Breast milk contains a variety of complex carbohydrates. The most abundant of which is lactose, which provides necessary energy for the infant's brain. The second-largest concentration of carbohydrates in human breast milk are non-nutritive components called human milk oligosaccharides or HMOs for short. So let's talk about HMOs, a fascinating part of the story of breast milk. HMOs are complex indigestible sugars. More than 200 different HMOs have been identified in breast milk, making them the third most abundant factor in human breast milk after lactose and fat. These things are superabundant, but here's the surprising part. They aren't there to feed the baby, instead, HMOs have a very special purpose, which is setting up and feeding and ultimately creating the conditions to select for a strong population of commensal, in other words, healthful bacteria in the infant gut.

The origin story of HMOs is similar to those of other examples of mutualism we find throughout the world of biology. Bacteria that benefit us, such as bifidobacteria, get a boost from our own biology as a function of breast milk. Whereas bacteria that harm us, pathogenic bacteria, are discouraged from establishing a foothold through a variety of mechanisms. These good bacteria then go on to help set up the immune system through the production of various signaling molecules, such as short-chain fatty acids, which are metabolites produced from the gut bacteria after they metabolize the HMOs. These substances prevent colonization of pathogenic bacteria in the gut. I previously spoke with experts on the gut microbiome, doctors Justin and Erica Sonnenberg who spoke a little bit on the topic of HMOs.

Dr. Sonnenberg: So babies that are fed formula, their microbiota looks very different than breast milk. And 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. And 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 is 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 that's a huge, makes a huge impact on that growing community. So all of 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.

And 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 we're say the age of five, many of these microbes will be with us throughout our entire lives. So we really want to get that community started in the best possible way.

Rhonda: And nature has come up with a way to get that microbial community started in the best possible way, breast milk, but that's not the end of the story of HMOs. They also serve as decoys to protect the infant from gut infections. In order for bad bacteria to cause infection, they must first target and bind to specific carbohydrates found on the cells that line the gut. However, the overall structure and shape of HMOs mimics that of bacterial targets, the bad bacteria bind to the HMOs instead preventing them from establishing themselves in the gut.

Another interesting quality of HMOs is their capacity to break down biofilms, sticky, slimy communities that bacteria create to protect themselves from antimicrobials and antibiotics. Not only that, they appear to enhance the activity of some antibiotics by increasing the membrane permeability of pathogenic bacteria. So, to sum up HMOs, they're the third most abundant factor in human breast milk after lactose and fat, and they play roles in establishing an infant's gut microbial community, a key component of the immune system. They also protect the gut from harmful bacteria by serving as decoys and breaking down the communities in which these harmful bacteria live.