Now that I’m nursing a newborn again, I’ve renewed my fascination with the science of breast milk. In particular, “lactocrine programming;” the idea that hormonal signals in Mama’s milk can “program” Baby’s body and behavior. So many new developments in the last few years since UJ was born!
Today’s question: Does breastfeeding make kids more fertile later in life?
Theories abound with evidence accumulating from various studies on humans and other mammals. Researchers Katie Hinde and Danielle LeMay, at Harvard and UC David respectively, offer some intriguing insights in their SPLASH! Milk Genomics blog.
The upshot: The first few days after birth are a crucial window for the development of reproductive tissues that will become part of the cervix, uterus, endometrium, and other parts. These tissues start growing when they receive signals from a hormone called relaxin (a multipurpose hormone, it also relaxes blood vessels and ligaments during pregnancy, opens the cervix, and much more).
There’s relaxin in the milk! report Hinde and LeMay, referring to a study on pig milk:
Here’s the crazy part: relaxin is delivered by the mother via her milk. Piglets that are allowed to suckle have relaxin in their blood stream, but not piglets fed a milk-replacer . Relaxin activity in pig milk is highest in the first few days of lactation, and is similar to findings from dogs and humans. Experimental manipulations have shown that as little as one colostrum feeding in the first 12 hours after birth can make a difference. For example, just a single colostrum feeding bout in the first hours after birth allows for typical cervical cell proliferation and development—an important predictor of future litter size.
But does access to milk really predict future fertility? It did in a recent pig study involving over 1,500 litters, say Hinde and LeMay:
Female pigs with limited access to maternal-origin hormones via milk as piglets had reduced litter size as adults. So, it is safe to conclude, at least in pigs, that the number of babies born in any generation was partly programmed by their grandmothers via milk hormones.
It’s mind-boggling, the possibility that access to breast milk in infancy — or at least colostrum (the thick milky fluid produced in the first days after birth, which also contains relaxin) may have an impact on our kids’ reproductive development.
Not that these preliminary (porcine) studies say anything conclusive about humans. For that, we’d need to address many more questions: Do women whose mothers never attempted breastfeeding have more fertility problems — or fewer kids — than their breastfed peers? Do they have a different growth trajectory in puberty? How does relaxin affect reproductive development in sons? Can breastfeed girls bear children later in life than those who weren’t breastfed? How long does a kid need to nurse to receive reproductive benefits — the first day after birth, a few days, the first week? More?
Further research is warranted. Even if turns out that fertility is only slightly enhanced among breastfed kids (that’s my bet, anyway), it’s more fuel for the breastfeeding movement.
Check out my video and excerpt from Chocolate Lovers on Big Think.
This is not an ad in the personals. It’s the opening line of the commentary in the straitlaced scientific journal, Proceedings of the National Academy of Sciences (PSAS). The authors, neuroscientists Liisa Galea and Cindy Barhain, intend to shock. Why, one wonders, would a man have no luck in love?
Findings from a new study suggest it may be your mother’s dietary exposure to bisphenol A (BPA).
Galea and Barha have all my attention now. Ever since my pregnancy, I have been tracking studies on BPA’s subtle yet shocking effects. One of the most common chemicals in the world, bisphenol A is found in the stuff we use every day of our lives. Soup and soda cans. Water pipes. Computers. Cell phones. Thermal paper receipts. Paper money. Even some baby bottles—at least in the U.S., because they are not banned here.
Much of the trouble with BPA lies in its ability to fool estrogen receptors into thinking it’s estrogen. Imagine a man doesn’t know that the woman he’s marrying is really an alien in drag, and you have a sense of the danger here. BPA disrupts any process that estrogen normally mediates, affecting brain, body, and behavior. It also tinkers with the way genes express themselves, turning up those that would otherwise be turned off or down. BPA exposure has been linked to breast cancer, heart disease, obesity, diabetes, attention-deficit disorder, increased anxiety, a decreased IQ in children and a low sperm count in men.
Pregnant women and new moms should be especially cautious. BPA has been found in umbilical cord blood and in breast milk. It crosses the placenta and flows in fetuses. Young bodies are especially vulnerable to pseudoestrogens. The toxin strikes us moms, too. Researchers worry that BPA may affect women’s brains in a way that alter their maternal instincts. In laboratory studies, BPA-exposed female rats are less likely to nurture their offspring—they lick them less—which in turn affects the emotional and cognitive system of their babies. They become more fearful and anxious.
And now, there’s more.
There is evidence that BPA emasculates males and makes them sexually undesirable. Galea and Barha’s opening lines in PSAS are tongue in cheek—they are describing a new study at the University of Missouri on the effects of BPA on deer mice—but the application to humans is implicit. Adult mice whose mothers were fed a dosage of BPA equivalent to what the USDA deems safe for pregnant women, were, well, different from other males.
“One of the prominent effects of early BPA exposure is that it eliminates a number of sex differences in brain and behavior,” the researchers wrote. It turned out that BPA-exposed males have impaired spatial ability (can’t find their way out of a maze or to their nest, considered unattractive to females). They also suffer from decreased exploratory ability (incurious and easily lost), and overall reduced attractiveness to the opposite sex. They may even smell different from their peers—in rodents, a sign of unhealthiness. Females are disgusted.
It’s not absurd to worry about similar effects of BPA-exposure on our babies. Men are not mice, but there is increasing evidence that BPA affects us as well, and in doses below the below the 50 µg/kg/day safety threshold in the United States. Almost every American pregnant woman (93 percent) has detectable BPA in her body, which is passed on to her fetus. The average BPA body burden of an American is high, alarmingly high, compared to other countries. We love our BPA-enriched Cokes and canned Campbell’s soups.
On a population level, how might BPA affect us? Might boys in the U.S. grow up to have poorer spatial skills—and, because it’s linked, weaker mathematical ability? Might they have little interest in exploring the world, preferring to hang out at home? Might our national temperament become more placid? Because BPA is lined with obesity and heart disease, will we become fatter and more sedate? And what about our sex lives?
Take a look at human history through the lens of hormones, as Harvard University’s Daniel Lord Smail did in his fascinating book, On Deep History and the Brain. Smail introduces a new view in which physiology and culture evolve symbiotically in a process driven by brain chemistry. Caffeine stimulated the body and mind, driving the industrial revolution and the modern corporation. Tobacco help us to focus and be calm. These substances changed the character of society. Now we have environmental toxins such as BPA (and other hormone disruptors such as phthlates and PCBs) that may also change our culture in subtle but very real ways.
BPA: Bad for your manhood. Bad for your sex life. Sensationalistic, sure—but would this get CEOs to pay attention? Hit them where it hurts.
Stubborn pushback—that’s the response from many corporations regarding BPA bans. The chemical is a mainstay in packaging, and to ditch it is disruptive for business. Coca Cola has famously refused to find an alternative. You can find BPA-free cans of beans from brands such as Eden, but not crushed tomatoes yet (in the meantime, buy them in glass jars). Avoid plastics that are marked with recycle codes 3 or 7; they may contain BPA. While Canada, Europe, and even China have banned the use of the chemical in baby bottles, the U.S. has not (although consumer demand has pushed many manufactuers to go BPA-free).
The good news, as I describe in my book, is that there is laboratory evidence that a diet high in folic acid and B12 may reverse at least some of the nasty effects of prenatal BPA exposure. How? One way that BPA tinkers with our systems is by attaching itself to strands of DNA and “turning on” certain genes (removing methyl groups) that are normally turned off—resulting in obesity, cancer, and other nasty effects. This is classic epigenetics—an environmental trigger affects the way that genes behave. Nutrients in green vegetables, beans, eggs, and soy may be protective (in those of us who include enough in our diet) because they turn off genes that BPA otherwise turns on.
Of course, the best protection is to turn corporations off BPA. That would really be a turn-on for us moms.
*If you like this blog, click here for previous posts. If you wish, check out my new book, Do Chocolate Lovers Have Sweeter Babies?: The Surprising Science of Pregnancy.
In this month’s Pregnancy & Newborn Magazine is an article on the science behind nesting. I offered theories that explain why pregnant women — especially those who are about to give birth — feel an odd and obsessive urge to clean and organize.
The best holiday gift: DO CHOCOLATE LOVERS HAVE SWEETER BABIES gets a STARRED REVIEW in Library Journal! Many thanks to Library Jrnl and the reviewer, Julianne Smith.
What a charm! Science writer Pincott (Do Gentlemen Really Prefer Blondes?) tackles some myths and legends associated with pregnancy and compares them to peer-reviewed research on the matter. The book covers such questions as: “Do men prefer babies who resemble them?” “What does a baby’s birth season predict?” and “Do bossy broads have more sons?” This is an enjoyable, insightful, and fascinating look at pregnancy that explains what we know and identifies what we don’t. In discussing topics from stretch marks to mama’s boys, Pincott takes a conversational tone, making the science readily available to all readers. An ideal acquisition for public libraries, a great gift for expectant parents, and the perfect choice for the doctor’s waiting room, this winning title deserves some talking up. Way more fun than What To Expect.
When our baby was six months old, my husband and I couldn’t walk down the street without stopping to accept a compliment. Baby was something of a local celebrity or a minor deity. Free pizza and donuts, the choicest table in the restaurant, a bracelet, stickers, sunglasses, a spot at the front of the line — these were some of the offerings made by the infant’s admirers. Ten months have now passed, and now she’s a toddler. To me, she’s as cute as ever, and she still receives a lot of kisses and waves from strangers. But, we’ve got to face it — she doesn’t attract the same the solicitous stop-in-the-street simpering that she did in her first year. How much longer will it be before everyone stops thinking she’s so adorable?
This question happens to be the subject of a new study on children’s faces. A trio of psychologists— Lu Zhu Luo, Hong Li, and Kang Lee — in China, and at the University of Toronto, recruited 60 men and women and showed then a large sample of children’s faces ranging from infants to 6-and-and-half-year-olds. The participants were asked to rate each face’s likeability (How much do you like the face?) and attractiveness (How attractive is the face?). The researchers wanted to know if younger kids would be given higher attractiveness ratings than older kids — and, if so, at what age does the cut-off happen from over-the-top adorable to merely cute?
The answer? Yes, it’s as expected. Men and women rated infants as cuter than toddlers, who, in turn are rated as cuter than young children. The big drop-off in cuteness appears to happen somewhere between preschool and kindergarten. The researchers identified it as approximately age 4 ½.
What happens to school-age kids?
Their facial structure changes, according to the Luo and his colleagues. Infants have a special set of features, such as a protruding forehead, a large head, a round face, big eyes, and a small nose or mouth. As a species, we have evolved to be attracted to babyish features and find them adorable. These cues make us feel soft and protective, whether or not we’re biologically related — which increases the likelihood of the baby’s survival. (Indeed, studies have found that infants that have tiny eyes, flat foreheads, and square faces, for instance, are less likely to receive attention.) Pleasingly, our tendency to prefer infantile faces even extends to infant faces of other races.
Facial cranial growth is gradual, as is a child’s independence from constant care-giving. Children’s faces lose some of their universal appeal right around the age that they don’t need it anymore to (merely) survive — somewhere around kindergarten-age. Incidentally, this interval — four to five years — is the same as natural birth spacing — when our foremothers would become pregnant with their next baby.
Not long ago, someone emailed me a new study on chocolate and temperament. The study came from a group of psychologists at North Dakota State University and Gettysburg College, and it involved personality differences between people who liked sweet foods versus bitter, spicy, sour, salty, or umami flavors. The researchers wanted to know, “Is having a sweet tooth related to having a sweeter disposition?”
Here’s a sample of what they asked their hundred or so volunteers.
Candy, caramel, chocolate cake, honey, ice cream, maple syrup, pears, raisins, strawberries, and sugar. How fond were they of these dessert flavors?
The volunteers rated their taste preferences on a scale of 1-10.
Following that, they completed an agreeableness scale. That is, they were asked to indicate the extent to which they behave in ways reflective of high (e.g., “have a soft heart”) versus low (e.g., “insult people”) levels of agreeableness.
Pleasingly, the researchers found a significant correlation. People who loved sweets were likelier to be more agreeable.
“Further proof that chocolate lovers would have sweeter babies!” my sender gushed. There’s logic here: If people with a sweet tooth really have a sweeter disposition then women who love sweets might have more agreeable babies because disposition is heritable. Equally valid, people who have sweeter dispositions would have a gentler parenting style, resulting in babies with sweeter dispositions. Ergo, chocolate lovers have sweeter babies. Natch.
Then the researchers took their study to the next level to see if sweet-toothed types not only test as more agreeable, but act sweeter too.
At the end of the session, participants were told that the study was over and that full participation credit would be awarded, thereby relieving them of any further obligations. However, it was mentioned that a colleague in the English department was collecting data on media preferences and was looking for volunteers.
Interestingly, people who had a sweet tooth were more likely to complete the voluntary survey, even though they weren’t getting any extra credit or compensation.
I wonder: does the mere suggestion of sweet food make people act sweeter? Does sweet food make people less angry and aggressive? The researchers claim that metaphors can be predictive about behavior and personality. Because “sweet” in English applies to both taste and disposition, does the correlation still apply in different languages?
How sweet would that be?
Among the great mysteries — and horrible injustices — of aging is that dementia strikes more women than men. No one knows why exactly. For years, scientists suspected that the culprit is estrogen. Implicated in memory formation, estrogen levels plummet after women reach menopause.
But here’s the catch. If low estrogen levels are behind age-related dementia in women, then why did a massive study by the National Institute of Health find that women who had hormone replacement therapy (HRT) were no less likely to experience cognitive decline than women who didn’t have it?
Because not all estrogens are created equal, nor are all brains. Motherhood may be the missing link.
This comes from an intriguing new study by Cindy Barha and Liisa Galea, neuroscientists at the University of British Columbia’s Brain Research Center. Barha and Galea knew that the female brain is highly plastic; it literally restructures itself in the course of pregnancy and caring for a baby. They were curious about whether motherhood might alter the brain in a way that protects against dementia under certain conditions.
One way to explore this is to study middle-aged rats that are genetically identical in every way with the exception of their reproductive lives. The scientists divided the rats into groups — virgins and mothers — and injected each with a form of estrogen: estradiol or estrone (the form in HRT). Later, they looked at the rats’ brain tissue to see whether new cells had formed — a process called neurogenesis — in the hippocampus where memories are formed. These new cells may reduce the risk of dementia.
It’s interesting, this contest between middle-aged virgins and mothers. Compared to virgins who had been injected with estrogens (and mothers who hadn’t), the middle-aged mother rats that took the hormone replacement therapy grew significantly more new hippocampal cells. The mothers — and only the mothers — grew many new neurons when exposed to the estrogens (especially a combination of estrone and 17a-estradiol). The hormones helped the mothers’ minds to remain malleable.
Why did the mothers have a seeming advantage? Anther mystery — which Barha and Galea say may be related to an enhanced ability of mothers’ hippocampuses, even late in life, to respond to estrogens. Such responsiveness may come from hormone exposure during pregnancy or afterward, or enrichment from the experience of mothering.
Interestingly, estradiol’s effect on the brain is mediated by BDNF (brain derived neurotrophic factor levels), which may be higher in mothers than in those who have not given birth. The researchers note that BDNF may help explain an assocation between high levels of estrogen exposure across a lifespan and a decreased risk of cognitive decline and Alzheimer’s disease. Among rats, those who have been mothers have a reduced age-related decline in spatial memory and other cognitive decline.
The scientists conclude:
Therefore previous reproductive experience, which is associated with altered hormone exposure and greater enrichment, can result in higher levels of plasticity in the middle-aged brain and may protect the brain from the deleterious effects of aging in females.
But there’s a very real caveat. Applied to humans, these findings are still highly speculative. First, the boost would depend on a woman taking hormone replacement therapy, which remains controversial due to its connection with breast cancer. (Why hasn’t HRT been shown to improve cognition thus far? It may because a combination of estrone and 17a-estradiol is much more effective than the current regimen of estrone and progestin.)
More importantly, there is no guarantee that new neurons will improve a person’s ability to learn and remember or decrease the risk of cognitive decline. As Barha and Galhea warn, all that new growth might even lead to a sort of jungle effect if not properly integrated. There are clearly other factors that determine what properties these new cells will have as they mature. Do some mothers put their new neurons to use more effectively than others? No one knows whether or how, but the question is fascinating. (More research is underway in Barha and Galea’s lab.)
What’s clear is that motherhood results in permanent changes in the brain. Will we mothers someday benefit cognitively more than childless women from new hormonal therapies? There’s a chance. Wouldn’t it be marvelous if a cognitive boost later in life would compensate for all the sleepless nights now?
*If you like this blog, click here for previous posts. If you wish, check out my new book, Do Chocolate Lovers Have Sweeter Babies?: The Surprising Science of Pregnancy.