Lactose intoleranceCauses




If you have a lactase deficiency, it means your body does not produce enough lactase.

Digesting lactose

After eating or drinking something containing lactose, the digested food passes from your stomach into your small intestine.

The lactase in your small intestine should break lactose down into sugars called glucose and galactose, which are then absorbed into your bloodstream.

If there's not enough lactase, the unabsorbed lactose moves through your digestive system to your colon (large intestine).

Bacteria in the colon break down the lactose, producing fatty acids and gases like carbon dioxide, hydrogen and methane.

The breakdown of the lactose in the colon, and the resulting acids and gases that are produced, cause the symptoms of lactose intolerance, such as flatulence and bloating.

Types of lactase deficiency

Primary lactase deficiency




This is usually after the age of 2, when breastfeeding or bottlefeeding has stopped, although the symptoms may not be noticeable until adulthood.

Secondary lactase deficiency


It can occur at any age, and may be the result of another condition, surgery to your small intestine or taking certain medication.


Possible causes of secondary lactase deficiency include:




Congenital lactase deficiency





Find out more about genetic inheritance

Developmental lactase deficiency

Some babies born prematurely (before the 37th week of pregnancy) have a temporary lactose intolerance because their small intestine was not fully developed by the time they were born.

This is known as developmental lactase deficiency and usually improves as affected babies get older.

Page last reviewed: 25/02/2019
Next review due: 25/02/2022