What’s in Breast Milk?
Water makes up about 88% of mature breast milk.
Lactose is the main sugar (carbohydrate) in breast milk.
Breast milk also contains a wide variety of sugars known as human milk oligosaccharides (HMOs) — around 200 types have been identified so far. These play a key role in developing the infant gut microbiome. Combined, these sugars provide about 40% of the total energy in milk. Glucose and galactose are also present, but in much smaller amounts.
Protein appears in various forms: whey, caseins (which come in different structures), free amino acids, and functional proteins like enzymes and immune proteins. Proteins contribute around 10% of the total energy in breast milk.
In mature milk, the whey-to-casein ratio is around 60:40, while in formula it’s typically closer to 80:20.
Fats in breast milk range from short- to very long-chain fatty acids. Their composition can vary depending on maternal diet and nutrient stores. Fats provide about 50% of total energy in breast milk.
These fats are found in structures called triacylglycerides, which are enclosed within the human milk fat globule—a complex structure that also contains proteins and sugars, contributing to breast milk’s overall bioactivity.
Vitamins and Minerals. Some vitamins in breast milk—such as Vitamin D, K, B12, and sometimes A—are dependent on the mother’s diet and nutrient stores.
Because of this, it’s often recommended that babies are supplemented with Vitamin D and K and that mothers continue taking prenatal supplements, especially if they have restricted diets or known deficiencies
Minerals in breast milk are generally less influenced by diet and are present in amounts suitable for infant needs. Iron, for instance, exists in low concentrations but is more bioavailable than the iron found in formula.
Bioactive Ingredients. Beyond basic nutrition, breast milk contains a wide range of functional components that support a baby’s immune system, digestion, and development. These include:
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Immune proteins
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Growth factors
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Antibodies
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Enzymes
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Live cells
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Antioxidants
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Bacteria
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Hormones
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And more
Together, these bioactive and nutritional components work synergistically to support healthy infant development.
How Breast Milk Changes Over Time
In the first few days before and after birth, breast milk appears in its early form: colostrum. Colostrum is higher in protein, contains high levels of immune proteins like lactoferrin and secretory IgA and is often called liquid gold for its powerful immune-supporting benefits
From Colostrum to Mature Milk. A few days postpartum, colostrum transitions into transitional milk. The main change is a rise in water content, which affects the consistency and color. Over time, breast milk evolves into mature milk, with increased lactose and fat concentrations to meet a growing baby’s needs and decreasing protein levels including a lower whey-to-casein ratio. These changes are believed to reflect the shifting needs of the baby. Early milk provides immune support for the transition from the womb. Later milk offers greater nutrition alongside continued immune and growth support
Changes During a Feed. Milk composition also changes during a single breastfeeding session. As the feed progresses, the fat content increases, meaning babies get more energy-dense, satisfying milk toward the end of a feed compared to the beginning.
Is Breast Milk Influenced by Maternal Diet?
Yes, some components are. As noted earlier, fat composition in breast milk is closely linked to the mother’s diet. For example:
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Women who eat more fatty fish (or take omega-3 supplements) have higher levels of long-chain omega-3s in their milk
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Those who consume more processed foods may have higher levels of trans fats in their milk
The effects of diet on sugars and protein in breast milk are less well understood, but some influence is possible.
Vitamins and Minerals
Certain vitamins—notably Vitamin D, K, and B vitamins—are influenced by maternal intake and stores.
As a result vitamin D and K supplementation for babies is recommended and mothers are advised to continue prenatal vitamins
In contrast, mineral levels like iron are more stable and typically sufficient for infants. Notably, breast milk iron is better absorbed than that in formula, despite being present in lower amounts.
What’s in Formula Milk?
The European Food Safety Authority (EFSA) sets guidelines for the composition of formula milk, based on research into breast milk. These guidelines define safe and appropriate ranges for various nutrients.
All infant formulas are required to include:
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Adequate protein, fats (including LA, ALA, and trans fats)
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Carbohydrates (including lactose)
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Essential vitamins and minerals such as calcium, iron, iodine, folate, Vitamin A, D, and K
EFSA also offers recommendations on which vegetable oils can be used to provide omega-3s and approved dairy and non-dairy protein sources
All brands are legally required to meet nutritional standards. In Europe, EFSA and EU legislation oversee this. In the U.S., it’s the FDA. Because of strict regulations, nutritional differences between brands are generally small. Variations may come from:
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Voluntary additions (e.g. DHA, taurine, nucleotides)
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Differences in ingredient sources (like specific oils or proteins)
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Processing methods
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The ratios of nutrients, such as the whey-to-casein ratio
Bioactive Components in Formula
Formula contains far fewer bioactive ingredients than breast milk. Recent efforts have focused on adding synthetic human milk oligosaccharides (HMOs) to formula—so far, two types have been successfully developed.
Some bioactive components occur naturally in formula made from cow’s or goat’s milk, including lactoferrin and alpha-lactalbumin. These may exist in different forms and concentrations than in human milk.
Structural Differences
The structure of nutrients in formula can differ significantly:
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Fats in breast milk are packaged in the human milk fat globule, which is uniquely suited to infant digestion and absorption
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Formula fats, especially those from cow’s milk, are packaged differently, with larger globules and different attached proteins
These differences can affect digestibility and nutrient absorption, with human milk fat globules being the most easily digested by infants.
References
Almeida, C. C., Mendonça Pereira, B. F., Leandro, K. C., Costa, M. P., Spisso, B. F., & Conte-Junior, C. A. (2021). Bioactive Compounds in Infant Formula and Their Effects on Infant Nutrition and Health: A Systematic Literature Review. Int J Food Sci, 2021, 8850080. https://doi.org/10.1155/2021/8850080
Ballard, O., & Morrow, A. L. (2013). Human milk composition: nutrients and bioactive factors. Pediatr Clin North Am, 60(1), 49-74. https://doi.org/10.1016/j.pcl.2012.10.002
Efsa Panel on Dietetic Products, N. a. A. (2014). Scientific Opinion on the essential composition of infant and follow-on formulae. EFSA Journal, 12(7), 3760. https://doi.org/https://doi.org/10.2903/j.efsa.2014.3760
Kim, S. Y., & Yi, D. Y. (2020). Components of human breast milk: from macronutrient to microbiome and microRNA. Clin Exp Pediatr, 63(8), 301-309. https://doi.org/10.3345/cep.2020.00059
Meng, F., Uniacke-Lowe, T., Ryan, A. C., & Kelly, A. L. (2021). The composition and physico-chemical properties of human milk: A review. Trends in Food Science & Technology, 112, 608-621. https://doi.org/https://doi.org/10.1016/j.tifs.2021.03.040
Perrella, S., Gridneva, Z., Lai, C. T., Stinson, L., George, A., Bilston-John, S., & Geddes, D. (2021). Human milk composition promotes optimal infant growth, development and health. Seminars in Perinatology, 45(2), 151380. https://doi.org/https://doi.org/10.1016/j.semperi.2020.151380
Ros, G., Aly, E., Ali Darwish, A., López Nicolás, R., & Frontela Saseta, C. (2018). Bioactive Components of Human Milk: Similarities and Differences between Human Milk and Infant Formula. In R. M. Barría (Ed.), Selected Topics in Breastfeeding. IntechOpen. https://doi.org/10.5772/intechopen.73074
Szyller, H., Antosz, K., Batko, J., Mytych, A., Dziedziak, M., Wrześniewska, M.,…Pytrus, T. (2024). Bioactive Components of Human Milk and Their Impact on Child’s Health and Development, Literature Review. Nutrients, 16(10).
