Milk tops in a supermarket

Hard to tolerate

The way we metabolise chemicals varies between populations

Human milk contains the sugar lactose, and babies make the enzyme lactase to digest it.

Some adults who don’t consume lactose stop making lactase, and they become lactose intolerant. Any lactose they drink is digested by the bacteria in their colon and can cause bloating and cramps.

In regions where people drink cows’ milk, lactase production often stays switched on. People in northern Europe, for instance, are often lactose tolerant, whereas people from China are more likely to be lactose intolerant.

The genetic changes that lead to the persistence of lactase have occurred independently several times and in different populations as people have domesticated milk-producing animals. The levels of alcohol dehydrogenase can also vary between people. This enzyme works in the liver to break down ethanol produced by bacteria that live in the gut, as well as that from alcoholic drinks. Heavy drinkers make more, but not enough to prevent the damage caused by the toxic ethanol and its breakdown products.

Mutated forms of alcohol dehydrogenase and aldehyde dehydrogenase, which are both involved in alcohol breakdown, are prevalent in East Asian populations. They have been linked to increased sensitivity to alcohol, including a facial flushing reaction, and even to the risk of alcoholism.

Lead image:

Steve/Flickr CC BY


About this resource

This resource was first published in ‘Food and Diet’ in June 2011 and reviewed and updated in August 2016.

Genetics and genomics, Health, infection and disease
Food and Diet
Education levels:
16–19, Continuing professional development