Mother’s milk mysteries

Mothers’ milk is so widely available and so important to babies’ development that it seems unthinkable it would hold any remaining mysteries. However, most milk researchers study the milk of other animals, seek to improve the formula for milk substitutes, or focus on what happens to mammary glands when they become cancerous. “How little we know about milk,” says Katie Hinde of Harvard University. “I find it actually to be very egregious.”

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Katie Hinde pulls a sample from her archive of rhesus macaque milk samples collected from rhesus macaques. Image courtesy of Cary Allen-Blevins.

A human evolutionary biologist, Hinde studies human milk when she can, but it is far more practical to study animal models for a number of reasons, including a mother’s schedule, diet, and cultural practices, such as allowing other women to nurse her baby. To tease out milk’s mysteries, Hinde started collecting and analyzing milk from several populations of rhesus macaques, including a population at the California National Primate Research Center, where she now has data from three successive generations.

“We know the fetus is exerting an influence on the mammary gland,” says Hinde, affecting the milk the mother produces after birth, and additionally that “milk changes composition across the 24-hour day.” Beyond that, there are hundreds, perhaps even thousands of constituents in milk “that are bioactive in the infant in ways we just don’t know,” Hinde says.

Given technological and ethical constraints, Hinde’s ideal study would involve studying the milk of rhesus macaque mothers eating different diets and then scanning the brains of their nursing young to understand how the baby macaques’ brains use the milk’s nutrients and develop.

“We know that milk is predicting babies’ behavior,” says Hinde, “and that behavior has to be mediated through the brain and the mind.” Understanding that causal relationship with a human study, however, would involve extremely large sample sizes to capture all of the natural variation in maternal styles, diet, and environment.