The World Health Organization currently recommends that all mothers breastfeed their babies for at least the first six months of life. These guidelines are based on extensive evidence that breast milk is the best nutrition for growing infants. However, there remains very limited understanding of the variability of breast milk and its 1,500 known components between different women, and whether the current prevalence of maternal obesity is altering the quality of breast milk. A new study from the University of Minnesota School of Public shows that the breast milk of obese mothers can be higher in leptin and insulin (both appetite regulators) and is associated with slower growing babies at age six months.

The study was published in the journal Pediatric Obesity. The paper is the first from the NIH-funded Mothers and Infants LinKed (MILk) for Healthy Growth study led by Professor Ellen Demerath and David Fields at the University of Oklahoma. The overall study aims to learn more about how breast milk is related to mother and infant weight gain and body composition.

“‘Breast is best,’ but the motto came from a period of history when we didn’t see a lot of obesity,” says Demerath, the paper’s senior author. “More than 50 percent of women now are overweight or obese when they give birth, and we’re trying to characterize its downstream effects for breast milk and infants.”

For the study, researchers determined the pre-pregnancy weight status — normal, overweight, or obese — of mothers. They also analyzed milk samples at one and six months of breastfeeding, and the body composition of the infants drinking the milk.

“The findings show that moms who were heavier at the start of pregnancy had higher levels of leptin in their milk, which is the hormone that tells us we’ve had enough to eat and makes us feel satisfied,” says Demerath. “Theoretically, the leptin from the breast milk then crossed into the infant’s system and depressed their appetite, which led to our observation of slower-growing babies who are smaller in body size. This may indicate the importance of breastfeeding for obese women in helping keep their infant growth rate within healthy ranges.”

The research also revealed a surprising gender relationship in that obese women produced milk with higher insulin concentration as well, but only for infant girls, not for those with boys.

The findings of the study show just how variable breast milk can be and why it’s important to understand it as a primary food source for infants.

“Building up a body of research on breast milk variation is going to be important for establishing which elements are important in breast milk’s myriad benefits for infants and how to optimize those elements for all moms,” says Demerath.

As part of her ongoing MILk study, Demerath plans additional research including tracking the body composition of breast-fed infants through later stages of growth. Further research will also examine the influence of maternal gestational weight gain, post-partum weight loss, pregnancy glucose levels and dietary habits to see how they relate to breast milk variation.