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The Evolution of Breast Milk

In the first months after birth, babies experience the most rapid growth of their lives. Your baby’s birth weight will more than double in the first six months. During this time, your baby’s nutritional needs are changing quickly, and the composition of breast milk is constantly changing to meet these changing needs.


Breast milk is initially high in protein to support an infant’s early rapid growth, then gradually decreases over the following months in sync with babies’ slowing growth rate.

Protein Content of Breast Milk


Energy requirements for infants, on a per kilogram body weight basis, decrease with increasing age. Breastmilk provides the right balance of protein, fat and carbohydrates to provide the energy your baby needs throughout the first few months of life.


During this initial period of rapid growth, the fat content of breast milk also quickly increases after the first few days of lactation, slightly increasing again over the first two months, before stabilizing or even decreasing.


One component of breast milk that progressively increases is the carbohydrate content, providing approximately 40% of the total energy in mature breast milk.

Months 0 - 6

From birth to six months, breast milk is the perfect, single source of nutrition to meet all of your baby’s dietary needs.

Did you know?

  • Your baby’s growth rate and milk intake are changing continually and are aligned by the ongoing adaptation of breast milk volume and composition.
  • The type of protein in breast milk changes over time, with whey decreasing and casein increasing, from less than 20% to about 50% of the total protein in mature breastmilk.
  • Breast milk is also packed with antibodies and contains good bacteria to protect your baby before his natural defenses are fully developed.

Protein and calories change from month to month

During the first few months of life, your little one will grow rapidly: your baby’s birth weight doubles in 6 months ! Babies gain an average of 1.3 ounces per day during the first month . Never again in your child’s life will they experience such a rapid growth period.

To fully harness this flying start, your baby needs an efficient fuel: your milk! Generally, from about the first to the fifth day of life, your baby ingests colostrum, an important yellow, “first breast milk”, rich in proteins, vitamins and minerals, ideal to give your baby their first start in life.

Next comes the transitional milk, which is rich in sugars and fats, gradually taking over until about day 14. Breast milk then becomes mature milk, from approximately the 15th day.

In the following months of lactation, as the pace of your baby’s growth declines a bit, the percentage of protein from milk decreases. The type of proteins also changes: during the first few months, an easy to digest group of proteins in the milk called "whey", make up the bulk of the milk. Whey is rich in several special proteins that support the development of your baby’s immune system. Over time, another type of protein called “casein”, which is also found in cow’s milk, increases and whey declines.

Fats provide energy for growth

Fats represent between 50 and 55% of the energy provided by mature milk, and are the nutrients that have the highest energy density. Protein and Lactose, the sugar found in milk, complements fat to provide needed calories.

The fat content of breast milk changes during the day and during each nursing session to fit the needs of your baby. Thus, the milk is richer in fat when your baby is likely to be the most active.

Antibodies and good bacteria reinforce its defenses

Breast milk contains immune proteins, including lactoferrin, which have positive effect on the immune system, and enhance the absorption of iron. Breast milk also contains antibodies, like immunoglobulin, that help protect your baby against many infectious diseases.

Breast milk also provides your baby with good bacteria (lactobacilli and bifidobacteria) which enhances intestinal flora, as well as non-digestible carbohydrates sugars that also contribute to the development of good bacteria in the gut to help strengthen his natural defenses.

Zinc and iron, two other essential elements

  • The milk you produce is rich in zinc during the first few months, to adjust to the rapid pace of growth of your baby. This mineral is essential to assimilate proteins, fats and carbohydrates.
  • Iron plays a crucial role in various metabolic processes (including red blood cell formation) and cognitive development. Before four months, your baby’s iron requirements are covered by the contribution of breast milk and reserves accumulated during fetal life. However, after four months, it’s possible that iron supplementation is needed. Ask your pediatrician for advice.

Months 7 - 12

During months seven to twelve, solids will gain an increasingly larger part of your baby’s diet.

Did you know?

  • As your baby becomes more active, breast milk alone is not always enough to cover all of his nutritional needs.
  • Milk, preferably breast milk, is still essential in feeding your baby, but introduction of other foods can begin as early as five months.
  • Fatty acids in breast milk can help to offset the lower fat levels in some of baby’s first solid foods.

Breast milk and solid foods create your baby’s new diet

After six months, your baby still continues to grow, but at a less rapid pace than in earlier months, gaining an average of half an ounce per day. He begins to taste his first vegetable purees and fruit compotes. Breast milk does not fill all of your baby’s nutritional needs on its own anymore, but, at eight months of age, still provides your baby a significant portion of his energy intake.

First solids

First solids for your baby consist mainly of cereals, fruits and vegetables, and then meat. His new eating regime gives him mainly carbohydrates and proteins. The protein and calories from breast milk are lower than when he was a younger infant.

Conversely, meals are not necessarily high in fat, particularly essential fatty acids. Although the energy provided by fat in breast milk decreases slightly to about 40% (from 50 to 55% in the first six months), your baby needs milk for the proper development of his organs and certain essential functions. Breast milk provides essential fatty acids to meet your baby’s growing needs, and complements your child’s first solid food.

The proteins that support immune function (immunoglobulin and lactoferrin), which are called "whey", decline steadily during the first six months. Now, the proportion of whey to casein (a milk protein) stabilizes at around 50/50.

Breast milk is also a good source of vitamins and minerals, which are not yet provided by the solids your baby is eating. Many essential nutrients are included in breast milk to ensure your baby continues to grow and develop.

Foods other than milk alter your baby’s digestive system

With the introduction of solid foods, your baby's digestive system changes dramatically. In addition, as your baby explores his environment, he puts objects in his mouth, exposing him to many different germs.

A smooth transition to solid food

The transition to a diet of mainly solid food from one that was all liquid is not done overnight. You must allow time for your little gourmet to get used to new tastes. Plus, the mechanism used for swallowing food is a very different experience for your baby.

As Baby approaches one year old, you may start to transition to toddler milk drink or to cow’s milk.