Building blocks and bioactive components—the proteins in breastmilk

Infancy is a critical period of life and sets the stage for lifelong health. During this unique stage in life, babies grow quickly, are constantly learning new skills and exploring new surroundings. In fact, in the 4 first months of life, most babies will have doubled their birth weight and, in 12 months, almost tripled it (1). The American Academy of Pediatrics recommends exclusive breastfeeding during the first 6 months of life and, because milk is the only source of nutrition babies receive during these early months, it must provide all the necessary nutrients to satisfy both a baby’s hungry belly and unique developmental needs (2, 3).

Breast milk varies widely depending on stage of lactation, time of day, length of a feeding session and in response to maternal and infant health and lifestyle reflecting the interconnected relationship between moms and babies (4, 5). Because breast milk is so unique to each mother-infant pair, there is no set definition or nutritional profile for human milk like you would see for most food products available in the grocery store. BIOMILQ is currently conducting a research study to understand how diet and lifestyle affects milk composition, to be able to compare the nutritional profile of cell-cultured human milk that’s made outside the body to breast milk made inside the body and contribute to expanding our knowledge of lactation. Until our study is complete, we want to highlight existing understandings of breast milk’s composition. Today, we’re highlighting one of the three macronutrients that help babies eat, sleep, play, and repeat: proteins.

Adequate protein intake is crucial for an infant’s growth and development because proteins help us break down food, absorb nutrients, repair body tissues, and support the development of the immune system among many other important functions (6).

There are two main groups of proteins in human milk: casein and whey (8). Caseins exist in structures called micelles, which are formed when many casein molecules come together with minerals to form spheres (9). When caseins reach the acidic environment of the stomach, they clump together and form curds. This process results in slower movement of these proteins through the stomach, slower digestion, and slower release of amino acids into the body (10). In addition to regulating digestion, caseins are also important for helping babies absorb micronutrients that are necessary for bone development such as calcium and phosphorus, regulating the immune system, and protecting babies from potentially harmful bacteria (9, 11-13).

In contrast to caseins, whey proteins are present in milk in liquid form and are easier for infants to digest (12, 14). The most abundant whey protein in breast milk is a protein called α-lactalbumin, which helps the breast tissue produce lactose (a sugar that provides energy, and helps with brain development and nutrient absorption) and provides infants with the essential amino acids, vitamins, and minerals they need to thrive. Additionally, many of the whey proteins also help to protect babies from potentially harmful bacteria and help their immune systems develop properly. One of these very important whey proteins is called secretory immunoglobulin A, or secretory IgA. Secretory IgA is the main antibody found in breast milk and is found in the highest amounts in early milk, or colostrum, when babies need the most protection against disease and infection (8, 13, 15). Some important whey proteins and their functions are listed in the table below.

The amount of protein and the types of proteins in breast milk change over the course of lactation.

Women who give birth to premature infants have higher amounts of protein in their milk compared to mothers who deliver term babies. For all women, the amount of protein in breast milk is highest in the first 1-2 months of life and protein levels in breast milk start to decrease when babies are 4- 6 weeks of age. Breast milk contains much more whey protein than casein but the ratio of whey to casein changes over the course of lactation. In colostrum, the whey/casein ratio is very high, about 90% whey and 10% casein, while in mature breast milk, the whey/casein ratio changes to about 60% whey and 40% casein. The high amount of whey proteins compared to caseins is one of main differences between cow’s milk and breast milk since the whey/casein ratio in cow milk is about 20% is whey and 80% casein. In cow milk based infant formulas, the amounts of casein and whey proteins are modified to more closely imitate breast milk, however, the amount of whey in infant formula may still be lower than human milk (8). In fact, breast milk contains lower amounts of casein than most other mammals and this is likely one of the reasons human infants grow more slowly (5).

While the importance of protein is well-recognized for infant growth, the complexity of breastmilk and all the functions of its diverse types of proteins are still not fully understood. This article highlighted the two main classes of protein found in breast milk, casein and whey, but there are many other proteins in breast milk that we do not yet fully understand. One of BIOMILQ’s research goals is to help better characterize the types of proteins found in breast milk, to determine their biological function, and how they help babies grow and develop over their first months/years of life.

References: 1. World Health Organization [WHO]. Child growth standards 2009 [Available from: 2. Eidelman AI, Schanler RJ, Johnston M, Landers S, Noble L, Szucs K, et al. Breastfeeding and the use of human milk. Pediatrics. 2012;129(3):e827-e41. 3. Fox P, Uniacke-Lowe T, McSweeney P, O’Mahony J. Production and utilization of milk. Dairy chemistry and biochemistry: Springer; 2015. p. 1-19. 4. Ballard O, Morrow AL. Human milk composition: nutrients and bioactive factors. Pediatric Clinics. 2013;60(1):49-74. 5. Andreas NJ, Kampmann B, Le-Doare KM. Human breast milk: A review on its composition and bioactivity. Early human development. 2015;91(11):629-35. 6. Haschke F, Haiden N, Thakkar SK. Nutritive and bioactive proteins in breastmilk. Annals of Nutrition and Metabolism. 2016;69(Suppl. 2):16-26. 7. Fennema OR, Damodaran S, Parkin KL. Fennema's Food Chemistry. 5th Edition ed: CRC Press; 2017. 8. Kim SY, Yi DY. Components of human breast milk: From macronutrient to microbiome and microRNA. Clinical and experimental pediatrics. 2020;63(8):301.

9. McMahon DJ, Oommen BS. Casein micelle structure, functions, and interactions. Advanced dairy chemistry: Springer; 2013. p. 185-209. 10. Huppertz T, Chia LW. Milk protein coagulation under gastric conditions: A review. International Dairy Journal. 2021;113:104882. 11. Karpen HE. Mineral homeostasis and effects on bone mineralization in the preterm neonate. Clinics in Perinatology. 2018;45(1):129-41. 12. Ahern GJ, Hennessy A, Ryan CA, Ross RP, Stanton C. Advances in infant formula science. Annual review of food science and technology. 2019;10:75-102. 13. Chatterton DE, Nguyen DN, Bering SB, Sangild PT. Anti-inflammatory mechanisms of bioactive milk proteins in the intestine of newborns. The international journal of biochemistry & cell biology. 2013;45(8):1730-47. 14. Martin CR, Ling P-R, Blackburn GL. Review of infant feeding: key features of breast milk and infant formula. Nutrients. 2016;8(5):279. 15. Lönnerdal B. Bioactive proteins in human milk—potential benefits for preterm infants. Clinics in perinatology. 2017;44(1):179-91.

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