How special is human breast milk? (Part I)
Last week the internet was aflutter with reports about Curtis a man who was planning to exclusively consume his wife’s breast milk. The couple had accumulated a stockpile of frozen breast milk that was no longer needed since weaning their baby. Dr. David Katz, director of the Yale Prevention Research Center, reportedly responded by saying that "the rapid growth and brain development of early childhood is best fueled by the high-fat, energy-dense elixir of breast milk." Its reported that Katz then further explained that breast milk has less value to an already developed adult. While the second half of Katz’s response is indeed supported by data, the first half demonstrated a wide-spread misconception about the specialness of human breast milk and provided the opportunity to present the first substantive post for Mammals Suck… Milk!
To cut to the chase, it turns out that human milk is indeed super special! Just like the milk of each mammalian species is super special. This is because mammalian mothers synthesize milk specific to their environment and for their infant’s developmental needs.
Unfortunately ‘environment’ is a pretty simple term for what is actually a very complex aggregate of inter-related selective pressures. For example the environment provides a mother’s diet, and we can predict that diet influences milk synthesis. But environment isn’t just what a mother eats, and how much, or even what she once ate. Environment also includes whether she has to be on the look out for predators while she’s eating it, how much she competes with other individuals in her social group to get food, whether some of the energy she gets from food has to be diverted to immune response and fighting pathogens, and whether that food is available year-round or seasonally. Similarly mother’s milk is specific to what the infant is using it for- in this way mother’s milk, its composition and volume, reflects the infant’s developmental priorities, which are incidentally also related in some ways to the infant's environment.
Tying all of this together we find an excellent example in hooded seals. To avoid hungry polar bears, hooded seals give birth on unstable, melting ice floes that can barely support the mother’s weight, so pups are weaned only four days after being born. These guys need a thick layer of blubber to stay warm and keep growing after their moms take off and before they are any good at catching their own fish. As a result, pups couldn’t possibly survive unless they get very, very high fat milk. Hooded seal moms have therefore been selected to produce milk that is on average >60% fat. They transfer 7 kilograms of milk fat each day of lactation.
However, environment and development aren’t exactly the whole story. Because closely related species evolved from a shared common ancestor, selective pressures on that ancestor from past environments and developmental priorities have shaped the genome of present day species. We can therefore expect mother’s milk to reflect, in part, phylogeny. And it turns out that human milk composition, as measured by percent fat, protein, and sugar, is very similar to apes and monkeys.
This brings us back to Dr. Katz’s incorrect statement. In actuality human milk has the same pattern of low fat, low energy density milk found in other primates. Humans produce breast milk that is on average ~4% fat, despite the fact that humans have substantial post-natal neurodevelopment. But we can step back even further and recognize that among mammals, primates are characterized by substantial neurodevelopment during infancy. Earlier we recognized that the developmental priorities of the infant predict the composition of milk. How then do we reconcile the dilute milk of primates with their post-natal neurodevelopment?
cont in Part II!
Comments
Post a Comment