Lactose Intolerance May Have A Lot To Do With Your Gut Microbiome

33:23 minutes

a black man in a blue tank pouring milk in a glass.
The mystery of lactose intolerance might be solved by your gut microbiome. Credit: Shutterstock

In the animal kingdom, it’s not normal to drink milk past infancy. It’s even more rare to consume milk from another mammal. But throughout history, humans have used dairy farming as a way to get calories and nutrition from creatures like cows, goats, and sheep. And a big perk: dairy products taste good. 

Evidence of dairying goes back to the early Neolithic era. Traces have been found in the historical record in Europe, Asia, and Africa, in ancient teeth and pottery. Lactase persistence, or the ability to consume dairy into adulthood, developed alongside this burgeoning industry. 

But here’s the catch: a large part of the population is still lactose intolerant, either from childhood or developed in adulthood. It’s estimated that about a third of the U.S. population is lactose intolerant, with a higher chance among certain ethnic and racial groups. 

There’s a lot to learn about the origins of lactose persistence and lactose intolerance, and much of that knowledge comes from the gut microbiome. Joining Ira to talk about this is Christina Warinner, associate professor of anthropology at Harvard University, based in Cambridge, Massachusetts.

Segment Guests

Christina Warinner

Dr. Christina Warinner is an associate professor of Anthropology at Harvard University in Cambridge, Massachusetts.

Segment Transcript

IRA FLATOW: This is Science Friday. I’m Ira Flatow. My physician told me the other day that she suspects that I may be lactose intolerant. Now, that’s not uncommon to debut in the later years, and I’m staying away from dairy for a while to test this out.

It’s not an easy thing to do to stop eating dairy products cold turkey because here in the old US of A, we love our cold glass of milk, our ice cream– oh, and I’m going to miss that cheese. Mm. But not everyone can enjoy these creamy treats, like me, if it’s true. It’s estimated that 30 to 50 million Americans– that’s a big number. 30 to 50 million are lactose intolerant, and your chance of being lactose intolerant depends on a lot of factors, including race.

Now, does that mean lactose intolerant people can no longer enjoy an ice cream cone? Well, that, among other questions, is what we’ll be discussing in this hour we call “What’s the Deal with Lactose Intolerance?” And joining me to tell us about it, from the history to modern treatments, is my guest, Christina Warinner, associate professor of anthropology at Harvard in Cambridge, Massachusetts. Welcome back to Science Friday.

CHRISTINA WARINNER: Thank you for having me.

IRA FLATOW: Nice to have you. Our number again, 844-724-8255 if you want to talk about lactose intolerance. We want to hear from you. We can’t answer any personal medical questions you have. That would be unethical.

But there is a really rich cultural, biological, and historical history of lactose intolerance that we can talk about. So what do you want to talk about? You make the call only if you make the call. Our number– 844-724-8255, 844-SCI-TALK, or tweet us @SciFri.

Let’s get right into this from the very beginning. Let’s start with a definition so that we’re all on the same page. Christina, what exactly is lactose intolerance?

CHRISTINA WARINNER: So lactose intolerance are a suite of symptoms that some people experience when they consume lactose. Lactose is the major sugar within milk, and the typical symptoms include things like diarrhea, cramping, and general discomfort.

IRA FLATOW: Mm-hmm. And what’s happening in the digestive system when someone who’s lactose intolerant eats ice cream, for example? What’s going on in there?

CHRISTINA WARINNER: Well. So lactose, being the main sugar in milk, is consumed by all mammals. Mammals consume milk. This is something that all mammals can consume in infancy. But actually, the natural state of mammals is to progressively stop producing lactase– the enzyme that breaks down lactose– as you age. It’s part of the weaning process.

And so actually, lactose intolerance or some of the symptoms of lactose intolerance reflect this sort of natural state of humanity that, at a certain point, you’ll stop drinking milk. You will no longer produce the enzyme that digests it. And what ends up happening is then the lactose passes through your digestive tract, it goes into your large intestine, where your gut microbiome is, and there the microbes will happily digest it for you.

However, unfortunately, they digest it by fermentation. And in doing so, they produce gases and acids which can upset your gastrointestinal tract. However, because the gut microbiome differs from person to person, there’s variation in lactose intolerance symptoms.

IRA FLATOW: Mm-hmm. And it’s obviously not a rare thing. I said before, 30 to 50 million people– wow.

CHRISTINA WARINNER: Yea, it’s very common worldwide. As I said, it’s actually the natural state of humans and mammals in general. Initially, lactose intolerance was presented as a kind of pathology, but we now know that it’s not actually pathological.

In fact, it is rather certain mutations that some humans have acquired throughout human history, human prehistory, that have changed their ability to digest lactose, allowing them to digest it longer into adulthood. We call that lactase persistence. So those people continue to produce the digestive enzymes that they did when an infant throughout their whole lives, making them more tolerant of milk sugars.

IRA FLATOW: You studied a fascinating case of Mongolia. And according to their DNA, people of this region should be lactose intolerant, but this is a big dairy culture. What’s going on there?

CHRISTINA WARINNER: Exactly. A lot of people don’t know about the dairy history of Mongolia, but it’s absolutely fascinating. So in Mongolia today, people there milk more species of livestock than any other place in the world. So they dairy the common dairy livestock species– so cattle, sheep, and goats– but also yaks and horses and camels and even reindeer. What’s very interesting about Mongolia is that people there do not have the genetic mutations that are known to enable lactose digestion into adulthood, and yet they have a dairy-based diet. And what seems to be happening there is a gut microbiome adaptation to long-term milk consumption.

IRA FLATOW: Because they’re surrounded with so many elements for your microbiome from all the lifestyles they do– I mean, the animals, the soil.

CHRISTINA WARINNER: Well, they have a really long history of dairying, so we can trace the history of dairying in Mongolia back to approximately 3,000 BC when it was introduced by migrating herders from the West. And so there’s a long and continuous tradition of dairying in Mongolia. However, as people move to the city and live more urban lifestyles, lactose intolerance is increasing because this gut microbiome adaptation, it’s not permanent or fixed. It really depends on the diet that you consume.

IRA FLATOW: So could that be part of the problem of 30 to 50 million people– that our microbiome needs a little tinkering with?

CHRISTINA WARINNER: Yeah, so lactose intolerance is not an on/off state. It actually exists on a spectrum. So some people who have these genetic mutations that allow them to produce the enzyme, lactase, what that means is that the lactose that they consume gets broken down into their small intestine, such that their large intestine just really never sees it. So they never experience those symptoms of lactose intolerance.

But many people lose that ability to produce the enzyme, and then they have this variable response. The population that has the highest amount of– or the highest rate of lactase persistence is Scandinavians, where approximately 80% of people produce this enzyme for their entire life, and then it declines from there. However, it’s not limited to Europeans. So we know that there are at least five different mutations that have arisen throughout human history in different parts of the world that occur, for example, in Europe, in East Africa, and through parts of the Middle East.

IRA FLATOW: Mm-hmm. Lots of people calling– 844-724-8255. Lots of folks are also tweeting us. So we have Natalie from Houston who wants to know if lactose intolerance is impacted by age.

CHRISTINA WARINNER: To some degree, yes, in part because your microbiome changes as you age. And so it will respond differently to the lactose that you consume. So a lot of people experience increased lactose intolerance as they age.

However, I will say, one thing that’s important is that different dairy products have very different amounts of lactose. So milk has a high amount of lactose. Ice cream has a high amount of lactose. But things like aged cheeses actually have very little lactose. So sometimes people become confused about whether or not they actually have a lactose sensitivity or another sensitivity to, for example, the proteins within cheeses and other dairy products.

IRA FLATOW: Which one of the aged cheeses? I’m wanting to know now.

CHRISTINA WARINNER: Well, most of the aged cheeses actually have very little lactose. So Parmesan has almost none. Also, even some of the aged soft cheeses, like brie, have almost no lactose.

IRA FLATOW: Wow. Wow. That’s good to know. Let’s go to the phones. Let’s see what kind of questions we have there. Kenny in Maryland, Welcome to Science Friday.

KENNY: Hi. Good afternoon.

IRA FLATOW: Hi there. Go ahead.

KENNY: So I was just going to bring up the point that I have a few friends that are lactose intolerant. And whenever I introduce them to goat milk and goat products, like goat butter and goat ice cream, or sheep– same thing– that they process that completely fine. But they cannot handle cow lactose anymore.


CHRISTINA WARINNER: Yeah, that’s really astute. Actually, a lot of people don’t know that different milks are compositionally quite different. They have different proportions or different amounts of lactose versus proteins versus fats. And for example, sheep milk is quite high in fats. And it is known that when you consume lactose in association with high amounts of fat, it actually slows down the digestion.

So even if you’re producing very little enzyme, it gives it more time to act and will reduce the symptoms of lactose intolerance. That’s why a lot of people who have sensitivity to lactose and maybe can’t drink a glass of milk might be able to tolerate ice cream better because again, the fat that’s in the ice cream actually slows down digestion and improves your ability to metabolize the lactose.

IRA FLATOW: Kenny, thanks for that observation.

KENNY: Absolutely. Have a good day.

IRA FLATOW: Yeah. Danielle on Twitter says, I’ve noticed that long periods without milk, I can then tolerate small amounts of milk. Does the enzyme replenish in the gut?

CHRISTINA WARINNER: Oh, that’s a great question. So the enzyme doesn’t seem to react at all or change at all in terms of how much milk you consume. So the enzyme is continuously expressed in infancy and then, in most people, declines through childhood, independent of how much milk they’re consuming.

But for those individuals that have lactase persistence or these mutations, they’ll just maintain this constant high level of lactase production. So that’s not what’s changing if you experience these kind of differences. More likely what’s changing your response to lactose is changes in your microbiome over time because your microbiome does react and respond to the foods that you eat because the foods that you eat are its major food.

IRA FLATOW: Have we been able to identify which bacteria or microbes are in our microbiome that would be beneficial to help us digest the lactose?

CHRISTINA WARINNER: That is a great question, and there is a lot of work going on to address exactly this question. We do know that certain bacteria are milk specialists, or they tend to really thrive in the presence of milk. There’s an entire group called bifidobacteria.

These bacteria are especially abundant in young children and infants, where they completely dominate the gut microbiome. But many people will maintain smaller populations of this group of bacteria into adulthood if they continue to consume milk. We don’t exactly know what role, however, they play in adult milk digestion.

IRA FLATOW: Mm-hmm. Let’s go to the phones again because we got some really interesting phones. One from Tracy in Meriden, Connecticut– hi, Tracy.

TRACY: Hello. Thanks for taking my call.

IRA FLATOW: Go ahead. Nice to have you.

TRACY: Thank you so much. I was wondering about raw milk because I had heard it said that those with lactose sensitivity can oftentimes tolerate raw milk as opposed to pasteurized, and that’s been true for me. So I was curious what you had to say about that.

CHRISTINA WARINNER: It’s a great question. So raw milk can still contain microbes, and those microbes contribute to the fermentation of lactose. So one thing that’s really interesting is for most of the period during which humans were dairying, they didn’t have any way of pasteurizing their milk or preserving it.

So if you milked a cow in the morning, by mid-afternoon it would already be turning to yogurt because it’s going to be fermenting from those bacteria. And what those bacteria are in part doing is they’re consuming the lactose. And they’re producing lactic acid, which is souring the milk as a byproduct.

And so when you’re consuming raw milk, you’re intaking higher levels of those lactic acid bacteria that help to break down the lactose. So that might be contributing to reducing somewhat the lactose content. But different raw milks contain very different amounts of lactic acid bacteria, depending on how the animals are raised.

IRA FLATOW: Tracy, do you consume raw milk regularly?

TRACY: I do. I get it from a local farm here.

IRA FLATOW: All right. Thanks for calling.

TRACY: Thank you so much. That was interesting.

IRA FLATOW: Thank you. What do we know about how the tolerance for milk drinking developed, Christina?

CHRISTINA WARINNER: Oh, this is a great question, and this is something I have spent the last 10 years trying to work on. It’s a very hard problem to answer if you think about it. Milk, as you might imagine, does not preserve very well in the archaeological record. It is a liquid. It decomposes very quickly.

But it turns out that it does leave a few traces behind. For example, milk fats will become embedded in ceramic pottery and different vessels, and those can be recovered to try to track milk throughout the archaeologically record. And what I’ve been working on in particular is that the dental plaque on the surface of your teeth actually becomes coated with milk proteins.

That calcifies while you’re alive. It’s actually the only part of your body that fossilizes while you’re still alive to form tooth tartar, which is the material you go and have your dental hygienist scrape off. But embedded in that tooth tartar are these milk proteins. And so one of the things you can do is study those proteins in the milk, and we can use that to actually track the origins and spread of dairying.

We know from this sort of research that dairying is at least 9,000 years old. It begins in the Near East, although where exactly, we are not precisely sure. It is going to be somewhere near Anatolia, which is where present-day Turkey is, or along the Levantine coast. Kind of in this general region, we believe, is where dairying begins.

And there from there it spreads. It spreads into Europe. It spreads south into East Africa. And then it spreads eastwards into Asia.

IRA FLATOW: Wow, wow. Wow, that’s fascinating. Fascinating talk with Christina Warinner, associate professor of anthropology at Harvard University in Cambridge– we’re talking about lactose intolerance. Das on Twitter says, as a South Asian, I noticed many of us are lactose intolerant. Do you know why?

CHRISTINA WARINNER: Yes. So the genetic mutations that allow lactase to be produced as an adult, they are present in some people in South Asia, but it’s not at a very high level. So most people of South Asian descent do not produce the enzyme as adults and so are more susceptible to becoming lactose intolerant.

And this has to do– these mutations themselves spread during prehistory through known human migration and interaction events. So what you’re experiencing is not uncommon. Lactase persistence is more common in the north than in the south, for example, within the Indian subcontinent.

IRA FLATOW: And that’s because?

CHRISTINA WARINNER: That has to do with the specific population and migration history of India.

IRA FLATOW: Hmm. Are there different ways lactose intolerance develops in people? I mean, do some people get it right away? Does it come in later in life with other people?

CHRISTINA WARINNER: Yeah, that’s a great question. Yes. It presents itself in a fairly heterogeneous way. Some people are very sensitive. Others are much less sensitive.

One thing that is really interesting– there have been scientific studies trying to understand if, for example, prolonged milk consumption will change your response. And it does. So for people who have– even people who have fairly severe lactose intolerance, if they continuously consume high amounts of dairy– that sounds very unpleasant, and I’m sure it is to undergo.

But after about a week, those symptoms will subside, and they’ll no longer experience diarrhea. What’s interesting is in Mongolia they actually have a type of cure that people go onto or a kind of treatment if they’re having gastrointestinal upset, where they’ll consume high amounts of dairy for about a week. And they find that they use this as a kind of purification means. And after that, they have improved dairy digestion.

IRA FLATOW: Do we know why that is?

CHRISTINA WARINNER: We believe it has to do with changing the gut microbiome. By feeding the gut microbiome high amounts of lactose, you actually shift the ecology or the proportions of the bacteria that are present. But narrowing down which specific bacteria are responsible is really tricky because there are more than 1,000 different bacterial species within the human gut.

IRA FLATOW: If you listen to the show, you know we love to talk about the microbiomes, so I’m glad we’re talking about it. A lot of folks on the phones– let’s go to the phones. Let’s see who we’re going to go to. Oh, let’s go to Carol in Chicago. Yeah, Carol in Chicago, welcome to Science Friday.

CAROL: Hey, Ira.

IRA FLATOW: Hi there.

CAROL: I heard you were just diagnosed possibly. And I wanted to take you out of your misery because I’ve been lactose intolerant for a long time, and there are these over-the-counter LACTAID pills. They’re fast-acting. You take one before you eat cheese, ice cream– whatever– pizza. And you have no problem.


CAROL: It’s a miracle little thing. I’ve been doing it for, like, 20 years. It’s amazing. I don’t how it works, but it’s great.

IRA FLATOW: That’s a great recommendation. Christina, why does that work?

CHRISTINA WARINNER: Well, it’s mimicking the production of lactase that would occur naturally in your small intestine. So it’s just replacing that function. So yeah, taking lactase enzyme pills in association with consuming dairy is a great way to minimize or eliminate the symptoms.

IRA FLATOW: Now, for the other side of the story, let’s go to Rachel in Brooklyn, New York. Hi, Rachel.

RACHEL: Hi. Thanks for taking my call. I’ve been lactose intolerant for about 10 years, since I turned 25– I guess a little more than 10 years. I have a very strange presentation of lactose intolerance from what I’ve discovered in talking to other lactose intolerant people.

I get stabbing pains in my stomach. I don’t have a lot of gas or diarrhea, but a lot of pain. And interestingly enough, LACTAID pills do not work for me, and I have no idea why. I’ve also been curious, noticing the impact of different types of dairy products on my stomach.

Melted butter is the worst. Melted cheese is extremely painful. But cheese that’s not melted, for some reason, is less painful. I just don’t really know why.

IRA FLATOW: All right, let’s get a reaction from Christina. Why might LACTAID not work for some people?

CHRISTINA WARINNER: It sounds like your intolerance might not be to the lactose itself but to something else because butter should have virtually no lactose whatsoever. So I suspect you might have an intolerance to another part of the dairy product and I’d talk to your doctor about that.

IRA FLATOW: Yeah, we can’t answer that one, but we can talk to Laura in Quincy, California. Hi, Laura.


IRA FLATOW: Hi, go ahead. You’re on Science Friday.

LAURA: Well, I became lactose intolerant– I’d say it was probably in my early to mid 30s. I went to the doctor. He told me avoid all kinds of stuff. And I avoided milk, and I was better. But then I went three years not eating gluten, and the lactose intolerance is gone.

IRA FLATOW: So what you thought was actually lactose intolerance turned out to be gluten intolerance.

LAURA: Yeah, and it made me lactose intolerant.


LAURA: Diarrhea was the symptom.

IRA FLATOW: Christina?

CHRISTINA WARINNER: Yeah, I think what you’re describing is not uncommon. What’s interesting is– so as I mentioned before, the symptoms of lactose intolerance are actually caused by how your microbiome responds to lactose. And depending on the composition of your microbiome, it can respond in different ways, whether it makes hydrogen gas, which makes you feel particularly bad, or if it makes you produce, for example, carbon dioxide gas, which can make you feel bloated but is not associated with as severe of symptoms.

The symptoms to gluten are also caused by a response to your gut microbiome. So it’s how your gut microbiome does or does not process gluten, also really strongly impacts the degree of symptoms you might have or the degree of response you have to gluten. So in both cases, those are two well-known food components where the microbiome plays a really large role in the particular symptoms you might have of intolerance.

IRA FLATOW: Speaking of the microbiome, probiotics and prebiotics– they’re a bit of a buzzy field right now. There have been some studies that show a positive correlation between probiotics and the ability to digest lactose. Do you think these are promising?

CHRISTINA WARINNER: I think it is complicated and often a lot more complicated than advertisements make it seem. So the things that we call– so probiotics are bacteria that can potentially aid in digestion. And so things like yogurt naturally come with these probiotic bacteria because they’re the bacteria that are digesting the components of milk.

And so the thing about probiotics, though, is that they have a very hard time surviving in your gastrointestinal tract. So first, they have to go through your stomach, which has a pH of 2. It’s super acidic, so they have to survive that. And then they have to be able to make it to your colon and establish themselves, which is very hard to do because it’s fully colonized with other bacteria.

In cases where it seems to be helpful, you have to take it with a food they actually consume. So they have a much higher chance of survival if you consume a probiotic, for example, in its natural food. So with yogurt, the yogurt itself helps protect the bacteria from being destroyed by the acid in your stomach.

And then once they’re in the colon, it actually helps to feed them so that they can survive. So probiotics work really well when taken in combinations with the kinds of foods that feed them. But if they’re taken in isolation, they have a really hard time surviving and establishing themselves.

IRA FLATOW: Mm-hmm. Now, as you were saying, not every region of the world has a culture of milk drinking. Where do we see a lot of historical milk consumption?

CHRISTINA WARINNER: So it begins, as I said, in the Near East and Anatolia. So kind of the Eastern Mediterranean is where milking seems to begin, but it isn’t a really large component of the diet. What’s interesting, too, which is I think one of the most fascinating puzzles that’s come out of recent ancient DNA and archaeological work, is there’s about a 4,000-year gap between when dairying first was invented and when the first genetic mutations arise that allow people to produce lactase for longer. So you can kind of think of it as effectively all of the original people who were consuming and making dairy products were effectively lactose intolerant in some way, and yet they could still do it.

IRA FLATOW: They were not just being– we wouldn’t know if they’re just in pain and eating and drinking dairy products.

CHRISTINA WARINNER: Well, we suspect that they weren’t in serious pain. But they didn’t have any special adaptations to it. They didn’t have any of the genetic adaptations.

I suspect that what is helping them is they’re consuming fermented products, as I said, in the absence of refrigeration and in pasteurization. Even within a few hours, raw milk will start to sour and already start to turn itself into yogurt and cheese-like products. And so there’s a natural reduction of lactose from that.

And also, they would have been utilizing much higher fat dairy products, and that also aids in the digestion. But it begins in the Near East. But where it really becomes important and where it comes to dominate the diet is actually in a region called the North Caucuses. This is a region between the Black Sea and the Caspian Sea.

It’s a really harsh region. You can’t grow agriculture there. And what ends up happening is people end up moving their dairy animals, who were previously farmers, where maybe the dairy made up a minor portion of their diet– they end up moving into this region and becoming full-time pastoralists– so just taking care of their animals. And they become the world’s first fully mobile pastoralists, meaning they have no permanent settlements but migrate with their herds to access different pastures.

This occurs approximately between 6,000 and 3,000 years ago. This is increasing intensification, and they are really the first population to really intensively utilize milk. They invent the wagon and many other technologies associated with the wheel. And that population expands both westwards into Europe and eastwards into Asia, and they spread this dairy technology. And it’s from this sort of Bronze Age expansion that dairy spreads to much of Eurasia.

IRA FLATOW: Would we suspect they became more tolerant, then, of the milk and the dairy products to so widely spread it around?

CHRISTINA WARINNER: That’s a great question. We do believe that one of the known mutations originates in these populations. But at the time when they’re doing this, it’s at a very low level. So this becomes a real question.

It’s actually later during the late Bronze Age and Iron Age that the levels of these mutations really start to increase in human populations. And we don’t know why it’s so late and why it doesn’t seem to be necessary during earlier periods. This is a real mystery.

IRA FLATOW: And is there a place that no one drinks milk, that we don’t see a lot of milk drinking going on?

CHRISTINA WARINNER: Yes, there’s many places around the world. So dairying was never developed in the Americas, for example, prior to European contact. It was never developed in Australia or in Oceania. But dairy does spread to large parts of Eurasia and Africa through these historic migrations and expansions.

IRA FLATOW: Interesting. Let’s go to the phones. Claire in Weybridge, Vermont, hi. Welcome to Science Friday.


IRA FLATOW: Hi there.

CLAIRE: Hi. I have a question about the enzyme you were talking about that is related to sustaining an ability to digest lactose, and I’m wondering how that is detected and whether it could be or already is part of a routine health screening.

IRA FLATOW: Oh, she’s from a cheese state, so she wants to know this.

CHRISTINA WARINNER: Yeah. So there’s two ways you can do it. So you can take a very simple genetic test to tell you whether or not you have the mutation that allows you to produce lactase. But more commonly in a clinical setting, people will take something called a lactose challenge test.

That doesn’t exactly measure the enzyme directly but a little bit more indirectly. So what you do is you consume lactose sugar in water, and then you breathe into a machine that measures the amount of hydrogen breath for the next two hours or so. And if the lactose passes very quickly into your colon, which will happen if you don’t have lactase production, then generally the bacteria will start to produce hydrogen gas.

And so you can measure that noninvasively by measuring your breath. So that’s the typical clinical test. But the problem with that test is it really does depend on the composition of your microbiome, so there is a bit of noise and a bit of error. So the more straightforward way to test for it is just to do a simple genetic test.

IRA FLATOW: Interesting. Thanks for the question, Claire.

CLAIRE: Thank you.

IRA FLATOW: You’re welcome. Does the origin of a dairy product change how our bodies react to it? Would my gut microbiome react differently from, let’s say, ricotta from Italy than to cow’s milk from Brazil?

CHRISTINA WARINNER: Oh, that’s a great question. I’m glad you brought up ricotta because ricotta is a really unusual dairy product. It’s actually not a cheese. So most cheeses, or what we call cheese, are typically essentially curds. So it’s the casein proteins in milk, which is a group of proteins that are easily coagulated to form the curd.

If you remember the nursery rhyme of “Little Miss Muffet sat on her tuffet, eating her curds and whey,” so that curd is what we use to make cheese. The whey– the liquid that’s left behind after you make cheese– actually still contains proteins. They’re just dissolved so that you can’t see them. But if you process it much, much more– and it’s really quite difficult to do– you can force those proteins to fall out of solution and into coagulate. And then you can turn that into ricotta.

IRA FLATOW: Wow. This is Science Friday from WNYC Studios, where you get to learn all about ricotta cheese like you never thought– it’s fascinating.

CHRISTINA WARINNER: Yeah, and the different dairy products– I mean, I think in the United States, we’re very familiar with a fairly narrow range of dairy products, mostly of European origin. But if you take a broader view, the number of different dairy products that are produced globally are incredible. For example, throughout central Asia and inner Asia, one very common dairy product is made by fermenting mare’s milk or horse milk.

This is called kumis throughout much of central Asia or airag in Mongolia. And this makes a kind of alcoholic milk beverage which is really widely enjoyed. That’s not the only alcoholic beverage you can make from milk. Also in Mongolia, they make another type of dairy product called shimiin arkhi, which is a type of distilled yak yogurt. And so there’s many different products that you can make. One of the reasons, though, that horse milk is so widely used to make alcohol is that of all the dairy animals, it actually has the highest lactose content, so it has the highest potential to produce alcohol.

IRA FLATOW: Well, what don’t you know yet that you’d like to know about? Wow, there’s wonderful little tidbits from all over the world. What would you like to know that you don’t know yet?

CHRISTINA WARINNER: Oh, I want to know so many things. One of the things I really want to know is where exactly milking begins and how it spreads. And this has been something that I am working on. I have a really talented post-doc and student working on this problem. But that’s one of the biggest questions I have.

But also, I am just absolutely perplexed by the fact that we have so much milk drinking. You have the creation of dairying systems. You have the expansion. You have even entire populations that are relying on milk as their primary dairy product, and yet we don’t have lactase persistence for much of this period of prehistory.

And then suddenly, around the Iron Age, the levels of lactase persistence– this mutation spreads like wildfire, and we don’t know why. And by the medieval period, for example, in Europe, it reaches present-day levels, which are pretty high. And what the process was is just completely elusive.

IRA FLATOW: And how do you find that out? By scraping the tartar off of old teeth that you do?

CHRISTINA WARINNER: Yes, I think that’s the brute force way to go about it is to reconstruct genomes across that period of time to try to understand the exact scale and the speed at which those genetic adaptations are changing, and then in parallel, to measure how much milk protein we have. There’s got to be an answer, and I think we just have to put together a really good team to solve it.

IRA FLATOW: Well, I was going to say, doesn’t seem to be a whole lot of people who are as interested as you are in doing this.

CHRISTINA WARINNER: I actually have people that– yeah. I think it is actually an area of great interest. I mean, this is actually a question that archaeologists and anthropologists have been debating and discussing for a really long time. And we keep coming back to it because on the one hand, it seems like a simple problem we should be able to solve, and yet it remains elusive.

IRA FLATOW: Do you lack the tools to nail this down or just being lucky in finding something that will pop out of some of your research?

CHRISTINA WARINNER: Well, I think a big change has been new technologies becoming available. So as I said, for a long time, it was just very hard to directly detect milk, and so we used a lot of indirect methods. The ability to detect milk proteins has been around for less than 10 years. So this is something that’s really ramping up, and I’m hoping that this will help resolve a lot of these longstanding questions.

IRA FLATOW: Mm-hmm. And so we’re going to be hoping that you can find out about the history of milking. Where’s the first place it started, if that’s what you want to know.

CHRISTINA WARINNER: Well, I want to know– yeah, a little bit. And I also want to know how it spreads into Africa because the trajectory of milking in Africa is also extremely interesting. And also, milking in the Arabian Peninsula also takes on an entirely different trajectory, where it seems to be associated with camel milking.

And one thing that’s really interesting there is that camels have a greater tolerance for salt in their water than humans do. So camels can tolerate saltier water than humans. So a lactating camel can function as a kind of desalinization plant for humans that are trying to cross a desert because finding freshwater oases is really difficult. They’re spread out. But if you have a lactating camel, you can access them.

IRA FLATOW: What a way to stop and end this conversation. Thank you for taking time to be with us today.


IRA FLATOW: Christina Warinner, associate professor of anthropology at Harvard.

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Kathleen Davis is a producer at Science Friday, which means she spends the week brainstorming, researching, and writing, typically in that order. She’s a big fan of stories related to strange animal facts and dystopian technology.

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Ira Flatow is the host and executive producer of Science FridayHis green thumb has revived many an office plant at death’s door.

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