person holding ice cream on a cone

The Biocultural Evolution of Lactose Tolerance

What is lactose?

Lactose is the main sugar in milk. It needs to be broken down into simpler sugars (glucose and galactose) in order for your body to absorb it.1 Lactase is the digestive enzyme that helps this breakdown happen.1

Lactose is a key source of carbohydrates for young mammals, and is one of the first sugars infants are exposed to after they are born (it is found in breast milk and most infant formulas).1 This is why it’s important for babies to be able to digest lactose.

What is lactose intolerance?

Lactose intolerance (also called lactase non-persistence) happens when your body doesn’t produce enough lactase, and therefore can’t breakdown/digest lactose into smaller, absorbable sugars.1 If you can’t digest lactose, then it accumulates in your gut and the bacteria in your colon start fermenting it, which produces gas and can lead to bloating and flatulence.1 The accumulation of lactose in the gut also attracts fluid, which can lead to diarrhea and additional bloating.1 However, not everyone who is lactose intolerant will experience these symptoms.1

Lactose intolerance is not the same as a milk allergy (which is where your body’s immune system has an adverse reaction to the proteins in milk – it has nothing to do with the sugar lactose).2

woman suffering from a stomach pain

What causes lactose intolerance?

There are a few types of lactose intolerance:

1. Primary lactose intolerance (most common) occurs when there is a drastic decrease in lactase production as you age, and your body cannot easily digest the amount of lactose in your diet.3 It is hereditary.

2. Secondary lactose intolerance can occur from an illness or injury to the small intestine (could result from cancer treatment, intestinal infection, intestinal surgery, Crohn’s disease, etc.).3 It is not hereditary.

3. Congenital lactose intolerance is rare but can occur in newborn infants producing very low levels of lactase as a result of genetics, or in premature infants who haven’t developed enough lactase producing cells in their small intestine.3

What causes lactase persistence?

As you age and replace milk with other foods, your body usually produces less lactase (the enzyme needed to digest lactose).3,4 However, some people are able to produce sufficient lactase into adulthood, and thus can digest lactose.4 This is known as lactase persistence (or lactose tolerance).4

How did humans develop lactase persistence (lactose tolerance)?

Primary lactose intolerance is actually a genetically determined dominant trait.1 You either have the gene that lets you continue to produce lactase into adulthood (lactase persistent – you can digest lactose), or you don’t.1 Around 10,000 to 6,000 years ago, lactase persistence arose independently in places like Northern Europe, Africa, and the Middle East.1,4,5

The beginning of dairy farming is some of the strongest evidence for this gene-culture coevolution.5 When the neolithic period began and humans started domesticating animals for their milk, there was a rapid rise in lactase persistence in places where milk was an important source of energy and nutrients.5 This was the case in Northern Europe, where it is assumed that the energy from milk products was relied upon to survive cold winters (fresh milk could be easily stored in the colder climates).1,5 Milk could also be relied upon if crops failed.1,6 Thus, lactase persistence was naturally selected for (meaning anyone who could digest milk had a genetic advantage).4,5

Places that didn’t domesticate animals for milk (like southern Europe and areas of Asia5) likely didn’t have this rise in lactase persistence because their livelihoods didn’t depend upon milk products. For example, the ancient Romans didn’t drink milk, and many of their Mediterranean descendants today are lactose intolerant.5

black and white cow on green grass field

Read more about Gene-Culture Coevolution and the Human Diet here

Why can some lactose intolerant people consume milk products without unpleasant symptoms?

Some people who do not produce sufficient lactase to digest lactose are still able to consume milk products without unpleasant symptoms.1,4 This may be because of differences in gut bacteria (specifically lactic acid bacteria).4 As well, some dairy products (like yogurt and cheese) have less lactose because they are fermented (the lactose has already been partially broken down), meaning lactose intolerant people can eat them without having unpleasant symptoms.1,4


  1. Willows, N. Gene-Culture Coevolution Lactase Persistence. NUFS 223: Cultural Ecology of Food & Health. University of Alberta, Fall 2021.
  2. Managing lactose intolerance. Unlock Food, updated 2019. (accessed 2022-02-13).
  3. Lactose intolerance. Mayo Clinic. (accessed 2022-02-12).
  4. Gerbault, P.; Liebert, A.; Itan, Y.; Powell, A.; Currat, M.; Burger, J.; Swallow, D. M.; Thomas, M. G. Evolution of Lactase Persistence: An Example of Human Niche Construction. Philosophical Transactions of the Royal Society B: Biological Sciences 2011366 (1566), 863–877. DOI: 10.1098/rstb.2010.0268 (accessed 2022-02-13).
  5. Arjamaa, O.; Vuorisalo, T. Gene-Culture Coevolution and Human Diet. American Scientist 201098 (2), 145-147. (accessed 2022-02-12).HHMI BioInteractive. The Evolution of Lactose Tolerance. YouTube, Aug 26, 2014. (accessed 2022-02-13).

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