When mammalian mothers produce milk for their babies, they provide more than just food. Mother's milk also helps develop an infant's immune system and contains signaling molecules that program how a baby will grow and respond to stressful situations.
Katie Hinde is a professor of human evolutionary biology at Harvard University where she heads the Comparative Lactation Lab and studies the signaling properties of mother's milk in humans, monkeys and other mammals. Hinde organized the "Building Babies: Development, Evolution, and Human Health" symposium for the AAAS Annual Meeting. She delivered her talk, "Food, Medicine, and Signal: Mother's Milk Programs Infant Development" as part of the symposium on Saturday, Feb. 15.
AAASMC: The title of your talk at the AAAS meeting was "Food, Medicine, and Signal: Mother's Milk Programs Infant Development." We all know how milk is food, but how does mother's milk act as a medicine and a signal?
Katie Hinde, assistant professor of human evolutionary biology at Harvard University: Milk contains both immunofactors and hormones. Immunofactors are critical for providing protection to infants before their own immune systems are mature. Even some complex sugars—oligosaccharides—have important interactions with the infants' intestinal bacteria. These sugars seem to have a role in establishing beneficial commensal bacteria. We have tens of trillions of intestinal bacteria that help us digest our food and are a crucial component of our defense against pathogens. Mother's milk is an important determinant of the infant's microbial community. In this way moms aren't just eating for two—they are eating for 2*200,000,000,000,000 (mom, baby, mom's bacteria and baby's bacteria).
Less understood are the hormonal components in mother's milk, but ingested hormones of maternal origin are associated with infant growth and predictive of infant behavior in rodents, monkeys, and humans.
AAASMC: You have studied the components of milk from hundreds of macaque monkeys. Which of the bioactive compounds that you have measured vary the most between monkeys? Do these variations correlate to developmental differences seen in the baby monkeys?
Hinde: Everything I have studied in monkey milk varies among individuals—milk fats, proteins, sugars, minerals, oligosaccharides and hormones, and the amount of milk also. Some of this variation is explained by the mother's weight, the mother's reproductive history (is she a first-time mom or has she had previous infants?). Some of the variation is related to infant characteristics, such as: Is she rearing a son or a daughter? These milk constituents are associated with infant growth and behavior.
AASMC: How do your monkey results compare to studies that have looked at the components of human milk? Or milk from other mammals?
Sometimes they are consistent. For example, across the mammals studied, first-time moms make less milk than reproductively experienced moms. In the rhesus monkeys, mothers of daughters produce more milk than do mothers of sons. Similar effects have been found in bank voles and cows (see our paper
published this week with dairy scientist collaborators). Unpublished data that I will be presenting at AAAS suggest that the hormone cortisol in milk increases nervousness in young in monkeys, but previous work in rodents suggests the comparable hormone (corticosterone) decreases nervousness. In humans, a single paper suggests that cortisol is associated with "negative affect" in daughters, but not sons. Taken together these studies suggest that these hormones are important, but the effects may vary among species.
AAASMC: Do the results of your research suggest that any new compounds should be added to baby formula so that it better emulates breastmilk?
Hinde: The results of my research suggest that we should be considering modification of infant formula to better replicate mother's milk and to match developmental priorities of sons and daughters.
AAASMC: What research questions are you exploring now?
In collaboration with an amazing interdisciplinary team of anthropologists, evolutionary biologists, and biochemists, we are investigating milk composition among multiple human populations living in a variety of nutritional and disease ecologies. These include pastoralists, horticulturalists, agriculturalists, and urban mothers and their infants from five continents. Lab work is ongoing so stay tuned for 2015 and read my blog: Mammals Suck' Milk