Breast Milk and the Blood Stream: How human milk is made

Yes, parts of breast milk come from blood! Just how is breast milk made? Today’s post covers the basics.

Let’s review some anatomy first, then go over how someone’s blood will influence their breast milk. Lastly, I’ll go over why this matters.

A few words on breast anatomy for making milk

(It’s possible for someone to produce milk without being pregnant, but this is usually done under the care of a health care professional and includes prescription medications. For this article, I’ll focus on pregnancy and delivery prompting the hormone changes, as that is far more common. And, for clarity, I’m also going to use the term “mother” when referring to the breastfeeding parent.). 

Humans have breast tissue in their chests. When someone is pregnant, hormones involved in the pregnancy trigger big changes in the breast tissue, preparing it to make milk. 

After delivery, there are even more hormone changes and the mother’s body prepares to release the milk (in order to feed a baby, for example). 

Usually a few days after delivery, we say the milk has “come in.” Once milk has “come in” and breast milk production is established, continued milk production is closely related to how often the breasts are emptied. The more often a baby feeds or a mom pumps milk, the more milk she’ll make. 

(The physiology involved with the first few days of breastfeeding is beyond the scope of this post. To be clear, there is milk present before the full supply comes in. It’s called colostrum and is perfect for newborns. See this article for more details). 

Alveoli make milk.

The parts of the breast that makes the milk are called the alveoli. They’re often referred to as little balloons. But, unlike balloons, alveoli have a lot going on inside. The inner lining is very busy making what will eventually be breast milk. 

So, some parts of the milk are made in the breast itself.

Other parts are absorbed from the mother’s blood as it passes by. 

This is how breast alveoli absorb something from the mother’s blood:

Blood travels past the alveoli. As the blood passes by, the alveoli extract what they need from the blood in order to make the milk complete.

Between what is made in the breast and what is absorbed from the blood stream, alveoli have what they need to make precise and excellent nutrition for a baby:

• proteins
• carbohydrates
• water
• vitamins
• minerals

It’s worth noting that some of those proteins make up part of the immune system. More on this below.

As for the blood stream, some things do not pass easily from the blood into breast milk. And some don’t get absorbed at all. 

How do nutrients get into the mother’s blood stream?

Let’s take a step back. If the alveoli absorb nutrients from a mother’s blood, then how do the nutrients get into her blood in the first place?

If a mom eats something, the nutrition is absorbed through the mouth, the stomach, or the intestines, depending on the food and nutrients involved. We can refer to the whole pathway as the gastrointestinal tract or GI tract. 

Once a vitamin (for example) is absorbed, it’s carried by the blood and delivered throughout the body. There’s a lot of regulation, filtering (hooray kidneys), and balancing so that the body can hum along in health even if someone isn’t eating a totally precise diet.

There are many (!)  exceptions to this, of course, as this is just an example. I have to say that some people do need to be precise and everyone should follow the guidance and recommendations of their own personal physician. 

But back to the blood . . .

The blood continues to travel around the body, including past the alveoli. The alveoli also absorb and exchange what they need to in order to make human milk. 

If something didn’t get into the bloodstream, it’s not going to get passed into the alveoli. 

Different things are absorbed and used by a mom’s body at different levels. This is why there isn’t a universal rule about maternal medications while breastfeeding. 

And! If a baby drinks breast milk, their own body absorbs it from their GI tract and then into their bloodstream. So, there are a few levels of “filtering” here. Usually, a medication taken by the mom will be at different levels in her blood compared to her breastfed baby’s. 

Not everything makes it from a mom’s GI tract into breast milk in equal amounts.

For some things, like caffeine, pretty low levels pass from the mother’s blood stream into the breast milk.

For other things, like toothpaste that is spit out, very little is expected to be absorbed into a mother’s blood stream, so almost none is expected to make it all the way into breast milk. 

And then, there are certain medications, like some chemotherapy for example, that do get absorbed into the blood stream and then pass on into the breastmilk. Because these could potentially be harmful to a baby, these are situations in which breastfeeding may not be recommended. 

Some things don’t travel into the breast milk at all.

On the other hand, there’s another more extreme example: fiber. Humans don’t actually absorb fiber into their blood stream from the gut. It has an important role in the GI tract itself, but it doesn’t end up in the blood. 

See where I’m going with this? Fiber from a mom’s diet won’t end up in the breast milk either. 

I have to point out the obvious now, for the sake of moms with fussy babies everywhere. If a mother eats cabbage or lentils or something else that gives her stomach a few rumbles, that same fiber won’t affect her breastfed baby. It doesn’t make it into the breast milk. 

A recap on alveoli, blood, and a mother’s GI tract

To summarize, the alveoli make the milk both from what they can make themselves and from what they can absorb from the mother’s blood stream. So, when we think of what a mom eats or drinks and how it affects a baby, we first need to think about how much of it gets in her blood from the GI tract.

 If we’re thinking about topical things like lotions or sunscreens, we must think about how much of something gets absorbed through the skin and into the blood before drawing conclusions about breast milk. 

What else influences what gets into human milk?

First, let’s give the breast tissue even more credit. When milk is made, it takes what it needs from the mom’s body in order to make amazing nutrition for her baby. Moms around the world eat mightily varied diets. And yet they all make milk to feed their babies that have really similar fundamental “ingredients.” For example, the same basic types of protein are made by the alveoli for the breast milk whether a mom eats red meat or tofu or something totally different for her protein source. 

A breastfeeding mother’s diet is also an entire other topic, but I mention it in order to take pressure off breastfeeding moms to eat a “perfectly.” Yes, for her own health, she may make certain nutritional choices (of course). But, most “adequate” diets with plenty of hydration will be good enough to make perfect breast milk. 

After those first few days after delivery, the volume of breast milk produced by most women is highly related to how often and how much milk is pumped or fed. This factor is usually more important than lactation cookies or specific supplements. 

See this post on expected pumped milk volumes for breastfed babies.

What influences amount of breast milk

The more milk that is removed from the breasts, the more the body will make. I’ve told parents that it’s like the baby is “placing an order for future meals.” Hormones that promote breast milk production are stimulated by the breasts being emptied. 

The opposite is also true. If breasts are constantly full (and not emptied) breast milk production will go down. (This makes sense, right? If someone isn’t feeding their baby with breastmilk, their body adjusts to not make it anymore). 

A few notes on immune properties of breast milk

First, we know that a mom’s immune system (largely made of blood cells traveling around in the, yes there it is again: the blood stream) can influence what is passed on to breastfed babies. Not all the proteins or parts of the immune system are passed from the blood into the alveoli. Some are. 

Secondly, we have some evidence that a baby actively sucking at a breast may influence the immune parts of the breast milk too. 

For example, if a baby has a cold, they are dripping with the cold virus, right? We’ve all seen the snot. When they suck at their mother’s breast, that direct contact may allow an exchange of sorts. It’s possible that it influences the breast to make milk that will help combat the virus. So, this is another way the milk adjusts for the baby. 

Summary: Breast alveoli make breast milk. They need a mother’s blood to complete the job.

Breast milk is made by alveoli in the breast tissue. They make some of it in the alveoli and also use “ingredients” from the mother’s blood stream. So, if something doesn’t get into the mother’s blood, that part of her diet won’t make it into the milk for her baby. Different medications get absorbed into the blood stream at different rates, so it’s important to know how her own medication and food regimens apply. 

Disclaimer: This contains no medical advice. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read on this website. This post may contain affiliate links. As an Amazon associate, I may earn a small commission on qualified purchases at no extra cost to you. Full disclaimer here.

Maya M. Mahmood, D.O., FAAP is a board-certified pediatrician and mom. She is passionate about parents having evidence-based information to help their families be healthier.  Subscribe to the newsletter for regular updates.

Thanks to Nadine Theunissen, IBCLC, CCBE, for reviewing this article.