What’s in Breast Milk?

Yvette Zhu
July 10, 2020

Look. I get it. There is the constant rally call of #breastisbest and we at SimpliFed truly believe in #normalizebreastfeeding. Breastfeeding is first choice when it comes to infant nutrition, but we also recognize that there are reasons for needing to supplement or replace and therefore we also believe #fedisbest. In this article, we’ll be delving into breast milk: what makes it so magical?

So what exactly is in breast milk?

The simple answer is that breast milk contains all the nutrients a baby needs for the first six months of life. It is composed of many different types of proteins, fats, carbohydrates, vitamins, minerals, water, and antibodies that can help boost the baby’s immune system. According to research, the exact composition of breast milk is highly variable and changes in response to various factors [1]. In other words, every mom’s breast milk is unique and tailored to match the baby’s needs as it grows! Pretty bada$$ actually.

Fats or lipids are the largest energy source in breast milk, contributing around 40-55% of the total energy. Most of the fats or lipids in breast milk are in the form of triacylglycerols. They are packed into tiny milk-fat droplets with the triacylglycerols found in the core. Besides fats or lipids, breast milk also contains over 400 different proteins that can perform a variety of functions. These proteins can be divided into three general categories, caseins, whey, and mucin proteins, with each serving important and distinct roles in the growth and development of the baby. Some provide nutrition for the baby, some stimulate the absorption of nutrients, and some can help fight microbes or even modulate the immune system. Also present in breast milk are a variety of different carbohydrates. Carbohydrates is the general category for different kinds of sugar, with a major one being lactose and the most abundant in breast milk. Another kind of carbohydrates is the human milk oligosaccharides (HMO). They function as prebiotics and can encourage the growth of beneficial bacteria to protect against pathogenic bacteria in the baby’s gut.

Antibodies are found in breast milk and are crucial in helping the baby develop its immune system. Antibodies, also called immunoglobulins, are a special type of protein that fight foreign invaders to the body, such as the bad bacteria that could cause diseases, viruses, and toxins. The predominant form of antibodies is called secretory IgA. They are found to be the most abundant in breast milk during the first few days after the baby is born and decrease over time. This shows that the baby is developing its own immune system and relies on the mom’s antibodies less and less as it grows. So the mom’s breast milk can adapt to the baby’s changing needs over time and provide the baby what it most wants!

From colostrum to mature milk

Right after the baby is born, breast milk is in a thick, yellowish form called colostrum. It has about the same ingredients that mature milk will have, but the proportions of each ingredient is different. For example, it has a high level of antibodies and white blood cells, almost like a “natural vaccination” to help boost the baby’s immune system. Besides antibodies, colostrum contains other proteins that can increase and protect the baby’s gut lining and help develop its digestive system. It also has more minerals and vitamins, including vitamins A, E, and K, than mature milk. Most babies will get all the nutrition they need from colostrum so don’t be worried if there’s not a lot of this colostrum milk at first because the amount will gradually increase over the first few days after the baby is born. The mature milk that is in a thinner, whiter form will start coming in about three days after

birth, but could take longer for first-time moms [2].

Why should I breastfeed?

Take a look at this post with an infant feeding infographic and worksheet to better understand your different feeding options and think through your infant feeding decisions

As mentioned earlier, every mom’s breast milk is “custom-made,” with ratios of each component tailored to meet the baby’s specific needs as it grows. Additionally, a number of health organizations, including the World Health Organization (WHO), the American College of Obstetrician and Gynecologist (ACOG), the American Academy of Pediatrics (AAP), the Center for Disease Control (CDC), and the American Medical Associations (AMA) all recommend breastfeeding for the first six month as the best choice for baby. Research has shown breastfed babies have lower risks of asthma, obesity, diabetes, diarrhea, vomiting, respiratory tract infections, ear infections, and many other illnesses. But breastfeeding is not just beneficial for babies, it can also have positive effects for moms. Breastfeeding can help reduce the risk of high blood pressure, diabetes, ovarian cancer, breast cancer, postpartum bleeding, menstrual blood loss, and even help moms return to pre-pregnancy weight faster [2, 3, 4].

However, the process of breastfeeding is not always smooth sailing and there are many other factors that play into the decision of breast milk vs. formula. This post mostly delves into what makes up breast milk but we also have a post dedicated to the composition of infant formulas. Breastfeeding has many benefits to both the moms and the babies, but infant formula contains all of the necessary nutrients for a healthy baby as well. Ultimately, this is a personal choice and we at SimpliFed are here to support you and believe in both #normalizingbreastfeeding and #fedisbest.


  1. Andreas, Nicholas J., et al. “Human Breast Milk: A Review on Its Composition and Bioactivity.” Early Human Development, Elsevier, 12 Sept. 2015, www.sciencedirect.com/science/article/pii/S0378378215001772
  2. “Breastfeeding.” Centers for Disease Control and Prevention, Centers for Disease Control and Prevention, 2 Mar. 2020, www.cdc.gov/breastfeeding/index.htm
  3. “Benefits of Breastfeeding.” AAP.org, www.aap.org/en-us/advocacy-and-policy/aap-health-initiatives/Breastfeeding/Pages/Benefits-of-Breastfeeding.aspx
  4. “Breastfeeding.” World Health Organization, World Health Organization, www.who.int/health-topics/breastfeeding#tab=tab_1
  5. “Breastfeeding Your Baby.” ACOG, www.acog.org/patient-resources/faqs/labor-delivery-and-postpartum-care/breastfeeding-your-baby
  6. “Breastfeeding 101: Q&A with Lactation Expert Nadine Rosenblum.” Breastfeeding 101: Q&A with Lactation Expert Nadine Rosenblum | Johns Hopkins Medicine, www.hopkinsmedicine.org/health/wellness-and-prevention/breastfeeding-101-qanda-with-lactation-expert-nadine-rosenblum
  7. “Breast Anatomy and Lactation (Video).” Khan Academy, Khan Academy, www.khanacademy.org/science/health-and-medicine/human-anatomy-and-physiology/reproductive-system-introduction/v/breast-anatomy-and-lactation