Discovering how breast milk influences a baby's gut bacteria could help scientists figure out the best way to feed premature babies, design better infant formulas and develop pre- and probiotics to promote lifelong health, researchers argue in a new article.

These advances, in turn, could have lifelong health consequences for babies.

Beneficial bacteria in a baby's gut are "absolutely critical for healthy infants," said Katie Hinde, a co-author of the new article and an anthropologist at Harvard University in Cambridge, Massachusetts, who studies lactation.

For babies, this bacterial community, or microbiome, "influences their immune system, it helps them digest their food and we now know that some of these bacteria are releasing chemicals that communicate with the baby's brain."

Limited knowledge

Every mother has heard it: Breast is best. In thousands of studies, breastfeeding has been associated with a host of positive health outcomes ranging from fewer ear infections to a slightly lower risk of leukemia.

But it's not clear why. Relatively little research has been conducted regarding what breast milk is actually made of and how its components work in the body, the researchers said. Breast milk composition varies between women, and does so even when it involves feeding daughters versus sons, but scientists don't know whether those differences are important.

And only a handful of the genes involved in making breast milk are known to scientists, said David Sela, a genomic biologist at the University of Massachusetts, Amherst, who was not a co-author on the new paper.

New insights

But researchers are beginning to untangle some of the mysteries. For example, in recent years, scientists have found that human breast milk has a much greater concentration and diversity of sugars called oligosaccharides, compared with the milk of cows and even our closest primate relatives, the researchers wrote in their article, which was published today (June 25) in the journal Science.

The human digestive system can't break down these simple sugars, so they pass intact into the gut and feed the trillions of microbes there. It turns out that a particular strain of of bacteria from the genus Bifidobacterium seem to flourish in their presence, and that some of these bacteria produce important compounds called short-chain fatty acids when they break down breast milk sugars.

Other studies have suggested that those short-chain fatty acids line the gut and can protect people against developing allergies. Short-chain fatty acids can also power the cells in the infant's colon, and have been tied to better immune function, the researchers wrote.

But breast milk isn't chemically uniform. For instance, a 2015 study in the journal Microbiome found that women with certain gene mutations produced oligosaccharides with slightly different structures, and that this in turn influenced which strains of Bifidobacterium proliferated in their babies' guts.

Understanding how a woman's genetics influence the milk she makes, and whether a baby's genes also play a role in the process, could help scientists match premature babies with donor milk that has the optimal set of nutrients for their growth, Hinde said.

Milk enhancers?

Understanding how the components of breast milk interact with a baby's gut microbiome could also help scientists design better infant formulas, or even design pro- or prebiotics that could steer the microbiome toward a healthier profile, Hinde added.

While it's clear that the microbiome affects a child's health, it's still not clear how breast milk may influence the microbiome over the long term, Sela said. After all, gut bacteria reproduce quickly and feed on whatever's coming into the intestinal tract. So when a baby is switched from breast milk to other foods, the change could potentially dwarf that early-life bacterial profile, he said.

"Breast milk enhances certain populations in the microbiome, but what exactly are they doing there, and if you take them away, would that collapse the whole system?" Sela said.

In addition, it's not clear that the same bacterial populations would be good for everyone. For instance, people eating a Western diet may benefit from certain bacterial communities, but these might not benefit people who have different eating patterns, Sela told Live Science.