In New Zealand, we drink a lot of milk, so you might be surprised to learn that most adults in the world are lactose1 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 stomach2 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.
After about 5 years of age, most people (about 75% of the world’s population3) stop producing the lactase enzyme4. 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 recessive5 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 gene6 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 mutation7 in the DNA near to the lactase gene changes the cytosine (C) nucleotide to a thymine (T). Individuals who have the thymine (T) nucleotide8 are lactose tolerant and can digest milk products in adulthood.
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.
Activity idea
Lactose intolerance – investigate the effect of the digestive9 enzyme lactase on a sugar found in milk called lactose. The digestive system10 condition11 known as lactose intolerance12 will also be looked at.
Useful links
Ask a geneticist about inheriting lactose intolerance – information about lactose intolerance13 written for high school students by The Tech Interactive, California, US.
Find out more about lactose intolerance, including the amount of lactose in milk products.
- lactose: A type of sugar found in milk and milk products.
- stomach: An organ of digestion with a sac-like shape located between the oesophagus and the small intestine. It temporarily stores food, mixes and churns it with gastric juice and allows digestion of some of the protein content of food to take place.
- population: In biology, a population is a group of organisms of a species that live in the same place at a same time and that can interbreed.
- enzyme: A complex protein that acts as a catalyst (speeds up chemical reactions) in specific biochemical reactions. For example, saliva contains an enzyme called amylase that can break down starch into simple sugars.
- recessive: Two copies of the allele need to be present in order for a recessive gene to be expressed.
- genes: A segment of a DNA molecule that carries the information needed to make a specific protein. Genes determine the traits (phenotype) of the individual.
- mutation: Occurs when the DNA is damaged or changed in such a way that it alters the genetic message carried by that gene.
- nucleotide: The basic structural unit of nucleic acids like DNA and RNA. A nucleotide consists of a base (adenine, thymine, guanine, cytosine or uracil), a sugar (ribose for RNA, deoxyribose for DNA) and a phosphate group.
- digestive system: The group of organs that are involved in the breakdown of food in the body, which includes the stomach and intestines.
- digestive system: The group of organs that are involved in the breakdown of food in the body, which includes the stomach and intestines.
- condition: An existing state or situation; a mode or state of being.
- lactose intolerance: The inability to digest lactose, resulting in discomfort and bloating.
- lactose intolerance: The inability to digest lactose, resulting in discomfort and bloating.
