Lactose Intolerance

Lactose is made up of two sugar units, glucose and galactose. Rich sources of lactose include cows’ milk, soft cheeses, custard, sweetened condensed milk and ice cream. To be absorbed in the small intestine, lactose must be hydrolysed into individual sugar units (glucose and galactose) by the enzyme, lactase. Lactase is predominantly found in finger-like projection (villi) of the jejunum. Therefore, lactase enzyme production is susceptible to conditions that cause cell damage to intestinal villi, such as gastroenteritis and untreated coeliac disease. 

In individuals who lack sufficient lactase to complete the breakdown of lactose, malabsorbed lactose attracts water into the small intestine and passes undigested into the large intestine, where it is fermented by gut bacteria. Fermentation of malabsorbed lactose can result in symptoms such bloating, flatulence, stomach cramps and diarrhoea. Lactose malabsorption that results in unpleasant symptoms is known as lactose intolerance.

Prevalence of lactose malabsorption varies a lot: affecting as few as 2% of people in Scandinavian countries and as many as 90% in parts of Asia.

Lactase enzyme insufficiency may occur for a range of reasons, including: 

• Non-persistence of the lactase gene expression. This phenomenon is relatively common, but prevalence varies considerably with ethnicity.

• Congenital abnormality where the genetic makeup of some people results in insufficient lactase production. However, this condition is considered very rare.

• Illnesses (such as gastroenteritis, untreated coeliac disease or parasitic infection), which can temporarily deplete lactase production. This is usually transitory, with absorption improving when the epithelium heals. Re-testing lactose tolerance may be worthwhile as lactose tolerance may change over time.