only source of food. Breast milk and milk formulae contain 6-7% lactose. If milk is given, the lactose which it contains may cause the symptoms mentioned earlier. However, the benefits of breastfeeding often outweigh the disadvantage of short term diarrhoea caused by lactose intolerance. If breastfeeding is not possible, soy or hydrolysed infant formulae may be appropriate. Soy formula is one of the several infant formulae available for this purpose and it does not contain lactose. Lactase levels fall in about 25% of older children (up to 90% in some racial groups). This is a normal event but means that some older children have lactose malabsorption which may persist throughout life.

Sucrose malabsorption

This is very much less common with less than 1 in 10,000 affected. These children are born without the enzyme sucrase and so are unable to digest table sugar. They usually have no problem while they are breastfed because milk does not contain sucrose.

Symptoms begin when they begin solids, some of which contain sucrose. Many fruits and vegetables also contain sucrose. Many syrups are flavoured with sucrose, including those used for antibiotics, sedatives and vitamins. In contrast to lactose intolerance which is temporary, sucrose intolerance is often permanent.


All of the symptoms caused by sugar malabsorption can be cured by removing the appropriate double sugar from the diet.

Lactose malabsorption

Because lactose is contained in breast milk and cow's milk, it is occasionally necessary to replace milk with a substitute which does not contain this sugar. There are several infant formulae available for this purpose. One type is made by adding the enzyme lactase, (obtained from yeast) to cow's milk. This splits the lactose to glucose and galactose before it is put into tins and bottles. Thus the infant does not need the enzyme lactase in the intestine and the sugar is completely absorbed.

Other formulae contain sugars which do not require lactase, and so are well tolerated by these children.

Your doctor will be able to guide you as to the method and timing of a return to normal milk. In children over 6 months of age this will usually be within 2-4 weeks. In younger infants it may be 1-6 months later. Lactose-free formulae will be used in the meantime.

Sucrose malabsorption

Sucrose can be removed from the diet of young children more easily than can lactose. Its removal does not require major changes like using a special milk formula, because breast milk and cow's milk do not contain sucrose. Your dietitian will provide you with diet lists and will suggest alternatives to foods which are restricted. In practice, this turns out to be a healthy diet and the children have excellent teeth.


Several tests are available to help decide about treatment of lactose malabsorption if recovery is not proceeding as expected.

The sugar content of the bowel motion can be measured by collecting a small amount of the fluid motion.

A newer test is measurement of breath hydrogen after a small dose of lactose is given by mouth. Undigested lactose feeds the normal bacteria in the large intestine. One of the gases they produce is hydrogen. This passes into the blood stream, is taken to the lungs and then passes into the breath. If excess hydrogen is detected in the breath, it suggests that the lactose has not been digested.

A third test used on rare occasions is a small bowel biopsy. This is a simple day patient procedure. During brief anaesthesia or sedation, a gastroscope, (flexible fibre-optic telescope) or a pea-sized biopsy capsule mounted on a tube, is passed to the top end of the small bowel. A scraping of the lining cells is collected. The enzymes, (lactase and sucrase) can then be measured in the specimen. If sucrose malabsorption is suspected, your doctor will probably suggest that this biopsy test be performed. Sucrose malabsorption is usually a permanent disorder and so it is wise to be sure about the diagnosis before suggesting permanent changes in diet.


Any donation towards our research would be most welcome. Donations of more than $2 are tax deductible. Please make your cheque payable to Gastroenterological Society of Australia Research Institute and send to:

Administrative Officer
Australian Gastroenterology Institute
145 Macquarie Street, SYDNEY NSW 2000
Tel (02) 9256 5455 Fax: (02) 9241 4586

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