We all know breast milk is incredible. From the Human Milk Oligosaccharides (HMOs) that help shape a baby’s gut microbiome to the cancer-fighting HAMLET cells, and everything in between. It’s the ultimate first food.
But what if breast milk is even more amazing than we thought?
Could it be sex-specific?
Groundbreaking Study
A study by Maria Lithoxopoulou et al., titled “From Mother–Fetus Dyad to Mother–Milk–Infant Triad: Sex Differences in Macronutrient Composition of Breastmilk” (Nutrients, 2025), explored this very question. Their discovery? Breast milk varies to the sex of the baby.
Researchers collected 102 milk samples from 51 Mediterranean mothers across four time points during three lactation stages; colostrum, transitional milk, and mature milk. They used mid-infrared spectroscopy to analyze the macronutrient content of each sample.
Key Findings by Lactation Stage
Colostrum (First 0–5 Days)
Milk for male infants contained approximately 60% more nitrogenous compounds, including:
- Crude protein: Total nitrogen-based nutrients that reflect overall protein-related nutrition.
- True protein: Actual proteins like lactoferrin and antibodies that support muscle growth, immunity, and tissue repair.
- Non-protein nitrogen: Nutrients like free amino acids and nucleotides that help build DNA, brain tissue, and gut health.
Other Findings
Lower carbohydrate levels were found in colostrum for boys compared to their later-stage milk, suggesting a temporary emphasis on protein. Milk for girls did not show this early dip in carbohydrate levels.
In male colostrum, all macronutrients were significantly interrelated, meaning if protein was high, so were fat and carbohydrates. This compositional synchronicity may indicate that milk for boys is biologically “tuned” to meet rapid early growth or immunity demands.
Transitional Milk (Days 5–14)
Milk intended for female infants had:
- Higher fat
- More total solids
- Greater energy content (calories)
This suggests a different metabolic focus at this stage, possibly linked to developmental patterns in baby girls. In contrast, milk for boys did not show these elevated energy values during this stage.
Mature Milk (After Day 14)
- At this stage, no statistically significant differences were found between the milk produced for male and female infants. By this point, the sex-specific tailoring seen in earlier stages appears to level out.
- Nutrient Shifts Across Stages
- Colostrum had higher nitrogenous compounds but fewer total solids than mature milk.
Strong positive correlations were found between fat and crude protein in both sexes during the transitional and mature stages, indicating the milk becomes more nutrient-dense as lactation progresses.
Why This Study Matters
This is the first-ever in-depth exploration of sex-based macronutrient differences in human milk across all three stages of lactation. It adds to our growing understanding of how breast milk is not a one-size-fits-all solution; it’s a dynamic, personalised form of nutrition.
Breast milk is not a one-size-fits-all solution; it’s a dynamic, personalised form of nutrition.
Practical Implications
It reinforces the awe-inspiring idea that a mother’s body can naturally adjust breast milk composition based on her baby’s sex.
It opens doors for improvements in neonatal nutrition, particularly for babies in NICUs or those unable to breastfeed.
In the future, we may see formula milk that is tailored specifically for boys or girls, helping babies receive nutrition closer to what nature intended.
How Does a Mother’s Body Know?
While the study doesn’t pinpoint the exact mechanism, scientists believe hormonal signals from the baby via the placenta may help a mother’s body “know” the baby’s sex even before birth. These signals likely influence how milk is made, enabling the incredible sex-specific tailoring seen in early lactation.
Final Thoughts
This research highlights just how attuned a mother’s body is to her baby’s unique needs. Breast milk, already nature’s most perfect food, turns out to be even more adaptive, intelligent, and individualised than we ever imagined.
References
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. Nutrients. 2025 Apr 23;17(9):1422. doi: 10.3390/nu17091422. PMID: 40362731; PMCID: PMC12073663.
