If someone is lactose intolerant it means that they’re unable to break down lactose. This is due to the fact that their small intestine is not able to produce enough of the enzyme lactase. In a healthy body lactase breaks lactose down in glucose and galactase otherwise it can’t be absorbed. In someone with lactose intolerance lactase stays in the gut and then passes onto the large intestine. Sugars don’t usually end up here and this causes two main issues:

• Diarrhoea: this is caused by lactose decreasing the gut’s water potential so water isn’t absorbed from the faeces and too much water ends up in the gut lumen instead.
• Flatulence: bacteria in the colon thrive on the large amount of lactose and produce acids and gases (such as carbon dioxide and methane) through fermentation.

Usually, an individual develops lactose intolerance as they get older, either as a teenager or as an adult. It can be genetic or brought on due to a number of reasons including infection, surgery, chemotherapy or pregnancy.