We tend to think of breastmilk as the biologically appropriate food for young infants. While this is true, it is so much more than nutrition. Your milk is a constantly changing, evolving product that nurtures, feeds, hydrates, supports the immune system, feeds the microbiome, exposes your baby to a variety of flavours, and so much more!

Let’s take a journey into the amazing milk that you are producing for as long as your baby is drinking it.

What's in breastmilk exactly?

On average, your milk has a stable percentage of macronutrients to optimally feed your baby and help them grow and thrive. Milk contains:

87% water. One interesting twist is that in hot climates, your body will slightly adjust the percentage of water contained in breastmilk. In these climates, babies will feed more frequently and take in a larger volume of milk over the course of the day. Total nutrient intake will remain constant while the amount of water taken in is increased to support hydration. This fabulous, natural adaptation is the reason that a breastfed baby under 6 months doesn’t need any additional water, as long as they have responsive, on-demand access to the breast.

7% lactose. Lactose makes up the majority of the carbohydrate profile in breastmilk. The relatively high lactose concentration provides energy for the developing body and brain. The lactose also helps maintain milk consistency and the absorption of minerals like calcium. There is also a special category of carbohydrates called human milk oligosaccharides (HMOs). For many years, researchers were baffled by this, as babies can’t digest HMOs, which makes them appear to be a redundant ingredient. However, more recent research has shown that HMOs function as prebiotics, feeding and encouraging a healthy microbiome.

4% fat. Fats are a very important component of breast milk. Fat not only supplies energy but also aids in the central nervous system’s development. In addition, as we all know, fat adds aroma and taste to food! There are over 200 types of fatty acids in breast milk; the exact profile can change with maternal diet. These fats each have different purposes; for example, short-chain fatty acids are a source of quick energy. Medium-chain fatty acids support the immune system, and long-chain fatty acids contribute to the development of a baby’s organs, tissues, and nervous system.

1% protein. Although there are more than 400 different types of protein in breastmilk, they can be categorised into two categories: casein and whey. Protein is a nutrient, but it also helps with the absorption of other nutrients and has immune-supporting properties. In colostrum, the whey/casein ratio is high at almost 90:10, but this gradually changes to 60:40 in mature milk.

Breastmilk composition changes as a baby grows in age.

Your breastmilk is tailored to your baby at every age. It is a uniquely bespoke product made by each mother to meet the precise needs of her baby. This is personalised nutrition at its best! There are some general changes that happen gradually over the first months of a baby’s life.

Colostrum: the first few days. This is produced during pregnancy and for the first 3–5 days after birth. It has the consistency of runny honey, and while it’s low in volume, it’s jam packed with all the good stuff. The purpose of colostrum is actually more connected to immune support than nutrition, as your baby meets the challenges of the outside world. Colostrum provides antibodies, good bacteria, HMOs, and other protective cells.

Transitional milk: the first few weeks Transitional milk is the first milk that appears after your milk ‘comes in’. At this time, available milk volume increases rapidly, which can explain the drastic change in breast size that some women experience! It has a higher fat and lactose content and more water than colostrum.

Mature milk: after the first month You are considered to have mature milk once your baby reaches approximately a month of age. The composition remains fairly stable at this point (in line with the proportions above).

Immune factors in breastmilk

Your breastmilk is jam-packed with antibodies and other bioactive cells. In fact, one study found there to be over 3,000,000 such cells in just one teaspoon of breastmilk, so there’s great value in any amount of milk your baby receives!

These immune factors aren’t static; they are a dynamic process. We don’t know for sure how this happens, but the hypothesis is that a baby signals their immune needs to their mum while they feed. During a breastfeed, the vacuum created by the baby while they are sucking causes some of the baby’s saliva to enter mum’s body in a ‘backwash’ type situation. If the baby has been exposed to an immune challenge like a virus, this saliva will trigger an immune response from mum’s mature immune system, which in turn will make immune cells (leukocytes) that will be passed back to the baby via the next portion of breastmilk. We know that leukocyte levels are found in higher concentrations in breastmilk when the baby is unwell, supporting their recovery through borrowed access to mum’s mature immune system.

What does breastmilk contain?

For our Baby Feeding Journey, Unii Expert Wendy Lever explains what’s in breastmilk.