The reason so many women have trouble making enough breast milk to feed their newborn is partly because a large proportion of them are prediabetic – insulin dysregulation undermines their milk supply, researchers from Cincinnati Children’s Hospital Medical Center and the University of California Davis suggested in PLoS ONE.

In a new study, which adds to their previous one, the scientists explain that insulin plays a major role in lactation success. Lactation is the secretion of milk by the mammary glands.

During lactation the human mammary gland (woman’s breast) becomes extremely sensitive to insulin. This is the first study to describe how this occurs, the authors wrote. They added that theirs is also the first study to show specifically which genes are switched on in a woman’s breast during breastfeeding.

Study leader, Laurie Nommsen-Rivers, Ph.D., explained that RNA sequencing revealed “in exquisite detail” the blueprint for making milk in the human mammary gland.

Dr. Nommsen-Rivers had demonstrated in a previous study that mothers with markers of sub-optimal glucose metabolism took longer for their milk to come down, suggesting that insulin plays a major role in lactation. Examples of sub-optimal glucose metabolism include being obese, having a very heavy newborn, and being at an advanced maternal age.

This latest study shows how a woman’s breasts become sensitive to insulin during lactation.

The milk-making cells in the human breast do not need insulin to take in glucose, which led most experts to believe that insulin played no direct role in lactation.

It is now evident that insulin does a great deal more than simply facilitate the uptake of sugars.

Dr. Nommsen-Rivers said:

“This new study shows a dramatic switching on of the insulin receptor and its downstream signals during the breast’s transition to a biofactory that manufactures massive amounts of proteins, fats and carbohydrates for nourishing the newborn baby. Considering that 20 percent of women between 20 and 44 are prediabetic, it’s conceivable that up to 20 percent of new mothers in the United States are at risk for low milk supply due to insulin dysregulation.”

Dr. Nommsen-Rivers and team managed to capture mammary gland RNA in samples of human breast milk. RNA, which stands for ribonucleic acid, refers to a chain of molecules that are blueprints for making specified proteins. They then created “the first publicly accessible library of genes expressed in the mammary gland based on RNA-sequencing technology.”

Their study revealed a number of genes expressed in human milk-making cells that are highly sensitive. An orchestrated switching off and on of several genes occurs during the transition period between the production of colostrum in the first days after giving birth and the secretion of greater quantities of milk in mature lactation, the scientists discovered.