Breast milk seems like a simple nutritious cocktail for feeding babies, but it is so much more than that. It also contains nutrients that feed the beneficial bacteria in a baby’s gut, and it contains substances that can change a baby’s behaviour. So, when a mother breastfeeds her child, she isn’t just feeding it. She is also building a world inside it and simultaneously manipulating it.
To Katie Hinde, an evolutionary biologist at Harvard University who specialises in milk, these acts are all connected. She suspects that substances in milk, by shaping the community of microbes in a baby’s gut, can affect its behaviour in ways that ultimately benefit the mother.
It’s a thought-provoking and thus far untested hypothesis, but it’s not far-fetched. Together with graduate student Cary Allen-Blevins and David Sela, a food scientist at the University of Massachussetts, Hinde has laid out her ideas in a paper that fuses neuroscience, evolutionary biology, and microbiology.
It begins by talking about the many ingredients in breast milk, including complex sugars called oligosaccharides. All mammals make them but humans have an exceptional variety. More than 200 HMOs (human milk oligosaccharides) have been identified, and they are the third most common part of human milk after lactose and fat.
Babies can’t digest them. Instead, the HMOs are food for bacteria, particularly the Bifidobacteria and Bacteroides groups. One strain in particular—Bifidobacterium longum infantis—can outcompete the others because it wields a unique genetic cutlery set that allows it to digest HMOs with incredible efficiency.
Why would mothers bother producing these sugars? Making milk is a costly process—mums quite literally liquefy their own bodies to churn out this fluid. Obviously, it feeds a growing infant, but why not spend all of one’s energy on filling milk with baby-friendly nutrients? Why feed the microbes too? “To me, it seems incredibly evident that when mums are feeding the microbes, they are investing on a greater return on their energetic investment,” says Hinde. By that, she means that setting up the right communities of microbes provides benefits for the baby above and beyond simple nutrition.
By taking up space and eating all the available food, B.infantis and its peers make it harder for pathogens—microbes that cause disease—to establish themselves. The HMOs deter these invaders more directly. Many pathogens launch their invasions by first recognising sugar molecules on the surface of intestinal cells. HMOs resemble those sugars, and so act like floating decoys that draw pathogens away from the gut itself. So, breast milk selects for beneficial microbes while also warding off harmful ones. It sets babies up with the right pioneers.
It’s important to get these first communities right. They steer the development of the immune system, creating a balanced set of sentries that can detect and respond to pathogens, without also going berserk at innocuous triggers like pollen or dust.
There’s also increasing evidence, at least in mice, that gut microbes can shape the early development of the nervous system. They can communicate with the brain via the vagus nerve—a long phone line that carries messages between the brain and gut. They can also release signalling chemicals like dopamine and serotonin. Through these means, they can affect an animal’s behaviour. Some groups have shown that mice which grow up inside sterile containers behave differently to their normal colonised peers: they tend to be less anxious and take more risks. And some teams have shown that specific microbes can reduce anxietyreduce anxiety in normal, healthy rodents.
These mind-manipulating properties might be really useful to mothers. Parenting is costly. It takes time and energy. It’s in a baby’s best interests to monopolise as much of that effort as possible, so they get the strongest start in life. Mums, however, have to divide their effort over many children, both present and future ones. If they expend too much effort on one, they might not be in good enough shape to have more. If they can wean their current infant earlier, they can have another sooner.
These aren’t conscious decisions, mind you. I’m not trying to portray mums as cold and calculating. But it’s important to note that from a cold evolutionary standpoint, mothers and babies have slightly conflicting interests. Simply put, infants will tend to demand more investment than is ideal for a mother to give, and evolution has crafted ways for infants to get that investment—think of smiling, crying, nuzzling, and tantrums. Similarly, mothers should have countermeasures for giving themselves an edge in these inevitable conflicts.
Hinde thinks that the HMOs might act as one such countermeasure. If these sugars can nourish specific microbes, and if certain microbes can change a baby’s behaviour, then mothers could potentially change the HMO content o their milk to influence their babies. The infants might become less demanding. They might be less active, and spend more energy on simple growth rather than on play or exploration. They might be less anxious and more likely to become independent earlier.
Again, this isn’t far-fetched. In her own research, Hinde showed that the milk of younger monkey mothers contains fewer calories but more cortisol—a hormone involved in stress. Babies that drink this cortisol-laden milk tend to be more nervous and less exploratory. They also grow faster. Perhaps these things are connected. The cortisol could be a mother’s way of saying: “Don’t waste the precious calories in my milk; focus on getting bigger.” And perhaps the HMOs might convey similar messages, via microbial messengers.
“I liked the paper,” says John Cryan from University College Cork. “It further emphasises the importance of microbiota-brain interactions in early life for health and development, and positions HMOs as positive drivers of such interactions. Indeed, this is very plausible.”
Plausible and, more importantly, testable. Scientists could see if different sets of HMOs promote the growth of bacteria that can affect the brain. What kinds of signalling chemicals are those microbes making? Do they affect parts of the brain involved in controlling emotions or motivation? Do these effects lead to noticeable changes in a baby’s behaviour? “Then we can look backwards at what are the HMOs that are really influencing the establishment and maintenance of those particular bacteria,” she says. If you load those specific microbes into germ-free mice, and load them with those specific HMOs, what happens?
“We can also look at the variation and abundance of those HMOs in various settings,” says Hinde. “Winning” the evolutionary conflict between parent and child might matter more to mothers who live in risky environments where food is scarce, or where they spend much of their energy on fighting diseases or evading predators. Likewise, younger mothers who are still growing might also fare better if they reserve more of their energy for themselves.
“This is likely to remain hypothetical for quite some time,” Hinde admits. “In humans, there’s hundreds of bacterial strains and oligosaccharides. Understanding what each one does will take forever, much less their complex interactions.” But as she writes in her paper: “An evolutionary perspective allows us to appreciate the essential tensions within the mother-infant dyad and recognize that the infant’s microbial ecology is a potential landscape for negotiating conflict and maintaining coordination. Among the many, many bacteria in the infant gut, may be lurking mother’s littlest helpers.”
Besides, as Hinde says, “Microbes are so hot right now.”
There will be more about milk, microbes, and mums in my upcoming book, I CONTAIN MULTITUDES, out next year.
Reference: Allen-Blevins, Sela, & Hinde. 2015. Milk Bioactives May Manipulate Microbes to Mediate Parent-Offspring Conflict. Evolution, Medicine, and Public Health. http://dx.doi.org/10.1093/emph/eov007