How genes affect mother’s milk and infants

Many factors affect the composition of mother’s milk, including a woman’s diet or even the time of day. Now, University of Minnesota researchers have added genetics to the list.

In a study published in the journal Cell Genomics, the research team analyzed genes in the milk-producing cells of mammary glands, which control what components, and how much of them, turn up in human milk. By shedding light on how a woman’s genetics affect her milk, their work points to ways of ensuring that infants get the best possible nutritional start in life.

“Our goal was to better understand the basic biology underlying variation in human milk and lactation, and how it may influence normal infant development,” says lead author Kelsey Johnson, a postdoctoral fellow in the College of Biological Sciences.

The study revealed that “hundreds of components of human milk are driven by genetic differences in women,” says co-author Ellen Demerath, a Distinguished McKnight University Professor in the School of Public Health.

In one major discovery, the team found that many genes are involved in determining levels of key beneficial components of milk. For example, they found many “new” genes that are involved in the production of certain sugars in human milk. Infants can’t digest them, but these sugars promote the growth of beneficial bacteria in their digestive tracts.

If one or more of these genes are more active in some women than others—or in the same woman under different circumstances—this may lead to milk that is richer or poorer in these sugars.

An inflammatory discovery

MIlk from women in the study also varied greatly in its content of a protein linked to inflammation.

That protein, IL-6 (interleukin-6), is released by some cells of the immune system in response to infection, such as bacteria in a minor cut. When released, IL-6 and related proteins trigger inflammation by “recruiting” other immune cells to pour into the infected area and fight the invading bacteria, viruses, fungi, or other pathogens.

That’s fine for a cut finger. But in the breast, these proteins can be released into milk by immune cells responding to infection—and causing inflammation—in the mammary gland. When they arrive in the infant’s gut, IL-6 and similar proteins can upset the normal balance of beneficial and harmful bacteria by revving up the immune system.

The researchers discovered this when they found that infants consuming milk high in IL-6 had lower abundances and growth of two types of beneficial bacteria in their digestive tracts at certain ages. Specifically, they found reductions in bacteria called Bifidobacterium at one month and Escherichia (the “E” in E. coli) at six months.

“Our results suggest that mammary inflammation, even when unnoticeable to the lactating individual, is a primary driver of variation in milk composition, with potential effects on the infant gut microbiome,” the researchers report.

The researchers also have evidence that maternal usage of probiotics—live microbes that confer health benefits—may reduce mammary inflammation and improve infant gut microbiomes, but a clinical trial would be needed to confirm this.

The University of Minnesota is a leader in understanding mother’s milk and how it affects maternal and infant health. For more on this work, visit the Human Milk and Nutrition Research Group.

The study, “Human milk variation is shaped by maternal genetics and impacts the infant gut microbiome,” is accessible here.