Study Reveals Effects of Delivery Mode and Diet on Infant Gut Microbiome

There is an important association between the way in which infants are delivered and fed, and the composition of microbiome (the overall communities of bacteria) in their intestines at six weeks of age, according to a recent Dartmouth-led study published in JAMA Pediatrics.

Following birth and at the beginning of feeding, the human gastrointestinal tract is colonized by a considerable variety of bacteria that have been shown to be important in immune system development and health outcomes. But while there is a growing body of research in adults linking gut microbiome composition to the risk of acquiring certain diseases, relatively little is known about how exposures shape the gut microbiome in infants and how it may impact lifelong health.

Led by researchers at Geisel School of Medicine at Dartmouth, the study examined relationships between the intestinal microbiome of six-week-old infants and both delivery (vaginal vs. cesarean section) mode and feeding (breast milk vs. formula) method, including supplementation of breast milk feeding with formula.

In one of the largest studies of the factors that determine infant gut microbiome composition, the research effort looked at a total of 102 healthy, full-term infants, using medical records to determine delivery mode, telephone surveys and diaries to ascertain feeding methods, and stool samples to measure microbiome composition.

The study, which is part of the Children's Environmental Health and Disease Prevention Research Center at Dartmouth funded by the National Institute of Environmental Health Sciences and the U.S. Environmental Protection Agency, yielded some interesting and novel results—both delivery mode and feeding were independently associated with microbiome composition, with delivery mode as strongly linked as feeding, even six weeks after birth.

“We knew from previous research that delivery mode is an important determinant of early microflora composition in babies in the first few hours of life, but we wanted to know if that pattern persisted,” explains Anne G. Hoen, PhD, a study co-author and an assistant professor of epidemiology and of biomedical data science at Geisel. “Other studies show that breast milk exposure is an important factor, but we wanted to directly compare delivery mode and breast milk feeding to see what their relative impacts were.”

The study was also the first to look at the impact of formula supplementation on microbiome composition (rather than only comparing infants who were exclusively fed formula with those who were exclusively fed breast milk). “Surprisingly, babies that were fed a diet that included a combination of formula and breast milk had microbiomes that more closely resembled the microbiomes of babies that were fed a diet of exclusively formula,” says Hoen, also noting that the exclusively breast-fed infants had microbiomes that were significantly different from either the combination-fed or the exclusively formula-fed babies.

“While we still have much to learn about the precise mechanisms of the health outcomes that are associated with colonization patterns in the gut—that’s something we’ll be looking at in future studies—it’s clear that newborns are probably the most important population to study when it comes to the microbiome,” says neonatologist Juliette Madan, MD, MS, a co-author of the study and an associate professor of medicine (pediatrics) at Geisel.

“Understanding the patterns of microbial colonization of the intestinal tract of healthy infants is critical,” Madan adds, “not only for determining the health impacts of specific and alterable early life risk factors and exposures, but also the potential consequences for both short-term and long-term health.”