Breast Milk Speaks: Science Writer Carl Zimmer Listens

You know breastfeeding is sacred and beyond the lens of scientific objectivity when one of our most respected science writers doesn't seriously critique a study purported to show further “benefits” of breastfeeding. Even Carl Zimmer cannot resist the lure of lactation. At the risk of once again losing out on the annual award for Best Female Science Writer, I must ask whether Zimmer's recent article in the New York Times strikes anyone else as less than a probing, unbiased perspective on empirical evidence - check it out, Mother's Milk, Nutrients, and A Message, Too. 

Is he referring to breast milk or manna from heaven? 

Milk is not just food. The more closely scientists examine it, the more complexity they find. 

Along with nutrients like protein and calcium, milk contains immune factors that protect infants from disease. It hosts a menagerie of microbes, too, some of which may colonize the guts of babies and help them digest food. Milk even contains a special sugar that can fertilize that microbial garden. 

[Forget for a second that formula too has protein and calcium. Forget those "diseases", because most, whatever they are, haven't been linked to breastfeeding, only such terrible diseases as ear infections and diarrhea. Forget too that babies all learn to digest food...these issues are the least of our problems today.]

Scratch that religious analogy, breast milk is not so much manna from heaven as a prophet:

Now, it turns out, milk also contains messages. 

Breast milk hasn't been just food for some time but now it speaks, foretelling the infant's future. It tells babies how to behave:

A new study of monkeys, published in the journal Behavioral Ecology, demonstrates that a hormone present in milk, cortisol, can have profound effects on how babies develop. Infant monkeys rely on cortisol to detect the condition of their mothers, the authors suggest, then adjust their growth and even shift their temperaments. 

Some monkey moms serve breast milk with lots of cortisol (i.e. the stress hormone). Some whip up the low-cortisol version. Newer moms (younger moms and/or those with fewer offspring) had the highest levels. Baby monkeys who drank breast milk with high levels of cortisol got the message to be fearful and less confident. Those who sipped low-cortisol milk got the message to be more outgoing and confident. Breast milk also told the high-cortisol babies to drink more or at least put on more weight (the exact message is unclear). 

Let me repeat that one bit. 

 Drinking breast milk appeared to make some infants more nervous and less confident.

How would you interpret that? 

Certainly not as a potential downside to breastfeeding. Oh no. Not that.

It's a benefit of breastfeeding if you're a breastfeeding researcher or reporter. Lots of cortisol prepares babies, it gets their cortisol receptors ready for exposure to future cortisol but more crucially it helps them cope with their negligent mothers (bad, bad mommy):

The babies fed high-cortisol milk develop a nervous temperament, focusing their limited energy on putting on weight. As a result, they grow faster, despite getting less energy from their inexperienced mothers.

Breast milk is so smart and benevolent it tells some baby monkeys they mustn't be too confident and out-going because they have to stick to mom and nurse as much as possible because mom is still young and small or because she's gonna be birthing other babies and kick you to the curb of the primate lab. 

Or as lead researcher Katie Hinde, PhD, head of Harvard's Comparative Lactation Lab told the New York Times: 

“Prioritize growth, kiddo. You can’t really afford to be exploratory and playful. Once you spend a calorie on that, it’s a calorie you can’t use to grow."

Science Daily did no better at explaining than Zimmer at describing the message or "signal."

Scientists are using this natural variation in breast milk quality and quantity to show that a mother's milk sends a reliable signal to infants about their environment. Science Daily

Enough with the monkeys. What about those other primates. What can humans take-away from the monkey talk? It’s not totally clear.

To his credit, Zimmer found an expert who thinks it's difficult to draw conclusions about how cortisol in breast milk might affect human babies. He also found an expert who said "it's important to explore the implications for humans" and either the guy didn't elaborate or CZ didn't think it important to follow up.

He did mention a study of cortisol in breast milk correlated with negative human infant temperament (as reported by moms). But again, he didn’t elaborate. So I checked it out. The speculative study found mothers with elevated cortisol in their breast milk reported their babies had more negative temperaments. Maternal depression or stress was also assessed via self-report survey (at 3 months) and no, it didn't predict levels of cortisol in breast milk (at 3 months). So mom’s emotional state didn’t predict cortisol levels in the milk – this seems strange but I’m no biochemist. Still, it would have been nice to know more about how cortisol levels relate to those in breast milk.

Of course it’s silly to talk about predictors of a child’s temperament (i.e. fearful/anxious) without mention of a possible direct connection to mom’s temperament. I only have the abstract of the human study but assume researchers checked whether a mother’s self-reported stress or depression also predicted infant temperament – and that a mother’s mood per say didn’t predict infant mood because it's not reported in the abstract (but this would contradict a fair bit of research). I could be wrong. I assume also that maternal cortisol levels also were not assessed because they're not reported in the abstract. The issue of a mother's cortisol levels (aside from breast milk levels) seems critical and yet, it's omitted from this discussion.

These are important questions surrounding elevated cortisol, in humans. But Carl didn't get into it. Readers are left to wonder and draw conclusions. Is there much if any evidence on how maternal stress impacts cortisol levels in general and in breast milk? I don't know because he didn't broach the topic. The issue of maternal mental health, anxiety and stress didn't figure into the miracles of breast milk. No, I couldn't voice my concerns at the New York Times because the article had no online comment section. 

I know, it's just about rhesus monkeys. 

Cue The Stones...I hope I'm not too messianic

Nobody is meant to act on this new discovery. 

Well, I am just a monkey man 

Especially not new, anxious moms under stress to breastfeed for a year.

I'm glad you are a monkey, monkey woman too...

UPDATE: I heard from lead researcher, Katie Hinde, see her comments below. I'm thrilled to report she supports a woman's choice to feed her baby breast milk or formula. As an advocate of more researchers writing for regular folk, I’m happy to see she pens the Mammals Suck...Milk blog, and one post alone contains a gif of Stephen Colbert and Big Bird. What do her results mean for human babies? She explains on her blog:

Human milk contains cortisol and has been linked to temperament. Breast-fed human babies have increased expression of cortisol receptors in their intestinal tract (kickass work by Sharon Donovan & colleagues). And in humans cortisol concentrations in milk are correlated with circulating cortisol in the mother’s bloodstream....

 What does it mean when cortisol is missing (formula) or cortisol in milk is turned to 11 with the knob broken off? We don’t know. Do we expect it to necessarily be catastrophic in typically developing kiddos? No, we don’t… because this one hormone, in one aspect of parenting, is just one of the rivets holding the plane wing on. One rivet can break, and the plane still flies… even two or three and the plane can get off the ground. Development is a multi-factorial system, and rarely is any one single aspect the linchpinSo moms, don’t let these findings stress you out. 

Did you hear that moms? And Carl Zimmer? 

Her stand on human feeding choice:  "Mammals Suck systematically supports moms, advocating for them to have ALL the options, be respected for whichever options fit their lives, and that ALL the options be better; better breast-feeding support, better formula, and the best breast milk science."  

Note her support for Momma Data friend and advocate for mothers, Suzie Barston, the Fearless Formula Feeder. (see her post, Manufactured Mommy Wars. Le Sigh. that mentions a FFF HuffPo article at the end). 


Katie Hinde said...

Dear Dr. Palumbo,
Thank you for your interest in our recent article on hormones in mother's milk.

You asked several questions about our study methods and interpretation of the data. I am happy to say that we published our paper open access so that all of that information is available.

I have also written a blog post contextualizing the data in terms of implications for humans and what is known about hormone receptors in human infant intestinal tracts. You can find my blog post over at "Mammals Suck... Milk!" at blogspot. I make it very clear that these findings should not "stress moms out" and explain why.


Polly Palumbo, Ph.D. said...

Hi Dr. Hinde,

Thanks for reaching out and making it clear moms shouldn't stress out. I do wish the New York Times would have stressed that point. Indeed you do make it clear on your fabulously-named blog. Most of my questions, however, pertain to the human baby study. I'm going to update this post with your comments about supporting a mother's feeding choice (from Mammals Suck...Milk). I see you've also linked to the HuffPo piece by my good friend, Suzie Barston, The Fearless Formula Feeder.