In the Netflix documentary Babies, the episode “First Food” features Dr. Katie Hinde, a scientist who pioneered research beginning in the early 2000s into the composition of breast milk. She discovered that the amount, composition, and energy density of breast milk differed depending on whether it was for a daughter or a son.
Thanks to the work of researchers like Dr. Hinde, we know that breast milk is an incredible living substance with immense physical, emotional, and иммунологический benefits, perfectly tailored to each baby’s individual needs over time. At the foundation of breast milk’s vital role in babies’ development is nutrition, and in recent years a number of scientific studies have taken up the question of whether an infant’s sex affects the nutritional makeup of the milk he or she receives.
Sex differences in breast milk: What researchers are looking for
When studying the nutritional content of breast milk, researchers generally look at the quantity of milk produced, the energy density of the milk (i.e., how many calories are packed into the same amount of milk), and the composition of the milk. Milk composition is typically broken down into the same categories of macronutrients that you’re used to seeing on food packages: protein, fat, and carbohydrates.
Researchers also have to take into account that lactation occurs in three stages. First (as early as 16 weeks into pregnancy), the mother’s body begins to produce colostrum, the “liquid gold” that sustains a baby during the first 3-5 days of life outside the womb. Then, up until about two weeks after giving birth, the mother produces transitional milk, which gradually turns white. Finally, mature milk begins being produced from around 10-15 days after giving birth for as long as the mother breastfeeds. Within each stage, milk production is dynamic, responding in volume and composition to the needs of the infant.
Some studies only include mature breast milk, while others compare samples of mature breast milk with colostrum, transitional milk, or both [1] [2] [3]. A Greek study published in 2025 presents a fascinating look at differences in milk produced for boys and girls across each lactation stage [4].
Breast milk changes over time for both boys and girls
The Greek study examined samples of colostrum, transitional milk, and mature breast milk from 51 mothers of male and female babies. The researchers compared levels of macronutrients and energy in the milk for male and female babies in each stage.
In line with existing research, they found that for both boys and girls, colostrum was higher in protein and lower in fat compared to mature breast milk [5]. Within this general framework, however, they noticed some significant differences between the milk produced for sons and for daughters.
Boys start with more protein (on average)
Researchers found that in the earliest days after giving birth, colostrum for baby boys had significantly more proteins and nitrogenous compounds than for baby girls at the same stage, while no significant differences were observed in the average concentrations of fat, carbohydrates, total solids (non-water content), and energy.
There was much more variation in the amount of protein and total solids that boys received in colostrum compared to girls.
Girls get a nutritional boost with transitional milk
In transitional milk, the samples from mothers of baby girls had similar levels of proteins but higher levels of fat, carbs, total solids, and energy than those for baby boys.
Differences between milk for boys vs girls level off over time
Over the course of each lactation stage, baby boys saw a steady increase in fat corresponding to a decrease in protein. For girls, the highest levels of fat, total solids, and energy were in transitional milk.
In mature breast milk, no major differences were observed in the macronutrient or energy concentrations in milk for boys and for girls.
Conflicting research
The authors of the Greek study note that, while their findings align with some previous studies, the research on this topic is limited, most of the studies are small, and there are conflicting results.
Например. Исследование 2020 года found that mature milk samples from mothers with daughters had higher fat content on average, whereas a 2023 исследование found that mature milk samples from mothers with sons had higher fat content [6] [2]. One исследование analyzed over 300 milk samples and found нет significant differences in macronutrient composition based on sex in either colostrum or mature breast milk [3].
Why such mixed results on sex-differentiated breast milk?
There are a few reasons why this research might be coming up with different answers when studying breast milk. The 2025 Greek study catalogs “the lack of standardized methods for human milk collection, variations in study designs, the limited number of studies on this issue, the small sample sizes used in some of the studies, and foremost to the significant variations documented in [breast milk] composition according to the circadian cycle, lactation frequency, and within-feeding BM composition variability.” The researchers who designed the Greek study controlled for as many variables as they could by including a uniform group of mothers, all of whom were healthy and of Mediterranean descent. They also collected data on the mothers’ dietary intake during the first three months postpartum.
Breast milk composition changes constantly to meet the needs of the infant in real time. This constant variability, while precisely what makes breast milk the perfect food for infants, also makes it difficult to isolate potential differences by infant sex. The Greek study accounted for some of these factors by having milk samples always collected about an hour after a full feeding and around mid-day, since we know that a mother’s циркадный ритм affects breast milk composition [7].
Breast milk composition changes constantly to meet the needs of the infant in real time. This constant variability, while precisely what makes breast milk the perfect food for infants, also makes it difficult to isolate potential differences by infant sex.
Many studies have also taken place in local settings; while this may help neutralize some environmental and genetic factors, it also makes it harder to extend the findings to broader populations.
Quantity vs density: do breast milk sex differences wash out?
In “First Food,” Dr. Hinde mentions a “daughter effect” where mothers who have a girl first produce more milk, even with their next child. In a исследование of rhesus macaques, she found that first-time mothers among the primates produced higher quantities of milk for female young but more energy-dense milk for males, resulting in equal available energy for both [8].
Whether that same trend applies to human mothers and babies remains to be seen. Studies have найдено that mothers of sons produce milk that’s up to 25% higher in energy and 39% higher in lipids than those with daughters [9]. However, these studies did not measure the amount of milk consumed.
Studies have found that mothers of sons produce milk that’s up to 25% higher in energy and 39% higher in lipids than those with daughters. However, these studies did not measure the amount of milk consumed.
What about twins?
A few researchers have tackled the issue of sex-specific differences in breast milk by studying the growth patterns of twins. They found that same-sex twins tended to be taller and heavier than opposite-sex twins in adolescence and young adulthood—but only if they had been breastfed.
By contrast, same-sex and opposite sex twins who were никогда breastfed did not show the same differences in height and weight in adolescence and young adulthood. The researchers suggest that this could be evidence that “sex-tailored” breast milk produced by mothers of same-sex twins was better able to meet their babies’ nutritional needs more effectively than the “mixed sex” breast milk produced by mothers of opposite sex twins.
Beyond macros
Macronutrients tell an important story about breast milk nutrition, but not the complete story. Breast milk also contains micronutrients like vitamins and minerals, hormones, and microbial and genetic material, and there is emerging evidence that breast milk also differs by the baby’s sex at the micro level [5].
A 2024 study found varying concentrations of different types of proteins in milk for newborn boys and girls [10]. Of 146 different proteins identified, 42 differed in amount by the baby’s gender. The researchers noticed a trend that boys received more proteins related to metabolic processes, while girls received more related to defense.
A study in Algeria investigating micronutrient contents of breast milk also found differences based on the baby’s gender [2]. For both boys and girls, total mineral content was highest in colostrum (more proof that it deserves the nickname “liquid gold”!); however, males received higher mineral amounts overall in breast milk, and females received higher concentrations of calcium and potassium.
Possible reasons for sex-specific changes in breast milk
One theory as to why baby boys and baby girls receive different nutrients in breast milk early on is their different growth rates. Boys tend to grow faster, reflected in the higher protein and overall energy content in the colostrum and milk they receive early on [1]. In “First Food,” Dr. Hinde refers to the higher calcium content in breast milk for girls as reflective of their faster skeletal growth and earlier maturation to adulthood.
Boys tend to grow faster, reflected in the higher protein and overall energy content in the colostrum and milk they receive early on. In “First Food,” Dr. Hinde refers to the higher calcium content in breast milk for girls as reflective of their faster skeletal growth and earlier maturation to adulthood.
Another factor could be the rapid hormonal, physiological, and biochemical shifts that occur after giving birth. During pregnancy, a mother’s гормон levels and even иммунная система differ depending on whether she is carrying a boy or a girl, and this affects her hormonal fluctuations postpartum [11]. The researchers in the Greek study suggest these hormonal differences may also explain the wide variance observed in the nutrient levels in colostrum for boys.
“Boy milk” vs “Girl milk:” An added consideration for donor milk?
Understanding sex-based differences in breast milk composition may lead to advances in matching new mothers with milk donors. The demand for donor milk has skyrocketed in the last few years; between 2017 and 2022, the number of NICUs using donor milk went from 50% to 90%. (And this doesn’t include grassroots milk sharing efforts through organizations like Human Milk 4 Human Babies.)
As we learn more about the vital role of breast milk—especially in the initial days after birth—it makes sense that more new moms and healthcare professionals prefer donated breast milk to formula. While matching donor milk based on factors like gender would be laudable, it’s worth noting that (at least in the Greek study) the individual milk samples varied far more within the male and female groups than between the two. In other words, there’s no reason to avoid donor milk from a mom with a baby of the opposite sex if you (or someone you know) needs donor milk.
Итог
We are far from exhausting our knowledge about breast milk and the amazing ways it adapts to nourish our babies from the first days and through the first months (or years!) of life. The good news for moms who are pregnant or nursing now is that your body is уже perfectly tailoring milk production to fit the exact needs of your baby.
Ссылки
[1] Powe C. E., Knott C. D. & Conklin-Brittain N. Infant Sex Predicts Breast Milk Energy Content. Am. J. Hum. Biol. 22, 50–54, 10.1002/ajhb.20941 (2010). https://doi.org/10.1002/ajhb.20941
[2] Khelouf N., Haoud K., Meziani S., Fizir M., Ghomari F.N., Boumediene K.M., Kadi N. Effect of infant’s gender and lactation period on biochemical and energy breast milk composition of lactating mothers from Algeria. J. Food Compos. Anal. 2023;115:104889. doi: 10.1016/j.jfca.2022.104889.
[3] Mangel L., Morag S., Mandel D., Marom R., Moran-Lev H., Lubetzky R. The Effect of Infant’s Sex on Human Milk Macronutrients Content: An Observational Study. Breastfeed. Med. 2020;15:568–571. doi: 10.1089/bfm.2020.0228.
[4] Lithoxopoulou M, Karastogiannidou C, Karagkiozi A, Zafeiriadou IE, Pilati E, Diamanti E, Kalogiannis S, Vassilopoulou E. From Mother–Fetus Dyad to Mother–Milk–Infant Triad: Sex Differences in Macronutrient Composition of Breast Milk. Питательные вещества. 2025; 17(9):1422. https://doi.org/10.3390/nu17091422
[5] Kim, S.Y., Yi, D.Y. Components of human breast milk: from macronutrient to microbiome and microRNA. Clin Exp Pediatr. 2020 Mar 23;63(8):301–309. doi: 10.3345/cep.2020.00059.
Ссылки Продолжение
[6] Hosseini M., Valizadeh E., Hosseini N., Khatibshahidi S., Raeisi S. The Role of Infant Sex on Human Milk Composition. Breastfeed. Med. 2020;15:341–346. doi: 10.1089/bfm.2019.0205.
[7] Qin, Y., Shi, W., Zhuang, J. et al. Variations in melatonin levels in preterm and term human breast milk during the first month after delivery. Sci Rep 9, 17984 (2019). https://doi.org/10.1038/s41598-019-54530-2.
[8] Hinde, K. Richer milk for sons but more milk for daughters: Sex-biased investment during lactation varies with maternal life history in rhesus macaques. Am. J. Hum. Biol. 2009;22 (4):512-519. https://doi.org/10.1002/ajhb.20917.
[9] Samuel T.M., Zhou Q., Giuffrida F., Munblit D., Verhasselt V., Thakkar S.K. Nutritional and Non-nutritional Composition of Human Milk Is Modulated by Maternal, Infant, and Methodological Factors. Front. Nutr. 2020;7:576133. doi: 10.3389/fnut.2020.576133.
[10] Bernardes-Loch R.M., et al. Human milk proteins differentiate over the sex of newborns and across stages of lactation. Клиническое питание ESPEN. 2024;62:144-156. https://doi.org/10.1016/j.clnesp.2024.05.013.
[11] Enninga E.A.L., Nevala W.K., Creedon D.J., Markovic S.N., Holtan S.G. Fetal Sex-Based Differences in Maternal Hormones, Angiogenic Factors, and Immune Mediators during Pregnancy and the Postpartum Period. Am. J. Reprod. Immunol. 2015;73:251–262. doi: 10.1111/aji.12303.
