Pumping may be linked to an altered microbial mix in breast milk

In the midst of breastfeeding my third child, I once started to calculate how many hours of my life I’ve spent attached to a breast pump. I quickly and smartly gave up, after realizing that “a lot” was a satisfying answer. Like many mothers who breastfeed, I’ve relied on pumping multiple times a day to express milk for my babies’ bottles. 

And probably like many mothers, I’ve wondered whether those bottles are the same as nursing directly. The answer, at least in some aspects, is no, according to a new study that compares the microbial makeup of pumped breast milk with that of milk received directly from the breast.

Milk from breastfeeding women who do not pump and milk from women who pump at all (exclusively, occasionally or infrequently) differed in their bacterial makeup, researchers reported February 13 in Cell Host & Microbe. That finding, taken from 393 mother-baby pairs in a larger dataset called the CHILD study, suggests that all breast milk is not, strictly speaking, the same.

Along with microbiologist Shirin Moossavi and colleagues, epidemiologist Meghan Azad of the University of Manitoba in Winnipeg, Canada, found two main differences between the mothers’ milk. Milk from women who pumped had more of the bacteria that can cause infections under the right conditions. It also had fewer bifidobacteria, which are generally thought to be beneficial. It’s too soon to say whether these bacterial differences are good or bad for infant health — or completely irrelevant. “We just show they are different,” Azad says.  

Azad and her colleagues hope to answer that question in a future analysis. “The babies in this study are now 8 years old, and we have continued following their growth and development,” she says.  

While scientists agree that breast milk is most definitely not sterile, they disagree on where the bacteria come from. Some researchers suspect that internal bacteria, such as that found in the gut, can travel to the breast and make its way into milk. Other researchers think that external sources, such as a baby’s mouth and anything it comes into contact with, can seed bacteria in the breast. Many scientists, including Azad, think both are true. 

The researchers found that milk for babies fed directly at the breast contains more bacteria typically found in the mouth, providing support for the idea that the babies’ oral bacteria are a source for the breasts’ bacteria. This baby backwash, which is thought to trigger infection-fighting proteins in the milk, isn’t a factor when a woman pumps. 

These bacterial differences between pumped and nonpumped milk are interesting, and I’m excited to see what scientists learn about them. But before women begin to worry about their pumped milk, keep this in mind: The overall variability of the bacteria among women was huge, the study showed, and pumping influenced just one small part of that. About 70 percent of the total variability among women couldn’t be explained by factors the researchers looked at. Of the remaining 30 percent, about 4 percent of the variability could be tied to pumping — a percentage that is small but potentially important, Azad says.

Other factors that influenced breast milk bacteria included the mothers’ BMI, whether a baby had siblings and the sex of the baby. Some ingredients of breast milk are thought to differ depending on whether it’s being made for a boy or a girl. 

The results help give a picture of standard breast milk that’s anything but. It seems the more we learn about it, the more it appears there is no single “normal.” 

These types of studies might also point out ways to tweak the microbial content of breast milk in a targeted way, perhaps by different pumping or storage methods. Azad emphasizes that the results should not make pumping mothers feel bad. “Pumping is hard work,” she says. “The last thing we want to do is discourage mothers who pump.”