Human breast milk contains complex, indigestible sugars called human milk oligosaccharides, or HMOs. More than 200 HMOs have been identified, and they are the third most abundant factor in breast milk after fat and lactose. The primary role of HMOs is to serve as prebiotics – compounds that induce the growth or activity of beneficial bacteria. They also serve as "decoys" to protect the infant from gut infections. HMOs are not present in infant formula, however. In this clip, Dr. Erica Sonnenburg describes the important role of human milk oligosaccharides in establishing a healthy gut in an infant.

Erica: So babies that are fed formula, their microbiota looks very different than breast milk.

Actually what we see is breast milk has a component of it. One of the major components of breast milk is this type of carbohydrate called human milk oligosaccharides or HMOs. For a long time, it was really a mystery why those molecules were there because we knew that humans can't digest human milk oligosaccharides.

So why would a mother put so much effort into creating these compounds and putting them in her milk if her baby can't even digest it? We'll come to find out it's actually the gut microbes that are digesting these HMOs. So in breast milk, there's not just food for the baby in the form of lactose and fats, but these HMOs that are food for the baby's growing microbiota. So the mother's feeding the baby and also her baby's growing microbiota.

And these HMOs are very specific for human milk and so far have not been able to be replicated in formula. And so that, we think is a large reason why the communities are so different. And then of course antibiotics, the average American child is on a round of antibiotics every year, and we know that makes a huge impact on that growing community.

So all these things that happen early in life could really set a child on a trajectory potentially for having potentially a very good, healthy, robust microbiota or potentially one that isn't as good. So I think as parents especially of new children, we need to be very mindful of the choices that we make early in a child's life because many of these microbes that we have by the time, say, the age of five, many of these microbes will be with us throughout our entire lives so we really want get that community started in the best possible way.