The gut microbiome of 6-week-old infants appears to be affected by both delivery method at birth and the way they are fed afterward, says research published online in JAMA Pediatrics.

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Babies fed only with breast milk have different gut microbiota than those whose diet includes formula milk.

"Gut microbiome" refers to the diversity of bacterial life that colonizes the human gastrointestinal tract. It develops after birth and after the start of feeding, and it has been increasingly linked with health outcomes in adults.

Meanwhile, cesarean delivery has been linked with obesity, asthma, celiac disease and type 1 diabetes later in life, and breastfeeding has been related to a lower risk of asthma, obesity, infection, metabolic syndrome, and diabetes, compared with formula feeding.

It is thought that exposure to microflora present during vaginal delivery could be related to the microbiome patterns in infants, but how this happens is unclear.

Previous small studies have indicated a link between the intestinal microbiome of infants, delivery mode and whether or not they were breastfed. Less is known, however, about how early life exposures shape the gut microbiome and its long-term effects on health.

In addition, there is evidence that human milk primes and matures the infant gastrointestinal system, potentially promoting a unique microbial colonization profile that could lead to healthy outcomes.

Vaginal delivery and breastfeeding may promote distinct microbiota

In the current study, researchers hypothesized that exposure to maternal vaginal microflora and/or to breast milk could cause specific microbes to be acquired in a particular order during the establishment of the core microbiome. This could represent a key mechanism underlying differences in immune development that influence lifelong disease risk.

Anne G. Hoen, PhD, of the Geisel School of Medicine at Dartmouth, Lebanon, NH, and coauthors examined associations between delivery mode and feeding method with the composition of the gut microbiomes of 102 infants.

The team collected information about the delivery mode from medical records, details on feeding from questionnaires and data about microbiome composition from stool samples.

The infants were born at an average gestational age of nearly 40 weeks; 70 were delivered vaginally and 32 by cesarean section (C-section).

In the first 6 weeks of life, 70 were breastfed, 26 received a combination of breast milk and formula, and six of them consumed only formula.

Results showed a link between the composition of the gut microbiome and the delivery mode. Differences in microbiome composition between infants delivered vaginally and infants delivered by C-section were equivalent or greater than the differences in composition by feeding method.

The microbiome of infants who were breastfed exclusively was different from those who consumed formula milk or a combination.

Those who were exclusively fed formula and those who were fed the combination shared a similar microbiome composition.

There have been no long-term longitudinal studies of the effects of early feeding method on the microbiome, but it appears that early feeding methods could have lasting effects on microbial community structure. These findings could provide one explanation for how breastfeeding benefits health both in childhood and in the long term.

The authors conclude:

"Understanding the patterns of microbial colonization of the intestinal tract of healthy infants is critical for determining the health effects of specific alterable early-life risk factors and exposures.

To this end, we have identified measurable differences in microbial communities in the intestinal tracts of infants according to their delivery mode and diet, with possible consequences for both short- and long-term health."

Limitations of the study include its small sample size and the population being from a single group in the US. Future studies could also take into account the exact proportion of the infants' diets and timing.

Medical News Today recently reported on research suggesting that people with anorexia have a unique gut microbiota composition.