Does the amount of protein and antioxidants in breast milk depend on the sex of the baby?

Scientists have investigated whether the milk of mothers with baby boys has the same levels of protein and antioxidants as the milk of women with baby girls.

The study was carried out by researchers from the Autonomous University of Madrid (UAM) and the San Carlos Clinical Hospital in Madrid, both entities in Spain.

The team led by David Ramiro-Cortijo (UAM) has shown that the amount of protein and antioxidants in breast milk depends on the sex of the baby. In other words, the sex of newborns influences the amount of protein and antioxidants in breast milk.

The results of the study indicate that, during the first month of lactation, the milk of mothers with male babies has higher levels of protein and antioxidants compared to the milk of women with female babies.

The study focused on the levels of fats, proteins and antioxidant systems in breast milk during the first month of lactation, as they are highly relevant for the survival and growth of babies, especially those born prematurely or with low birth weight.

Thus, a higher content of macronutrients and antioxidants in the milk of mothers with male babies could have a benefit that counteracts the “male disadvantage”.

“Therefore – the authors of the study detail -, the influence that neonatal sex could have on the bioactive compounds of breast milk should be considered to establish the pattern of infant nutrition during the neonatal period, even more so in those babies born under conditions adverse or who need donated milk”.

Differences in the levels of proteins and antioxidants in breast milk according to sex. The female sex was considered as the reference. (Image: UAM)

Statistical data and analysis

At the Hospital Clínico San Carlos, 48 ​​women and their 55 newborns were recruited anonymously and voluntarily in the hours immediately following delivery.

The sociodemographic data of the women and their clinical data related to pregnancy were collected, as well as parameters such as weight, height, and percentages of fat and muscle using bioimpedance.

The researchers also had access to the characteristics of the delivery and those of the newborn’s first moments of life, such as the Apgar score, the weight of the newborn or its sex.

“The women -explain the authors- were asked to provide a 5 mL sample of milk at 7, 14 and 28 lactation to analyze fat (Mojonnier method), proteins (Bradford method) and various antioxidant systems: total antioxidant capacity (ABTS and FRAP methods), thiol groups, reduced glutathione, superoxide dismutase and catalase activities (spectrophotometric and fluorimetric methods). Molecules of oxidative damage such as lipid peroxidation and protein oxidation were also determined in milk (spectrophotometric methods)”.

Statistical analyzes consisted of a succession of linear mixed models with random effects that were adjusted for maternal anthropometry, standardized neonatal growth scores, and gestational age.

Using these models, the researchers determined the main effects of neonatal sex, lactation period, and their interaction with gestational age on protein, fat, and antioxidant levels.

Possible influence of sex hormones

Previous studies on the influence of gender on milk composition have yielded contradictory results.

“Like our data on Spanish women, studies on American and Korean women show the same trend; the milk of mothers of male babies had more caloric content than that of women with female babies.

“However,” the researchers continue, “these results have not been found in Kenyan women. In this case, it has been postulated that it is due to socioeconomic and cultural factors that interact with the sex of the baby and influence the amount of compounds in breast milk.”

The work explains these results in terms of the Trivers-Willard hypothesis, which predicts the care that parents invest in their children is different depending on the reproductive potential of the offspring.

“In addition to maternal socio-cultural, nutritional and anthropometric aspects, a third factor that may explain the results of our study is the influence of male sex hormones during pregnancy and postpartum, which participate in the development of the mammary gland and which may account for greater glandular thickening in women with a male fetus. This aspect deserves further investigation”, conclude the authors of the study.

The study is titled “Influence of Neonatal Sex on Breast Milk Protein and Antioxidant Content in Spanish Women in the First Month of Lactation.” And it has been published in the academic journal Antioxidants. (Source: UAM)