Human breast milk stands out among that of other mammals: it has five times as many types of HMO as cow’s milk, and several hundred times the quantity. Even chimp milk is impoverished compared to ours. No one knows why this difference exists, but Mills offers a couple of good guesses. One involves our brains, which are famously large for a primate of our size, and which grow incredibly quickly in our first year of life. This fast growth partly depends on a nutrient called sialic acid, which also happens to be one of the chemicals thatB. infantisreleases while it eats HMOs. It is possible that by keeping this bacterium well fed, mothers can raise brainier babies. This might explain why, among monkeys and apes, social species have more milk oligosaccharides than solitary ones, and a greater range of them to boot. Larger groups mean more social ties to remember, more friendships to manage, and more rivals to manipulate. Many scientists believe that these demands drove the evolution of primate intelligence; perhaps they also fuelled the diversity of HMOs.
An alternative idea involves diseases. Pathogens can easily bounce from one host to another, so group-living animals need ways of protecting themselves against rampant infections. HMOs provide one such defence. When pathogens infect our guts, they almost always begin by latching onto glycans – sugar molecules – on the surface of our intestinal cells. But HMOs bear a striking resemblance to these intestinal glycans, so pathogens sometimes stick to them instead. They act as decoys to draw fire away from a baby’s own cells. They can block a roll call of gut villains including Salmonella; Listeria; Vibrio cholerae, the culprit behind cholera; Campylobacter jejuni, the most common cause of bacterial diarrhoea; Entamoeba histolytica, a voracious amoeba that causes dysentery and kills 100,000 people every year; and many virulent strains of E. coli.They may even be able to obstruct HIV, which might explain why most infants who suckle from infected mothers don’t get infected despite drinking virus-loaded milk for months. Every time scientists have pitted a pathogen against cultured cells in the presence of HMOs, the cells have come out smiling. This helps to explain both why breast-fed babies have fewer gut infections than bottle-fed ones and why there are so many HMOs. “It makes sense that they would need to be diverse enough to handle a range of pathogens, from viruses to bacteria,” says Mills. “I think it’s the amazing diversity that provides a constellation of protections.