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) normally produced in your small intestine that is used to digest lactose.

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

Digesting lactose

After eating or drinking something containing 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 (other types of sugar), which are then absorbed into your bloodstream. If there is not enough lactase, the unabsorbed lactose moves through your digestive system to your colon (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 such as flatulence and bloating.

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 worldwide. This type of lactase deficiency is caused by an inherited genetic fault that runs in families.

Primary lactase deficiency develops when your lactase production decreases when your diet becomes 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, surgery to your small intestine, or taking certain medication.

Secondary lactase deficiency is the most common cause of lactose intolerance in the UK, particularly in babies and young children.

Possible causes of secondary lactase deficiency include:

gastroenteritis – an infection of the stomach and intestines
coeliac disease – a bowel condition caused by an intolerance to a protein called gluten
Crohn's disease – a 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

The decrease in the production of lactase in secondary lactase deficiency is sometimes only temporary, but it may be permanent if it's caused by a long-term condition.

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 an inherited genetic fault that means affected babies produce very little or no lactase

The genetic mutation responsible for congenital lactase deficiency is passed on in what is known as an autosomal recessive inheritance pattern. This means both parents must have a copy of the faulty gene to pass on the condition.

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 it usually improves as affected babies get older.

Bacteria: Bacteria are tiny, single-celled organisms that live in the body. Some can cause illness and disease and some are good for you.
Blood: Blood supplies oxygen to the body and removes carbon dioxide. It is pumped around the body by the heart.
Enzymes: Enzymes are proteins that speed up and control chemical reactions, such as digestion, in the body.
Gene: A gene is a unit of genetic material that determines your body's characteristics.
Genetic: Genetic is a term that refers to genes. Genes are the characteristics inherited from a family member.
Intestines: The intestines are the part of the digestive system between the stomach and the anus that digests and absorbs food and liquid.