Lactose intolerance is usually the result of your body not producing enough lactase. Lactase is an enzyme (a protein that causes a chemical reaction to occur) normally produced in your small intestine that's used to digest lactose. 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

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 as your diet becomes less reliant on milk and dairy products. 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

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 adverse reaction 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 cancer treatment
• 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's 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's caused by an inherited genetic fault that means affected babies produce very little or no lactase.

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.