Rethinking Nutrition from the Start

Why Breast/Chest Feeding Deserves the Spotlight in Science and Public Health

by Department of Nutrition
August 24, 2025

Every August, National Breastfeeding Month invites us to celebrate nursing parents and their dedication. But it also offers an opportunity to reconsider how science and public health approach one of the most foundational aspects of human development: nutrition in early life.

What makes a baby healthy—and what does the latest research say?

It starts with diet. The World Health Organization (WHO) and the American Academy of Pediatrics (AAP) recommend exclusive breastfeeding for the first six months to protect infants against infections and chronic diseases while supporting optimal growth and development.

For researchers at UC Davis, the answer begins with a deeper look at human milk.

“I’ve always been interested in how to define health,” says Dr. Carolyn Slupsky, a researcher and professor at UC Davis. “And who better to define that from, but a baby.”

Dr. Bruce German, shares a similar perspective. “I decided to look at what evolution viewed as nourishing as food,” he explains. “Once that decision was made, lactation became utterly compelling and obvious.”

Dr. Jennifer Smilowitz, who studies the impact of diet in the first 1,000 days, says, “I remain humbled and in awe of the new functions research continues to uncover about human milk. It not only nourishes and protects, but also communicates with the infant, guiding development in ways no other food can. Beyond its unmatched nutrition, human milk is also a medicine, a protector, and a teacher.”

A Dynamic System, Not Just a Mix of Individual Nutrients

Unlike infant formula which contains about 30 to 40 nutrients and ingredients, human milk is a living, adaptive fluid—evolved to meet an infant's changing needs. Human milk contains over 1,000 nutrients and complex biologically active molecules, many of which interact and synergize within the infant. Its composition shifts over time to support immune development in the early weeks and cognitive and physical growth in later months. According to Dr. Slupsky, “There are many bioactive components in human milk, and we are only just beginning to understand them.”

Microbiome Magic: How Milk Shapes the Infant Gut Microbiome

One of the most groundbreaking discoveries in lactation science is the role that human milk plays in supporting the development of the infant gut microbiome—the community of microbes that help regulate digestion, immunity, and even brain development.

“There is little evidence that human milk has any effect by itself on most bacteria,” says Dr. German, alluding to the fact that something else must be happening within an infant’s gut microbiome.

The real marvel lies in a unique component of human milk: human milk oligosaccharides (HMOs). HMOs are complex carbohydrates that are the third most abundant nutrient in human milk. What is astounding is that, although these complex carbohydrates are abundant in human milk, infants themselves cannot digest them. Why would evolution preserve the genes for producing HMOs over millennia if infants were not using them? Well, it turns out they serve a highly specific evolutionary purpose—feeding a specific and protective bacterium called Bifidobacterium longum subspecies infantis (commonly known as B. infantis).

“Breast/chest feeding enriches the gut microbiome with beneficial bifidobacteria, which are essential for building proper immunity and gut development,” explains Dr. Slupsky.

This discovery, pioneered by UC Davis researchers, helped lay the foundation for our current understanding of the co-evolutionary relationship between human milk and the infant gut microbiome. But there’s a catch: many modern infants are no longer colonized by B. infantis, largely because medical practices such as C-section deliveries, antibiotic use and formula feeding– all of which have unintentionally disrupted the gut microbiome with effects that carry across generations.

Human Milk and the Gut Microbiome: Why It Matters

Unlike infant formula, which lacks HMOs, human milk selectively feeds B. infantis. When B. infantis is fed HMOs it colonizes the gut and dominates the infant gut microbiome. The by-products of HMO consumption by B. infantis, including metabolites and organic acids, thwart the growth of harmful microbes, support metabolism and guide the infant’s developing immune system. B. infantis colonization with an exclusively human milk diet is associated with protection against atopic conditions such as allergies, eczema and asthma.

But colonization is not automatic. Infants historically acquire B. infantis from horizontal transmission from baby to baby. But this happens only if babies have B. infantis to pass to their peers.

German adds, “Without an explicit inoculation with B. infantis, most modern infants will never get it.”

B. infantis exposure does not result in colonization alone–you also need human milk to deliver B. infantis’ preferred food–HMOs. One study from UC Davis researchers and colleagues found that a country’s breastfeeding history impacts B. infantis colonization patterns. Globally, babies that live in countries that have a short breastfeeding history have an absence of B. infantis in their gut.

On the flipside, infants from countries that have a long breastfeeding history have a gut microbiome dominated by B. infantis.

According to Dr. Jennifer Smilowitz, “This global analysis is the first to show that B. infantis is at risk of extinction during historical periods of breastfeeding disruption. The gap in colonization may have lasting effects on health, and it’s something we have the tools—and the responsibility—to address.”

A team of researchers led by Dr. Smilowitz conducted a landmark clinical trial combining B. infantis supplementation and exclusive breastfeeding in newborns and found colonization of B. Infantis restored the gut microbiome, improved gut biochemistry and function and reduced inflammation. In a follow-up study, B. Infantis persisted for one year as long as infants were still consuming human milk (with solids) but not formula.

Support for Human Milk Feeding

Despite the unequivocal evidence on the protective role of an exclusive human milk diet and the national and international recommendations for exclusive breastfeeding during the first six months, only one in four U.S. babies meet this standard. Breast/chest feeding rates in the U.S. remain low due to systemic barriers that are intricate and multifaceted.

While breast/chest feeding is the biological norm for infant feeding, it is also a learned skill—for both parent and infant—that requires time, guidance, and support to master.

"Successful breast/chest feeding doesn’t happen in isolation.” Dr. Smilowitz points out, “It is not just about the parent and child alone; it is shaped by a network of support, resources, and knowledge.”

Parents need knowledge, confidence, and hands-on skills, while friends and family can offer encouragement and practical help. Clinicians play a key role in troubleshooting latching and positioning, and workplaces must offer time and space for pumping to sustain milk supply.

Beyond the home and clinic, cultural acceptance of breastfeeding in public and policies like paid family leave make it possible for families to prioritize breastfeeding during those critical early months.

The Long-Term Stakes of Early Nutrition

What happens when an infant does not develop a healthy gut microbiome early in life?

“This is an area that we are actively researching,” says Dr. Slupsky. She continues, “UC Davis research has shown that infants who are breast/chest-fed have fewer gastrointestinal issues and less inflammation in the intestine—if the babies have bifidobacteria in their gut.”

These findings underscore the urgent need for long-term studies that follow gut microbiome development from birth into adulthood—something that is still lacking in much of the scientific literature.

What Science Still Needs to Ask

At UC Davis, researchers are continuing to push the boundaries of what we know about human milk and the early-life gut microbiome.

“I think that there is so much more regarding the proteins in milk and how they can activate the gut microbiome,” states Slupsky.

And while nutrition science continues to explore personalized and precision-based approaches, Dr. German argues that nature provided the first—and perhaps most refined—model of precision nutrition: human milk.

“Human milk is the epitome of personal nourishment,” he suggests. “If scientists really wanted to be serious about precision health, they would start by exploring human lactation.”

Slupsky points to emerging technologies that are helping make sense of this complex system. “The ability to integrate multiple data types will accelerate our understanding of human milk –in this regard, some new AI technologies will be very helpful.”

As science continues to unravel the intricate biology of human milk, one truth remains clear: this dynamic, living system is far more than nutrition. It is a vital foundation for lifelong health.

Final Thoughts: Why Human Milk Research Matters

Human milk is not just food—it is a dynamic biological system that shapes the gut microbiome, metabolism, immune function, and brain development. And yet, despite its profound importance, lactation science remains underfunded, understudied, and often dismissed as a niche topic.

“We are only beginning to understand the complexity of human milk,” notes Dr. Smilowitz.“ Given its profound influence on lifelong health, it deserves the same scientific attention as any other vital system in the body. To truly protect the health of future generations, we must also prioritize and reform public policies that support breast/chest feeding.”

As we reflect during National Breastfeeding Month, the message is clear: to improve public health, we must start earlier—at birth, with human milk, and with the microbes that help shape lifelong health.