How Breast Milk Nutrients Differ Depending on Gender
Any mom of boys will tell you that they are completely different than girls. They are loud, rough, and always on the go. Yet, they can be the biggest snuggle bugs and give the sweetest kisses, too. It also seems that nursing moms of boys produce breast milk that is richer in fat and protein, while girls tend to get more milk. In September 2012, a study at Michigan State University discovered that among 72 mothers in rural Kenya, moms with sons generally produced richer milk (2.8 percent fat compared with 0.6 percent for daughters).
A study done at Boston University in 2010 quantified the nutrient and energy density in breast milk of affluent mothers in Massachusetts. The study revealed that mothers of boys produced milk that had 25% greater energy content than mothers of girls.
Although there is no clear-cut evidence to determine why breast milk nutrients differ between a mom of a baby boy versus the mom of a baby girl, there is evidence showing that the changes in a woman's body begin when the baby is in utero.
This finding is fascinating to me, being a mom of three boys who have all been breastfed up through and often past the first year of life. Scientists feel that these findings may help explain the 40-year-old evolutionary theory known as the Trivers-Willard hypothesis.
The Trivers-Willard hypothesis states that there is a much greater parental investment in boys when economic times are good and a bigger investment in girls when times are hard. This is on the assumption that males of parents in good conditions are more likely to find mates and have children of their own, carrying on the family lineage. Sons of families experiencing poor conditions have less of a chance in finding a mate, leading to a “reproductive dead end.” This theory is especially played out in polygamous societies, such as Kenyan villages. In such societies, a son will either grow to be a strong, popular male with many wives and children or a poor man with no family. Wealthy parents invest in their sons because their gamble could give them many grandchildren. On the other hand, poor parents will not heavily invest in their sons because it is unlikely to pay off, as their offspring start at the bottom of the socioeconomic ladder. These families are more likely to invest in their daughters since they are more likely to produce children.
Another theory in why breast milk makeup is different for baby boys is based around research done on rhesus monkeys. A female rhesus with a daughter monkey produces milk containing greater amounts of calcium. Scientists believe that this is adaptive in that it accelerates development in female monkeys, allowing them to begin reproduction at an early age.
Rhesus monkeys, for instance, tend to produce more calcium in the milk they feed to daughters who inherit social status from their mothers. Male monkeys do not need to reach sexual maturity as fast as females since their only limit on how often they reproduce is based on the amount of females that they can win over.
Although there is no clear-cut evidence to determine why breast milk nutrients differ between a mom of a baby boy versus the mom of a baby girl, there is evidence showing that the changes in a woman's body begin when the baby is in utero. Scientists will continue to learn how breast milk impacts an infant's development and how milk is personalized for specific infants. This research could help improve the elements in baby formula. It could also help in better matching donated breast milk to ill and premature babies.