Digestive Health

Updated 08 May 2017

Lactose intolerance

Lactose intolerance is the inability to digest components of milk.


What is lactose intolerance?

Lactose intolerance is a condition caused by inability to digest one of the carbohydrate components of milk, specifically lactose, due to a lack of the intestinal enzyme lactase, which is essential for the efficient metabolism of lactose (milk sugar).

This condition can first show itself in the neonatal period with vomiting and profuse diarrhoea after milk ingestion, which resolves when lactose-free fluids are fed to the infant. However, it can also show up only in adulthood.

What causes lactose intolerance?

It is not known why some people are deficient in the enzyme lactase.

Because of the lack of lactase, the lactose in milk is not broken down. It remains in the gut, where it causes fluid retention which results in diarrhoea. This, together with bacterial fermentation of the milk sugar in the colon leads to abdominal cramps, distension, and gaseous, acidic stools.

A transient lactase deficiency may develop after an infant has undergone a protracted period of diarrhoea or has had a large segment of small bowel removed.

Who gets lactose intolerance?

Lactase deficiency occurs normally in about 75% of adults in all ethnic groups except those of northwest European origin for whom the incidence is less than 20%.

Although statistics are unreliable, most nonwhites gradually lose the ability to digest lactose between the ages of 10 to 20 years. It affects 90% of Orientals, 75% of blacks and Indians, with a high incidence among people from the Mediterranean region.

What are the signs and symptoms of lactose intolerance?

A child who cannot tolerate lactose will have diarrhoea and fail to thrive.

An adult may have bloating, flatus, nausea, diarrhoea and abdominal cramps. When lactose absorption is impaired by deficiency of the enzyme lactase, the resulting diarrhoea may be severe enough to purge other nutrients before they can be absorbed.

A history of milk intolerance may be obtained in patients with lactose intolerance. Some people recognise this early in life and consciously or unconsciously avoid eating dairy products, thus making a diagnostic history more obscure. In others, symptoms may simulate irritable bowel syndrome.

How is lactose intolerance diagnosed?

The diagnosis may be suspected if acidic stools (pH lower than 6) are passed or if, after a glass of milk, the patient develops abdominal cramps, distension and watery diarrhoea within 20 to 30 min. It is further substantiated by an oral lactose tolerance test, and can be absolutely confirmed by the finding of low lactase activity in a bowel biopsy specimen.

How is lactose intolerance treated?

The disorder is readily controlled by a lactose-free diet, or often simply by avoiding milk drinks. A child who lacks the enzyme lactase can absorb fructose. If a lactose-free diet is continued, oral calcium supplements should be given.

(Reviewed by Dr I.V. van Heerden)


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Dr. Estelle Wilken is a Senior Specialist in Internal Medicine and Gastroenterology at Tygerberg Hospital. She obtained her MBChB in 1976, her MMed (Int) in 1991 and her gastroenterology registration in 1995.

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