Lactose intolerance

Causes of lactose intolerance

Lactose intolerance is usually the result of a lactase deficiency. Lactase is an enzyme (protein that causes a chemical reaction to occur) that is normally produced in your small intestine.

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

Digesting lactose

After eating or drinking something that contains lactose, it passes down your oesophagus (gullet) and into your stomach, where it is digested. The digested food then passes into your small intestine.

The lactase in your small intestine should break down the lactose into glucose and galactose (another type of sugar), which are then absorbed into your bloodstream. If there is not enough lactase, the intact, unabsorbed lactose moves through your digestive system to your colon (your large intestine).

Bacteria in the colon ferment (break down) the lactose, producing fatty acids and gases such as 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.

Types of lactase deficiency

The main types of lactase deficiency are outlined below.

Primary lactase deficiency
Primary lactase deficiency is the most common cause of lactose intolerance. This type of lactase deficiency is genetically inherited (it runs in families).

Primary lactase deficiency develops when your lactase production decreases as a result of your diet being less reliant on milk and dairy products. This is usually after the age of two, when breastfeeding or bottle-feeding has stopped, although the symptoms may not be noticeable until adulthood.

Secondary lactase deficiency
Secondary lactase deficiency is a shortage of lactase caused by a problem in your small intestine. It can occur at any age, and may be the result of another condition or surgery to your small intestine, or it can be caused by some medications.

Possible causes of secondary lactase deficiency include:

  • coeliac disease, a bowel condition that is caused by an intolerance to a protein called gluten
  • gastroenteritis, an infection of the stomach and intestines
  • Crohn's disease, a chronic (long-term) condition that causes inflammation of the lining of the digestive system
  • ulcerative colitis, a long-term condition that affects the large intestine
  • chemotherapy, a treatment for cancer
  • long courses of antibiotics

Certain conditions and treatments can cause a decrease in the production of lactase. Sometimes the deficiency is temporary, but if it is caused by a long-term condition it may be permanent.

It is also possible to develop secondary lactase deficiency later in life, even without another condition to trigger it. This is because your body’s production of lactase naturally reduces as you get older.

Congenital lactase deficiency

Congenital lactase deficiency is a rare condition that runs in families and is found in newborn babies.

It is caused by a genetic mutation (a gene change), which stops one or more processes of the body from working in the right way.

Read more about genes and how they can affect our health.

The genetic mutation is passed through families in what is known as autosomal recessive inheritance.

Familial lactase deficiency

Familial lactase deficiency is similar to congenital lactase deficiency, and is also found in newborn babies. However, with this type of lactase deficiency, your body is able to produce a substantial amount of lactase, but the enzyme does not work and cannot break down the lactose.

As with congenital lactase deficiency, the genetic mutation responsible for the production of lactase is passed on in autosomal recessive inheritance.

Last updated: 11 February 2014

Continue to next section: Diagnosing lactose intolerance