Breast Milk Is Understudied. What Are Scientists Learning Now?

FLORA LICHTMAN: This is Science Friday. I’m Flora Lichtman. If you’ve found yourself scrolling through the online breastfeeding world, you know that people have a lot of strong opinions about breast milk. It’s a magical elixir. Breast is best. Look at how big my baby got after three months of exclusive breastfeeding is a video I get served a lot more than I’d like.

But what exactly do we about the biology of breast milk? Does breast milk really adapt to a baby’s needs? Does it confer immunity? How does making breast milk impact the breastfeeder? Here to spin through the science of this very sophisticated substance are two experts in the field, Dr. Shelley McGuire, professor at the University of Idaho and director of the Margaret Richie School of Family and Consumer Sciences, based in Moscow, Idaho. And Dr. Deepshika Ramanan, assistant professor at the Salk Institute for Biological Studies, based in San Diego, California. Welcome to you both to Science Friday.

DEEPSHIKA RAMANAN: Thank you for having me.

SHELLEY MCGUIRE: I echo that. It’s great to be here.

FLORA LICHTMAN: Shelley, let’s start with you. You have researched breast milk for decades. To me, it seems like a pretty important substance for human existence. How much attention has it gotten from science?

SHELLEY MCGUIRE: Oh, golly, what a question. Thanks for asking that. Yeah, this is the most important food that Mother Nature has ever created. And it seems like we should know everything about it, but we don’t. We know more about what’s in a strawberry than what’s in human milk. And that’s not because there aren’t people interested in it. And that’s not because there aren’t great scientists. But the funding for this topic really has been not where it should be. And I could go on about that for hours.

FLORA LICHTMAN: Well, why is it special?

SHELLEY MCGUIRE: Well, if you think about it, which most people don’t, but if you think about it, human milk or breast milk is the only food that’s been evolutionarily designed to feed humans. Everything else that we eat has been co-opted by humans for human consumption. But this is a food that is so special because it’s the only food of its type. And we really need to understand what is in human milk because that is what is really evolutionarily designed to start humans off. It’s probably the most important food that we have.

FLORA LICHTMAN: You’ve made this point that we have doctors for other body parts, but not for this. Will you talk about that?

SHELLEY MCGUIRE: Yeah, again, something that most of us don’t think about, but there are neurosurgeons, there are foot doctors, there are heart doctors, but there’s no such thing as a specialty for the mammary gland or breast. And to me, that seems a little odd. I think that is based in the fact that it is, of course, a women’s issue, and there’s so much other stuff that’s around breasts and breastfeeding that I think it just hasn’t been a topic that we’ve been all that interested in tackling, unfortunately. And I think it’s time we do that.

FLORA LICHTMAN: Shika, anything to add?

DEEPSHIKA RAMANAN: Yeah, and I would say I’ve been studying breast milk immunity for the last few years, and what got me interested in that is my own experience of becoming a mother. And I think part of the reason why we don’t have a lot more research on this is because we don’t have enough women doing the work, and we don’t have enough mothers doing the work. And I think that brings the point of having more women in science and more mothers in science to ask these questions that people didn’t think of asking before.

FLORA LICHTMAN: Let’s talk about immunity in the role of breast milk. You researched this, Shika. What do we know about how that works, how breast milk can transfer immunity to babies?

DEEPSHIKA RAMANAN: Yeah, so it’s been known for a few decades that breast milk has antibodies that are protective for the babies. And mothers pass these antibodies in the breast milk. And what it means is, if mother receives a vaccine or an infection to a certain bug or a pathogen, then they develop antibodies against that pathogen, and then they provide those antibodies through breast milk to their babies because babies actually don’t start making antibodies until they’re a little bit older. So until the babies are able to make their own antibodies, mothers will provide this passive immunity.

But what’s actually striking is that antibodies are not the only protective thing in breast milk. There is just so much more going on. And breast milk is bioactive. There are so many live immune cells in breast milk. There are so many other factors in breast milk, including the oligosaccharides, which each mother custom-makes for their baby, which is just amazing.

FLORA LICHTMAN: What do you mean by that? Tell me more.

DEEPSHIKA RAMANAN: Yeah, every woman is custom-making the breast milk that their baby needs in terms of how much fat is in the milk, in terms of how much protein is in the milk. We don’t know a lot about the science behind this. We know that this does happen, but we still have a long way to go in understanding how this actually happens and what are the signals that the baby’s giving the mom, and how do the moms then respond to this by making this custom milk for babies? I mean, this is just fascinating.

SHELLEY MCGUIRE: I agree. I’d like to just add a little bit on to that. What Shika is saying is absolutely correct. There is this massive variation in milk composition among women, even within a woman. The milk she produces at the beginning of a feed is different from the end of a feed. Milk produced around the world is different. But the thing is, we don’t know if that matters. So it’s a very interesting thing to say that milk is customized to the baby, but we don’t know if that matters. I consider things like that really interesting fairy tales right now that need a lot of research to understand.

DEEPSHIKA RAMANAN: Yeah, I absolutely agree. We’ve been studying immune cells in milk, and we just had a story come out yesterday about how we found these immune cells that are migrating from the intestine of the mom to the mammary gland during pregnancy and breastfeeding. But we don’t know what these immune cells mean for the baby. We’ve seen in human milk that there’s a lot of these immune cells.

FLORA LICHTMAN: So not just antibodies, but actually live, like, T cells.

DEEPSHIKA RAMANAN: Yes, these are live immune cells. And what we specifically found is T cells, yes, correct.

SHELLEY MCGUIRE: And I’ll add on to that. In addition to immune cells that migrate, bacteria migrate. We used to think that human milk was sterile. Guess what? It’s not. Nothing is sterile. And we think that some of those bacteria are actually coming to the breast from the mother’s intestine.

FLORA LICHTMAN: Why would they be coming from the intestine?

SHELLEY MCGUIRE: So our whole systems are set up to process the environment through our gastrointestinal tracts. And so our gastrointestinal tracts have so many microbes in there. And it’s a reflection of what we’re coming into contact with in our environment.

So if you think about it, the mom comes in contact with these microbes. Maybe she wants to teach the baby that these microbes should be tolerated and are OK. So they’re transported to the breast, they’re put into the milk, the mom says, OK, these are OK bacteria. Don’t respond to them with some sort of a bad immune response. So again, I’m weaving a fairy tale here about what we think, but we don’t know, to be honest.

DEEPSHIKA RAMANAN: Yeah, we don’t how this might be helpful for babies and what it is doing to the immune system in the babies. But the other thing I think we don’t think enough about is what does that do for the mothers? Because you now have this huge influx of immune cells that are coming from in the intestine or even other parts of the body to the mammary gland now, to the breast tissue. And you potentially also have microbes.

And there has been a lot of work showing how breastfeeding has been helpful for mothers. So it speeds postpartum recovery. And also some correlative evidence saying that for every year a woman breastfeeds, they are less likely to develop breast, uterine, and ovarian cancers later on in life, as well as other chronic diseases such as heart disease and type 2 diabetes. But we still don’t know a lot about how is this process protective. And it obviously doesn’t protect everybody. So how does it protect some women but not others? And there’s a lot left to do in this area as well.

FLORA LICHTMAN: If this turns out to be more than a correlation, that actually it is protective for parents to breastfeed, could this be an evolutionary adaptation? Now that you have a baby, you got to be kept alive to take care of it.

SHELLEY MCGUIRE: Yeah, I think you’re right on with that. Of course, this is evolutionary. I mean, this is the only way that babies were fed for a very, very long time. And so we have evolved to do things well. Mother Nature is a smart thing. And this process does have to be protective to the baby and to the mom in the long run. Otherwise, it’s way too much energy to invest in a biological process.

FLORA LICHTMAN: I have to come back to this, this idea that breast milk is not sterile, that it’s bioactive, it’s filled with microbes. Why did I spend what felt like years of my life sterilizing flanges?

SHELLEY MCGUIRE: Well, OK, there’s a good reason for that. Maybe we’ve gone overboard. But we have no reason to think that the bacteria that are in human milk cause a problem. But we do have– there is something called foodborne illness that you can get from other things.

FLORA LICHTMAN: I am familiar with that, yeah.

SHELLEY MCGUIRE: It’s not a ridiculous idea. But this idea that everything that we feed our babies should be sterile, I think we’ve debunked that now.

FLORA LICHTMAN: Shelley, how much variety is there in breast milk across the world. Is it like Budweiser? It’s pretty consistent no matter where you’re drinking it?

SHELLEY MCGUIRE: It is not like Budweiser in a lot of ways. It’s very different. We did a study a few years ago. We collected milk in nine different countries around the world. And so many things are different. And the question is, does it matter? I would like to think that it does. I would like to think that that milk produced by those moms, let’s say, in rural Ethiopia, is setting those babies up for success in rural Ethiopia, and that milk in Spain is setting babies up for success in Spain. But again, that’s one of these fairy tales that we would like to be able to support with science.

FLORA LICHTMAN: I think a lot of people who are breastfeeding also end up thinking a lot about what they’re consuming, and they’re really cautious about what they put in their bodies because they don’t want it passed along. And it seems like there’s a lot of uncertainty around this. Shelley, I know that you’re studying maternal cannabis use. What have you learned and why are you focusing on that?

SHELLEY MCGUIRE: Oh gosh, what a hot topic. So recreational cannabis has just recently been legalized in a lot of states. And like so many things, we don’t really know the impact on milk composition or infant outcomes. And so when cannabis became legalized in the state of Washington a few years ago, my colleague Courtney Meehan, who’s at Washington State University, we put our heads together and we said, we need to understand this because right now, the recommendation is that breastfeeding women not use cannabis. And that’s based on just being super, super careful. It’s not based on science.

And so what we do know so far is that women who use cannabis during breastfeeding, the active component THC, does show up in milk. But again, we have all this variation. In some women, it shows up a lot. In some women, it hardly shows up. In some women, it lasts a long time. In other women, it disappears.

So we’re just learning the basics about that. We know nothing about potential impacts on the baby at this point. There is a lot to learn. A lot about alcohol right now, but almost nothing about cannabis. So we’re keeping an open mind because another thing I’ll tell you that we’ve learned is that at least in our experience, women are using cannabis for all the right reasons. They feel like it’s safer than prescription drugs, let’s say, for anxiety and postpartum depression. And they really want to know whether it’s OK to use or not. But we just don’t know.

FLORA LICHTMAN: I mean, there is a stigma that applies to cannabis that does not apply to antidepressants or pain pills, which are prescribed.

SHELLEY MCGUIRE: Yeah. But a lot of those drugs also haven’t been studied a whole lot in breastfeeding women. So we’ve got a lot of work to do.

FLORA LICHTMAN: I think there can be a lot of pressure on people to breastfeed. I’m sure you’ve come across this. A lot of people can’t for a lot of reasons. A lot of people choose not to for a lot of good reasons. You all are breast milk scientists. You’re human milk scientists. What is your take on this?

DEEPSHIKA RAMANAN: You know, as a mom, myself, we have enough mom guilt. I don’t think we need to be putting more guilt on ourselves. If I’m not breastfeeding the baby, then I am setting up my baby for a lifetime of failure in terms of immunity or something like that. We’ve got to do what’s best for us and the baby. I mean, and if that means that you are going to give the baby formula, that’s absolutely fine.

The truth is, like Shelley said, we still have a long way to go in terms of figuring out what is it about breast milk and what is in breast milk that is protective, or how is it impacting the health of the babies? And once we have more information on that, hopefully, we can make formula better, and maybe we can even offer women a way to become better producers because one of the reasons why women don’t breastfeed or can’t, it’s because they can’t breastfeed.

And maybe we can find a way to offer women the option if they wanted to breastfeed and they can’t, that they could maybe take a certain probiotic or a certain dietary supplement that would help with that. But my take is, we have a long way to go. And if, right now, you are struggling with breastfeeding, don’t stress about it too much. Just offer your baby formula, and it’ll be OK.

SHELLEY MCGUIRE: I’d like to follow up a little bit on that. One thing I want to make crystal clear is that we do know that breastfeeding saves babies lives in areas of the world where there is no safe alternative. So I want to be very– I want to be crystal clear about that. And we do know that there are benefits of breastfeeding.

But what we also know is that babies, at least in places like the United States, can do quite well on other forms of nutrition. And so we just have to make sure we’re putting all of this in perspective and looking at the big picture.

FLORA LICHTMAN: I mean, you kind of alluded to this, but is there a world where understanding breast milk better actually helps us make a better formula?

SHELLEY MCGUIRE: Oh, gosh, yeah, absolutely. I mean, I tell you what, I have fabulous colleagues that work for companies that make formula, and that’s exactly what they want to do. And they are trying to use what we know about human milk composition in the most ethical, scientific way to make their formulas better.

DEEPSHIKA RAMANAN: Yeah, and we’ve come a long way. I mean, I think 10-15 years ago, all of the formulas were based on cow milk. And to some extent, they still are. But if you go to the supermarket right now, you see that there are a lot of formulas that are designed to be closer to human milk. And we still have a long way to go on that. But there has been a change in the last few years, I would say.

FLORA LICHTMAN: We’re just about out of time. Do you have final thoughts on this, things you want our audience to, or things you don’t know about breast milk that you’re dying to learn?

DEEPSHIKA RAMANAN: I will say, when we pick up an ingredient in the grocery store, when we pick up a food, we know exactly what’s in it. And being that breast milk is the first food that most babies get, and we don’t know enough about it, there is still so much left to find out about the wonders of breast milk.

SHELLEY MCGUIRE: Yeah, I agree. I am actually very, very positive about the future of this sort of research. I think there’s a renewed interest in infant health, infant nutrition, child health, and I just have a lot of hope that we’re going to see a resurgence and a renaissance of human milk research in the next few years.

FLORA LICHTMAN: Shelley, I hope you’re right.

SHELLEY MCGUIRE: Yeah, me too.

FLORA LICHTMAN: Thank you both for taking the time today.

SHELLEY MCGUIRE: You’re very welcome. Thanks for having us.

DEEPSHIKA RAMANAN: Yeah, thank you for having us. This was great.

FLORA LICHTMAN: Dr. Shelly McGuire, professor at the University of Idaho. And Doctor Deepshika Ramanan, assistant professor at the Salk Institute for Biological Studies.

Copyright © 2025 Science Friday Initiative. All rights reserved. Science Friday transcripts are produced on a tight deadline by 3Play Media. Fidelity to the original aired/published audio or video file might vary, and text might be updated or amended in the future. For the authoritative record of Science Friday’s programming, please visit the original aired/published recording. For terms of use and more information, visit our policies pages at http://www.sciencefriday.com/about/policies/