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The unique biology of human breast milk


The role of each of these sugars and why their composition changes during breastfeeding is still a scientific puzzle. It's likely connected to the infant immune system and developing gut microbiome. A Review of what's known and the different jobs of human breast milk appears April 19 in Trends in Biochemical Sciences.

Breast milk is often an infant's first meal. However, many of the sugar molecules in the milk are not meant to feed the baby. Infants are born sterile of any bacteria in their guts, but within a few days they have millions, and after a week there are billions. The sugars that come from mother's milk are usually the first compounds that these bacteria have to chew on, a free lunch that is intended to culture specific bacterial species.

"The first impact breast milk has is favoring the colonization of the gut by specific bacterial groups that can digest these sugar molecules. Infants don't have the machinery to digest these sugars so they are literally for the bacteria — it's like a seeding ground, and breast milk is the fertilizer."

Thierry Hennet PhD, Department of Physiology, Zurich Center for Integrative Human Physiology, University of Zürich, Switzerland, and review co-author.

Human breast milk also helps lay the foundation for the new baby's immune system. After birth, milk is rich in antibodies and molecules that slow the growth of harmful bacteria and coordinate white blood cell activity.

After one month, when the infant begins developing an adaptive immune system of his or her own, the composition of breast milk transitions so that levels of maternal antibodies drop by more than 90 percent. There is also a sharp decrease in the diversity of breast milk sugars, indicating less selection for bacterial species. Instead, mature human breast milk has an increased number of fat and other nutrients that support infant growth.

Despite the many functions of breast milk, children can grow up healthy with limited supplies or without ever being exposed. This raises controversial questions about what is normal when it comes to breastfeeding.

Breast milk clearly reduces infant mortality and significantly decreases a newborn's risk for gut and airway infections, but there's little support for longer-term benefits.

"We have to be careful about giving any recommendations," says Hennet, who co-wrote the Review with Lubor Borsig, also a physiologist at the University of Zurich. "On one hand, breast milk is the product of millions of years of evolution and certainly possesses the optimal nutrients for a newborn. But, how long does the newborn really need this supply? We feel families should make that decision, and not scientists."

What researchers can do is continue to work on understanding the role of all of the different molecules in breast milk, something that has become much easier with advances in gene sequencing technologies. The next few years are likely to yield new understanding of the hormones within human breast milk and the exact role of the bacterial populations that it cultures in the infant gut.

More than just a gold standard, breast milk has been shaped by millions of years of evolution that has resulted in a perfect multifunctional fluid. In fact, beyond the supply of nutrients and vitamins, breast milk provides bioactive factors, including immunoglobulins, cytokines, antimicrobial proteins, hormones, and oligosaccharides, which work in concert to fortify mucosal immunity, shape the gut microbiota, stimulate body growth, and even regulate birth spacing in mothers. Breast milk is a rich fluid that fulfills multiple tasks, as discussed in this review.

Beyond the biological functions of breast milk addressed in this review, the act of breastfeeding itself is the topic of emotional discussions related to the philosophical question of motherhood. Should society encourage breastfeeding simply because it is ‘natural’? Is a woman who stops nursing her baby after 3 months a bad mother? Does breastfeeding depreciate the economic and social status of women [79]? Similar provocative questions keep the general debate on breastfeeding alive and remind us that the discussion on breast milk transcends biology (see Outstanding Questions). Breast milk is ultimately why Carolus Linnaeus, as the father of seven children, chose the term Mammalia to define our own class of animals in the tree of life.

This paper was supported by the Zurich Center for Integrative Human Physiology and the Swiss National Science Foundation.

Trends in Biochemical Sciences, Hennet and Borsig: "Breastfed at Tiffany's" http://www.cell.com/trends/biochemical-sciences/fulltext/S0968-0004(16)00045-1

Trends in Biochemical Sciences (@TrendsBiochem), published by Cell Press, is a monthly review journal that keeps readers informed about recent advances in biochemistry and molecular biology through succinct articles. Learn more: http://www.cell.com/trends/biochemical-sciences. To receive Cell Press media alerts, please contact press@cell.com.
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Apr 25, 2016   Fetal Timeline   Maternal Timeline   News   News Archive   

Breast milk provides more than just nutrients and vitamins to the baby. Recent research
demonstrates the complexity of breast milk, including immunoglobulins, cytokines, antimicrobial
proteins, hormones, and oligosaccharides. All of these molecules act together to fortify immunity,
shape baby's gut microbiota, stimulate body growth, and even regulate birth spacing in mothers.
Image Credit: Thierry Hennet




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