A mother's immune status depends on her feeding strategy

Researchers at the University of California, Santa Barbara, have found that mothers’ immune status changes postpartum depending on how they feed their babies. According to a paper published in the journal Scientific Reports, certain inflammatory proteins — substances released as part of the immune response — peak at different times of day depending on whether mothers breastfeed, pump, or formula feed.

"This is a great study; there are so many unanswered questions about maternal health in the postpartum period," said Amy Boddy, a biologist and evolutionary theorist in the UCSB anthropology department and senior author of the paper. It's a rare, in-depth look at immunity from a postpartum mother's perspective, which she hopes will serve as a starting point for future research.

In fact, she said, most of the research on the effects of breastfeeding has focused on the infant, with numerous findings about the benefits of breastfeeding for the child’s immunity and development. In the long term, mothers who breastfeed also have a lower risk of developing certain cancers and diabetes.

But what about women in the first months and years after giving birth? To explore this, Boddy, lead author and co-lead researcher Carmen Hove, and team followed 96 women in the Seattle area who had given birth in the previous six months and collected their saliva twice over a 24-hour period: once before bed and again in the morning after waking up.

Because the COVID-19 pandemic had just begun and everyone was in quarantine, the researchers found themselves in an unexpectedly ideal experimental situation in which the mothers' environments were strictly controlled for infections that might skew immune measures.

"It was kind of a perfect natural experiment because we were looking at immune function, and no one was reported to be sick," Boddy said. The goal was to track cyclic levels of five types of proteins (called CRP, IL-1β, IL-6, IL-8 and TNF-α) that indicate inflammation, a marker of immune response.

"Breastfeeding has been shown to trigger a complex inflammatory response," Boddy explained. "Inflammation isn't always bad — the breast is remodeling, functioning, and doing things in the body."

The diurnal patterns of these proteins mean that, generally speaking, their concentrations are usually higher in the morning and lower in the evening. What the researchers were interested in was identifying unusual levels in the normal fluctuations of these proteins and how they corresponded to the infant feeding strategies of new mothers.

For several proteins, there was no measurable difference in morning and evening levels, regardless of whether the mothers pumped or breastfed. However, for C-reactive protein (CRP), the researchers found that levels peaked in the evening in women who were actively breastfeeding, reversing the normal diurnal trend.

"We expected that low lactation rates would be associated with a relatively high morning peak in CRP and vice versa," Howe said. "What we found was that among mothers who reported heavy lactation, whether breastfeeding or pumping, CRP was higher at night." Further research is needed to determine the exact effects of this unique pattern in lactating mothers.

"We don't know exactly what's going on," Boddy said. "It could be that incomplete breast drainage is causing inflammation." Or perhaps the inflammation is a healing response from pregnancy. Maybe incomplete drainage is a behavioral change due to stress. Maybe the stress is a result of disrupted sleep associated with around-the-clock breastfeeding schedules.

"We don't have a cause and effect relationship, it's just an association," she said. "This study shows that there is a unique immune profile, and we need to study that further."

This study highlights the true complexity of postpartum breastfeeding. Breastfeeding is part of an ongoing physiological dialogue between mother and newborn that benefits the infant, Boddy said.

"In evolutionary biology, there's a concept called maternal-fetal conflict. The idea is that when you have two bodies in one maternal unit, the baby always wants a little bit more than the mother can give," she explained. This study delves into the gray area of postpartum health from a maternal perspective, particularly around breastfeeding and immunity.

Indeed, despite the ideal promoted by institutions like the World Health Organization that "breastfeeding is best," the researchers found that even among their sample of educated, relatively affluent women, there was a mix of feeding strategies that highlighted the challenges of exclusive breastfeeding.

"There was a lot of pushback, mostly from breastfeeding mothers, about time constraints. Our society doesn't make it easy for us to breastfeed and support lactation," said Boddy, who breastfed both of her children and found it "difficult to achieve breastfeeding goals."

Additionally, there are no clear guidelines on when to stop breastfeeding. When do the physiological and other benefits to the mother in this ongoing dialogue begin to decline? Can this information provide insight into other trends, such as maternal mortality?

The researchers hope to study this topic more deeply and on a more individual level to identify additional patterns in postpartum health and breastfeeding, such as the influence of various hormones involved in lactation.

"I think this study opened up more questions than it answered. We'd like to follow some of these same women throughout their postpartum experience," Boddy said. "It's always been a challenge to figure out the best way to feed our babies, and breastfeeding is so demanding."