What is in breast milk?
Water makes up about 88% of mature breast milk.
Lactose is the main sugar (carbohydrate) in breast milk.
Milk also includes an extensive variety and volume of sugars called human milk oligosaccharides (around 200 have been identified thus far). These play an important role in developing the gut microbiome. Together these sugars provide 40% of the total energy in milk. Glucose and galactose are also found in breast milk but in very low amounts.
Protein exists in multiple different forms, including whey, caseins (of which there are different structures), free amino acids and functional proteins such as enzymes and immune proteins. These contribute around 10% of total energy.
In mature breast milk the whey to casein ratio is around 60:40 whereas in formula it is often around 80:20.
Fats in breast milk include short to very long chain fatty acids. Breast milk fat composition can vary depending on maternal diet and nutritional stores. Fats contribute the remaining 50% of total energy in milk. The fats within breast milk come in structures called triacylglycerides that are further bound within the human milk fat globule. These globules also contain proteins and sugars and contribute to the overall bioactivity of breast milk.
Vitamins and minerals. Some vitamins in breast milk are dependent on maternal diet and maternal stores, such as Vitamin D, K and B12 and sometimes A. Therefore, it is often recommended that babies are supplemented with Vitamin D and K in the first week of life and that mothers remain on their prenatal supplements, especially if they are known to be deficient or have restricted diets.
Mineral concentrations in breast milk are generally less influenced by maternal diet and exist in quantities suitable for infant needs. Iron, for example, despite existing in fairly low concentrations it has increased bioavailability compared to the iron found in formula.
Bioactive ingredients. In addition to the nutritional composition, breast milk contains many functional components. Each plays a role in supporting a baby’s transition to the world, by boosting their immune response, supporting digestion and aiding growth. These include immune proteins, growth factors, antibodies, enzymes, live cells, antioxidants, bacteria, hormones and many more. The mix of bioactive and nutritional components in human milk complement each other to support infant development.
Breast milk changes over time
Just before and for a few days after birth, breast milk is in colostrum form. Colostrum contains more protein than later milk, and has high levels of immune proteins such as lactoferrin and secretory immunoglobulin A. Colostrum, also known as liquid gold, has powerful immune supporting properties and aids healthy growth and development.
Milk composition changes a few days postpartum, from colostrum to transitional milk. The main change occurring at this point is the increase in water content, leading to the change in consistency and colour. The composition continues to change over time until it becomes mature milk. Lactose and fat concentrations increase as baby grows. Overall protein concentrations as well as the ratio of protein (whey:casein) declines as milk matures.
The reason for the changing nature of milk is believed to be in response to the changing needs of babies. Initially, they require immune support as they transition from the protected environment in the womb. Then their nutritional needs increase alongside their requirement for immune and growth support.
Breast milk composition also changes over the course of a feed. As a breast feed progresses, the fat content of milk increases. Therefore, babies receive a higher energy and more satiating milk at the end of a feed vs at the beginning.
Are breast milk ingredients influenced by maternal diet?
Yes, some are.
As mentioned previously, fats in breast milk are believed to be related to what mum consumes in her diet. Studies have shown that milk from those who consume more fatty fish contains higher levels of long chain omega-3s, this was also true for women who consumed these fats via supplementation. Other studies have found that those who consume more processed foods have had higher levels of trans fats in their milk.
Sugars and protein may have some impact on breast milk composition but this is not as widely explored as the relationship between dietary and breast milk fats.
Certain vitamins in breast milk are dependent on maternal diet and maternal stores, such as Vitamin D, K and some B vitamins. Therefore, it is often recommended that babies are supplemented with Vitamin D and K in the first week of life and that mothers remain on their prenatal supplements.
Mineral concentrations in breast milk are generally less influenced by maternal diet and exist in quantities suitable for infant needs. Iron, for example, despite existing in fairly low concentrations in breast milk has increased bioavailability compared to the iron found in formula.
What is in formula milk?
The European Food Safety Authority (EFSA) provides recommendations for the composition of formula milk based on breast milk research. They advise healthy ranges for concentrations of a long list of nutrients. These are based on concentrations in breast milk as reported in scientific studies, however, concentrations in formula may not exactly match breast milk.
All infant formula must contain appropriate concentrations of protein, fat (including LA, ALA and trans fats), and carbohydrates (including lactose). They must also contain appropriate levels of minerals and vitamins (including calcium, iron, iodine, folate, Vitamin A, Vitamin D, and Vitamin K).
Additionally, the EFSA provide advice on appropriate sources for fats and proteins, e.g. what vegetable oils can be used for additional omega-3 fatty acids and what dairy and non-dairy protein sources should be used.
All infant formula is made to the same high standard as all brands are legally obligated to provide a nutritionally complete product. The EFSA provides guidance in Europe, while the FDA oversees this process in the US. The EU has legislation to ensure infant formula standards are met.
This means that the nutritional difference between brands is often negligible. The EFSA advises on compulsory ingredients and others which are voluntary additions, therefore certain brands may add additional ingredients to their recipe such as DHA, taurin and nucleotides.
There are significantly less bioactive components in formula. Attempts to supplement infant formula with some of the important bioactive ingredients in human milk have been made recently. These include artificially created human milk oligosaccharides (2 have been developed thus far).
There are, however, naturally occurring bioactive compounds in formula coming from cows and goats milk. Examples include proteins such as lactoferrin and alpha-lactalbumin, some exist in slightly different forms to those found in human milk and often in different concentrations.
The form of some of the nutritional ingredients in formula differ to that in breast milk. For example, fats in breast milk are packed into the human milk fat globule. Formula based on cows milk includes a larger milk fat globule, with different proteins and other components attached. The differences in these structures impact their digestibility and absorption, with human milk fat globules being the easiest digested by babies.
References
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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
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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
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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).
