Genes and lactose intolerance

In New Zealand, we drink a lot of milk, so you might be surprised to learn that most adults in the world are lactose intolerant and can’t consume dairy products.

Inheriting lactose intolerance

The ability to digest milk or milk products is inherited from our parents. All babies drink milk, but after they’re weaned, some lose the ability to drink milk without feeling ill. This is known as lactose intolerance.

If you are lactose intolerant and you drink a glass of milk, you may become nauseous or have stomach cramps, bloating and diarrhoea. The amount of milk that causes this reaction varies from person to person.

Causes of lactose intolerance

Lactose is a sugar that occurs naturally in milk and most dairy products. Lactose is readily broken down in the gut by an enzyme called lactase. Babies produce large amounts of lactase until they are weaned.

Lactose is broken down by lactase

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Lactose (the sugar in milk) is broken down by lactase into glucose and galactose.

Rights: The University of Waikato Te Whare Wānanga o Waikato

After about 5 years of age, most people (about 75% of the world’s population) stop producing the lactase enzyme. Without lactase, they can no longer digest milk, and they become lactose intolerant. Typically, people who originate from Africa, Asia or the Pacific stop producing the lactase enzyme after weaning and can’t consume milk or milk products in adulthood. However, there are a number of biotechnology solutions developed to help people who are lactose intolerant to consume dairy products.

Find out more about treating lactose intolerance.

Lactose intolerance is a recessive trait

Lactose intolerance is a recessive disorder. For a recessive disorder to show, both copies of the gene, called alleles, have to be identical. Individuals who have a cytosine (C) residue on both alleles close to the lactase gene do not produce lactase in adulthood and are lactose intolerant.

Genetic mutation helps digest lactose

So how do most New Zealanders consume milk without feeling ill? A single point mutation in the DNA near to the lactase gene changes the cytosine (C) nucleotide to a thymine (T). Individuals who have the thymine (T) nucleotide are lactose tolerant and can digest milk products in adulthood.

Genotypes of lactose tolerance and intolerance

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Most people in the world are lactose intolerant, but a genetic mutation where a cytosine (C) nucleotide in a person’s DNA is replaced with a thymine (T) nucleotide allows them to digest milk.

Rights: The University of Waikato Te Whare Wānanga o Waikato

People who can digest lactose often originate from northwest Europe (like many New Zealanders) and some parts of Africa. The genetic mutation that allows them to do this arose relatively recently, about 12,000–10,000 years ago, as farming became a major source of food.

Find out more about a genetic test for lactose intolerance.