#7: How we adapted to milk, and how we adapted it to us

This week: A dive into lactose intolerance.

This is my seventh post of Scientific Discovery, a weekly newsletter where I’ll share great new scientific research that you may have missed. Check out the About page if you’re interested in why I’m writing this.

If you’re like me, you might have thought that digesting milk was a binary: either you were able to do it or you weren’t.

But that’s not true, and I only realised it when I discovered four years ago that I have partial lactose intolerance!

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And after reading much more into the topic, I found out about lots of interesting things about how humans adapted to digest milk, and how we adapted it to be digestible to us.

How we’ve adapted to milk

Milk is a great source of calories and nutrition, but it can be a major pain. Its sugar – lactose – is indigestible for many people around the world.

Newborns and infants produce an enzyme called lactase that can break down this sugar. But as they age, only some children and adults (who are called ‘lactase-persistent’) continue to produce it.

For others, the lack of lactase can mean drinking milk becomes intolerable, causing symptoms like constipation, bloating and diarrhoea.

In people with lactase persistence, lactose is broken down into glucose and galactose in the small intestine. But in other people, it’s fermented by other bacteria in the large intestines instead – where it’s transformed into hydrogen, fatty acids, carbon dioxide and methane. The release of these gases results in bloating and constipation. The remaining lactose also changes the osmotic balance in the large intestines, which pressures more water into being released, leading to diarrhoea. (Abbreviations: LP = lactase persistent, LNP = lactase non-persistent) Ségurel and Bon (2017)

Today, the share of adults who produce lactase varies widely around the world, although we still lack good estimates of its frequency in many countries.

Lactose tolerance (being able to digest lactose) is common in Europe, South Asia and West and East Africa, while it seems to be much less common in East Asia, and Central and Southern Africa.

To see how it developed, let's first look at milk consumption in history.

It’s estimated that milk consumption became widespread in Europe from 7,000 BC onwards (shown below). Milk drinking grew as nomadic and pastoralist cultures migrated into the continent and spread the practice of dairy farming.

This map shows relative estimates of dairy consumption in Europe in ancient history, based on analysis of fat residue in ancient pottery. Regions in red had most of their pottery samples containing dairy residue, while yellow regions had few or none. Evershed et al. (2022)

But the genetic mutations that allowed people to digest this milk only began to spread thousands of years later. You can see this in the maps below.

Across Europe, Asia and North Africa, lactase persistence is largely determined by a single genetic difference (13.910*T) near the LCT gene, which encodes the lactase enzyme.

This change in a single base of DNA, from a C to a T, quickly spread across Europe starting around 3,000 years ago. It means that most people in Europe today continue to produce lactase into adulthood.

These maps show the frequency of the 13.910*T genetic variant. This variant has a very strong effect on whether people in Europe are able to digest lactose. The maps are estimated based on ancient DNA samples. Today, most people in North and West Europe have the variant that allows them to produce lactase as adults and digest milk. (Abbreviations: BP = before present) Ségurel et al. (2020)

In the Middle East and East Africa, there are other genetic variants that help people continue to produce lactase. They also influence the same LCT gene, but are much less common. You can see them below.

These maps show the frequency of other genetic variants that increase lactase persistence. Their frequency is shown in darkening shades of red (it seems like the scale on the fourth panel has typos). The dots show where ancient dental samples were taken, which were used to determine how common milk consumption was in those regions historically. Bleasdale et al. (2021)

But hold on a second. Why did it take thousands of years after people consumed dairy for the Eurasian lactase mutation to become common? How did people digest milk before that?

Part of the answer is that even without lactase, people can consume small quantities of milk without symptoms – by consuming it slowly or with the help of different intestinal microbes that are more efficient at breaking lactose down.

And milk from different animals contains different levels of lactose. Some types have low levels of lactose that wouldn’t have caused major problems.

How much lactose is present in different types of milk? FAO (2013) Some milk has low lactose content, which may have helped people with lactose intolerance. For example, it’s believed that Sámi populations, which herd reindeer, have lower rates of lactase persistence because reindeer milk contains less lactose.

But there are bigger reasons too.

Milk isn't the only source of food for people with lactose intolerance, of course. But when other sources of food were not available, due to drought or crop disease, the ability to digest it could have become pivotal.

People who farmed crops in sedentary cultures might suffer more in these situations. But people in pastoralist cultures, for example, might be able to migrate into other flourishing areas and rely on milk products from their herd. A major event like this might have given them a survival advantage and set off the spread of the genetic variant.

And there's more. Aside from these ways by which we adapted to drink milk, we also adapted milk to make it more digestible to us.

In many cultures, milk is commonly fermented into other dairy products, like yoghurt, kefir and cheese, for preservation. These have lower lactose content or contain microbes that help digest it.

How much lactose is present in different dairy products? Some fermented dairy products, like cheese, have lower levels of lactose, which makes them more digestible to people with lactose intolerance. Other fermented dairy products like yoghurt contain microbes that break lactose down. Gerbault et al. (2014)

It's really cool to think about how people in history developed fermentation techniques that made milk digestible when they didn’t have the enzymes to help them.

But today there are even better ways of doing it, because we can pin down the reason that milk indigestion develops.

Thanks to the discoveries that lactose produced indigestion, that lactase broke it down, and that various microbes produced lactase too, we can now mass produce the lactase enzyme with moulds, yeast or bacteria.

Then, we can use it to process milk and turn it into lactose-free milk.

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Or we can just take lactase in a tablet with a meal, without changing what we drink or eat. That’s pretty neat!

An illustration showing how lactose can be broken down into lactose-free milk before it’s packaged.

So the ability to digest milk isn't a binary. It hasn't been a binary for a long time – because of cultural techniques to ferment milk – but it is even less so today, with the ability to mass produce the lactase enzyme.

And we also have lots of other milk sources and milk substitutes without lactose too – like coconut milk, almond milk, soy milk, and oat milk – which avoid animal suffering and the environmental impacts of dairy farming.

As someone who loves cheese, I think that's a great thing.

More links

Here’s some more great stuff I’ve been reading:

• At Works in Progress, we’ve started a new Substack series of personal content from our authors. The first is by Stuart Ritchie on how science is usually done backwards, and his experience with turning it back around.

• I’ve been looking for a study like this for months. Vaccination against smallpox ended in most of the world in the 1970s; since those vaccines also protect against monkeypox, some people alive today already have some immunity to it. This great paper estimates of how much protection each country already has against monkeypox (not much).

I’m guessing you want to see this map without clicking the link, so here it is! It shows the share of people in each country who are estimated to be vulnerable to monkeypox, based on smallpox vaccination rates and timings and the age of the population. It was surprising to see that countries like India and Nigeria, which were among the last to stop vaccinating against smallpox, have high susceptibility. But I think this is because they have a larger younger-aged population today. Taube et al. (2022)

• Why are bats responsible for so many zoonotic diseases? An episode on This Week in Virology.

• A podcast episode on the life story of Joseph Lister, who pioneered antiseptic surgery, told by two comedians.

• It’s estimated that vaccine mandates at US universities reduced the country’s total number of Covid deaths by 5% in autumn last year.

• A major crystallography database is retracting

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  nearly a thousand entries of crystal structures after they were discovered to have been faked. And PLOS ONE is retracting more than a hundred papers after discovering that they went through fake or manipulated peer review.

• When will we have a universal coronavirus vaccine, if ever? Hilda Bastian has a great roundup of all the research on it.

And that’s all!

I hope you subscribe if you haven’t already, and share all this cool stuff you've learnt about digesting milk with your friends.

I won't have a post out next week, as I’m away travelling, but I'll be back after that.

See you next time.

– Saloni

[Updates:

6/8/2022: The caption for the first figure initially said lactose intolerance led to the release of carbon monoxide – that should have said carbon dioxide! An embarrassing accident, which I've corrected.

7/8/2022: This post was updated to mention alternative sources of milk and milk substitutes without lactose that reduce animal suffering and the environmental impacts of dairy farming.]

1. Thanks to 23andMe – it’s only a problem for me in large quantities (I have one of the less common variants that helps break down lactose), so I probably would have continued drinking dairy and feeling sick afterwards for years without realising why.

2. Note that milk that’s treated with lactase before it’s consumed tends to taste different (possibly sweeter due to the increased glucose content), and also tends to have a shorter shelf-life. This doesn’t affect lactase tablets, however.

3. Oops, I initially had the wrong link here. That’s now corrected.