The mother holds the baby at her breast in a chair.

Many women struggle to breastfeed. Scientists are starting to ask why.

We know more about cow’s milk than human milk. But new research about the makeup of breast milk and the factors influencing its production could change the lives of millions.

Jennifer McClure is one of the many women who have faced challenges in breastfeeding. After her daughter's birth, she was hospitalized and had to supplement with bottle-feeding—which Esme preferred despite Jennifer's best efforts and stopped breastfeeding at eight months old.

Even in her groggy, post-sedation haze, Chandra Burnside was adamant about breastfeeding her firstborn son. It was May 2010, and the then 29-year-old lobbyist had just given birth in an emergency caesarean section in a Virginia hospital. Upset that the delivery hadn’t gone as planned, Burnside was determined to get breastfeeding right. After all, decades of research have shown that breast milk confers vital nutritional and health benefits to babies, including protection from illnesses like diabetes and from sudden infant death syndrome.

But that didn’t go according to plan either. Burnside both nursed and then pumped her breast milk around the clock to keep her supply flowing, just as she had learned in the 45-minute class she had taken while pregnant. But after a couple weeks, her son still wasn’t gaining weight. The pediatrician urged her to feed him more; she was advised to supplement with formula if she couldn’t produce enough breast milk. But Burnside refused to give up on breastfeeding exclusively.

Many women struggle with breastfeeding. According to the U.S. Centers for Disease Control and Prevention, more than 80 percent of new mothers start out attempting to breastfeed. Yet after three months, a typical amount of time U.S. women take for maternity leave, less than half are still exclusively breastfeeding—and only a quarter do so for the six months that the American Academy of Pediatrics recommends. Many begin supplementing with formula or switch entirely.

Although estimates suggest that only about five to 10 percent of women are physiologically unable to breastfeed, many more say that they’re either not making enough or there’s something nutritionally lacking with their milk that keeps the baby from thriving. Yet there has been surprisingly little research into how lactation goes awry—and, as most experts will tell you, similarly little support for women who are trying to breastfeed. In contrast to the dairy industry, which has funded extensive studies of lactation in cattle, researchers have barely scratched the surface with human milk.

In recent years, however, research is gaining momentum as scientists investigate factors like genetics, environmental exposures, and diet, hoping to yield answers for future generations of mothers.

“The science is evolving so fast, I think the next decade is going to be really cool to be in this field,” says Shannon Kelleher, a biomedical and nutritional sciences researcher at the University of Massachusetts Lowell.

Seeking her own answers, Burnside went to see an endocrinologist to figure out if she might have insulin resistance, which she heard could cause low milk supply. Although tests revealed that she had some markers for polycystic ovary syndrome, which can cause insulin resistance, the endocrinologist told her she didn’t need medication to improve her insulin sensitivity.

(Women’s health concerns are dismissed more, studied less.)

Ultimately, Burnside joined a support group where she found encouragement—but no answers. She continued to breastfeed, but reluctantly supplemented with formula. “I was still really floundering,” she says.

The biology of breastfeeding

Breastfeeding may seem simple to the casual observer—a woman lifts the child to her breast and the baby latches on and takes over from there, right? But as mothers know, lactation is a complex process that can go wrong in any number of ways.

“It’s really a finely tuned orchestration of different hormones that are binding to their very specific receptors and driving very specific reactions,” Kelleher says. Anything that interferes with these reactions “will shut down lactation, sometimes within hours.”

Breasts only become fully mature during pregnancy, which floods the body with a cocktail of hormones that prompts the milk-making machinery to develop. Kelleher likens mammary glands to a bunch of grapes: the milk ducts are the stems and the hollow spaces where the milk accumulates—the grapes—are called alveoli. There are about a dozen of these clusters in each breast, and each one contains two types of cells. The cells inside the alveoli produce milk, and muscle cells surrounding these structures contract, pushing the milk into the ducts.

When the baby is born, the removal of the placenta triggers a sudden drop in the hormone progesterone, which turns on milk production.

It takes another complex sequence of events to release the milk. When a baby suckles at the nipple, it activates sensory nerve impulses in the mother’s body that release both prolactin and oxytocin. These hormones then encourage the cells of the mammary gland to release milk. To keep the lactation process going, the baby must nurse regularly or the mammary gland will return to its pre-pregnancy state.

How breastfeeding goes wrong

By the time Burnside was pregnant with her second child, in 2012, she had a much better understanding of breastfeeding: Her struggles feeding her firstborn had inspired Burnside to switch careers and enroll in a nursing program to study lactation.

“In my mind that was going to be the big game-changer,” she says. Unlike her first pregnancy, Burnside went into the delivery room armed with knowledge of all the potential “booby traps,” as she calls them, that can get in the way of making enough milk. Those include waiting too long feed the newborn child and offering formula instead of the breast in the earliest days of life.

“That’s a very critical time for establishing lactation,” says Parul Christian, director of the human nutrition program at Johns Hopkins Bloomberg School of Public Health. Experts recommend breastfeeding within an hour of birth to start the hormone signaling process. Offering formula also deprives the newborn of colostrum—the first form of breast milk the body produces for two to four days after birth, which is packed with vital nutrients, antibodies, and antioxidants.

Many women can overcome their milk supply issues with support and education, says Ann Kellams, a pediatrician at the University of Virginia and president of the Academy of Breastfeeding Medicine. Like Burnside, most first-time parents receive only basic training in breastfeeding—and most medical schools offer little training on the science of lactation. During her own pediatric residency, Kellams says, her hospital’s lunchtime education sessions on breastfeeding were led by formula company representatives. (In February 2023, a series of papers published in The Lancet further outlined these and other marketing strategies of the commercial milk formula industry.)

If parents and doctors were better informed, Kellams argues it may put them more at ease. For one, they might worry less about a low milk supply if they understood that the amount of milk they produce varies with the baby’s stage of development—and sometimes the baby doesn’t need much. And while many parents supplement with formula when milk seems low, this can backfire and drop milk production further.

“You have to have been signaling from the start every time your baby is hungry in order for your body to know that it should and needs to and will produce milk,” Kellams says. “It can take weeks to build back up your supply. It’s not like a light switch that you just turn on and off.”

Sometimes the challenge might also be on the baby’s side. Conditions like a tongue tie—when a band of tissue tethers the tip of the tongue to the floor of the mouth—can keep a baby from adequately stimulating the nipple.

New parents shouldn’t be forced to navigate all these potential issues themselves, Kellams says. She advocates for access to lactation consultants, who can troubleshoot, as well as institutional support like paid maternity leave, which makes the nonstop feeding-and-pumping routine more feasible.

(A glimpse into the lives of families under Sweden's generous parental leave policy.)

But even access to excellent health care isn’t going to be enough for all women. It wasn’t for Burnside. She made it through about two weeks of breastfeeding her second child before the pediatrician warned her that something more needed to be done. At two weeks, infants typically drink two to three ounces every couple of hours. Burnside was still short about six ounces of milk a day—and nobody had any idea why.

It’s not you; it’s biology

Kelleher argues that biology may trigger struggles with breastfeeding in ways that science is only just beginning to probe.

There are several medical conditions that are known to interfere with lactation: Breast surgery—whether a mastectomy, enlargement, or reduction—can destroy the architecture of the mammary gland, and there’s also a rare condition in which women don’t develop enough mammary tissue during puberty. Thyroid issues, diabetes, and polycystic ovarian syndrome can all impact hormone levels, disrupting the delicate interplay that’s needed to keep milk flowing. And chronic stress has been found to deplete the body of the energy it needs to make milk.

(Debunking popular myths about polycystic ovary syndrome.)

But Kelleher says there are other biological factors that can affect a woman’s milk supply. Of these, researchers know the most about diet. Obesity and malnutrition both affect the body’s hormone levels, and Christian says that a mother’s diet can influence the fat and vitamin profile of her milk. This is why many breastfeeding women take nutritional supplements and are encouraged to consume a healthy diet and avoid sudden calorie deficits.

Kelleher says there’s increasing curiosity about the role that antioxidants might play as well in reducing oxidative stress, a state in which rogue electrons in the body “basically start attacking different parts of the cell.” If those electrons kill cells in the mammary gland, that can shrink the alveoli and return them to a pre-pregnancy state. Antioxidants like fenugreek, a common ingredient in lactation supplements, are thought to help stabilize those electrons.

When it comes to understanding the impact of genetics on lactation, however, Kelleher says that “we’re millennia behind the dairy industry.” Years of research have helped to identify genes in cattle that promote higher protein content or greater milk supply. By contrast, Kelleher says, there have only been sporadic studies in humans.

Kelleher’s own research has focused on how genetic mutations affect zinc transportation in the mammary gland. The mineral is highly concentrated in colostrum, suggesting its importance for newborns. She also points to another recent study from researchers at Penn State University that showed how one variation in a gene that produce the protein lactadherin is associated with low milk volume. But it’s still unclear why.

“We don’t even know what this protein does in the mammary gland yet mutations in it are associated with low milk volume,” she says. “That seems like to me kind of an important thing to understand.”

Similarly, Kelleher points out a lifetime of environmental exposures to chemicals, microplastics, and other harmful substances might impact both the quantity and quality of milk humans produce. And it’s incredibly difficult for scientists not just to distinguish which of these exposures might have caused harm.

(Microplastics are in our bodies.  How much do they harm us?)

“There are any number of things that can go wrong, that do go wrong, and we don’t yet understand it for a variety of both social as well as political and financial reasons,” she says.

The future of research

Historically it’s been difficult for researchers to get funding to investigate the biological factors that affect breastfeeding. That’s partly due to the same gender discrimination found elsewhere in health care, but Kelleher says resolving breastfeeding challenges generally doesn’t seem urgent to funders who consider formula an adequate back-up in an emergency. Recent formula shortages have exposed the weaknesses in that argument, but the tides seem to be turning all the same.

Technology has taught scientists in recent years that human milk “is rich with not only nutrients but all these bioactives that influence the health of the baby, its growth and maturity, and its development,” says Christian. She argued for gaining a better understanding in a paper published last year with researchers from the Bill and Melinda Gates Foundation and the National Institutes of Health.

And funding is beginning to trickle in. In 2020, the Gates Foundation supported the creation of the International Milk Composition Consortium, which is focused on how to optimize the nutritional value of human milk. Around the same time, the National Institutes of Health established its own working group on breastmilk ecology. Kellams says the Academy of Breastfeeding Medicine is also developing an agenda to address the key parental questions about lactation.

“You’re not doing it just for the sake of the cool science involved,” Christian says. Better insight into the biology of human milk could be life-changing for millions of women around the world and their children—particularly those in low-income settings where malnutrition is common.

For Burnside, any revelations that come of this research will be too late for her family. Three years ago, she gave birth to her third child, boosting her own mastery of breastfeeding by becoming a certified lactation consultant.

Burnside had experienced a postpartum hemorrhage—a rare condition in which a woman experiences heavy bleeding in the days after birth that is known to delay lactation. When her milk finally came in, it was still about four to six ounces short. She’ll never know for sure, she says, whether it related to the hemorrhage or was part of a broader biological problem.

“I had skills and ability to advocate for myself and a work situation where I could pump as many times as I wanted,” she says. “I had all of that and still ended up with a big question mark at the end.”

Editor's note: This story originally published on June 17, 2022. It has been updated.

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