Expressing milk: does it affect its composition?

Unlike infant formula, which is standardised within a very narrow compositional range, the composition of human milk is dynamic and varies throughout a feed, across the day, over the course of lactation, and between mothers and populations.

Hindmilk, defined as the last milk of a feed, can contain two to three times the fat concentration of the milk found in foremilk, defined as the first milk of a feed [1]. During each breastfeeding session, the milk expressed first is thinner and contains more lactose, which quenches the baby's thirst, whilst hindmilk is creamier and contains more fat to meet the baby's needs [2].

Furthermore, one study found that the fat content of breast milk was significantly lower during night and morning feeds compared with afternoon or evening feeds [3].

That said, don't worry about which milk your baby receives at the breast — things tend to balance out naturally with on-demand feeding: trust your body and your baby.

Furthermore, melatonin (the hormone that plays a role in regulating sleep) is a normal component of breast milk, with concentrations that are higher at night (peaking around 3 a.m.) than during the day.
Some authors suggest that mothers should breastfeed in the dark at night to avoid a reduction in the melatonin content of breast milk, which could disrupt the infant's sleep. It has also been suggested that, for women who express milk for their baby, milk expressed during the day should be kept separate from milk expressed in the dark, so as not to disturb the baby's sleep cycle [4].

What about exclusive pumping?

Due to its high fat content, hindmilk is more viscous, which may explain why it can be difficult to express with an electric pump. Research findings have shown that mothers who used hand expression had a higher fat content in their milk than women who used only an electric breast pump [5]. Themixed feeding is also possible.

To find out how to express breast milk, have a look at our article on the subject.

Try to pump for long enough at each session to avoid your baby receiving mostly foremilk and not enough hindmilk [6]. The frequency of this phenomenon has not yet been documented and warrants further research.

During breastfeeding, the baby's saliva reacts with breast milk to produce reactive oxygen species, whilst simultaneously providing precursors of growth-promoting molecules. Breast milk therefore plays more than a simple nutritional role in mammals, as it interacts with the infant's saliva to produce a powerful combination of stimulating and inhibitory metabolites that regulate the oral — and therefore intestinal — microbiota. As a result, the milk-saliva mixture appears to represent a unique biochemical synergy that strengthens the early innate immunity of newborns [7].

During milk expression, milk passes from the nipple through a plastic flange, through a valve, and is collected in a bottle. If not cleaned thoroughly, these surfaces can harbour bacteria that may be transferred to the milk during pumping [8].

A study conducted a randomised crossover trial at home to determine the effect of using a woman's own pump and collection supplies on the microbiome of pumped human milk, compared with using a hospital-grade pump and sterile supplies. They provide the highest level of evidence to date that pumping equipment can dramatically alter the microbiome of pumped human milk. On average, milk collected by women using their own pumping equipment shows higher levels of bacteria compared to the sterile control. However, it is not yet known whether this has an impact on the child's health. [9].

Nutrient loss varies depending on the nutrient and the storage method. 
To find out how to freezing breast milk and on storing breast milk, check out our articles!

For vitamin C, loss occurs rapidly, even during the bottle-feeding process with freshly expressed human milk [10]. Expressing milk reduces its vitamin C content during storage [11].

When breast milk is stored at temperatures typical of domestic freezers, lipids are hydrolysed, immunological cells are destroyed, and antioxidant activity is reduced [12].

One study also showed a significant decrease in fat content after 7 days of storage at -20°C, and a decrease in energy value after 15 days [13].

Furthermore, microwave thawing of frozen milk leads to a marked reduction in the anti-infective factors in milk [14]. It is never recommended to reheat breast milk in a microwave. 
To find out how to thawing breast milk, have a look at our article on the subject.

The concentrations and activities of the main host defence proteins are reduced in frozen human milk compared to fresh human milk. The stability of the various protein components of human milk appears to be extremely variable, including those with antimicrobial activity. Lysozyme, lactoperoxidase and IgA concentrations are all modified by storage at -20°C (concentration decreases of 32%, 66% and 51% respectively). In correlation with the reduced protein concentrations, the activities of defence proteins are diminished by storage at -20°C for 4 weeks, with a decrease of 50 to 69% [15].

As a result, expressed milk may not offer the same nutritional and anti-infective benefits as milk obtained directly from the breast. The consequences of these differences for infant health are unknown and warrant further study.

Expressing milk has an impact on the composition of breast milk. The composition of breast milk will vary, particularly with storage, but also because a baby feeding directly at the breast causes the milk composition to change too!

However, although there are changes, it is not yet well understood whether this has an impact on the newborn.

But expressing milk and giving it to your baby by bottle is already wonderful, and can offer many of the benefits found in breastfeeding "directly at the source".

If you're exclusively pumping, mama, know that you're giving your child the very best — the benefits of breastfeeding are amazing. You can be proud of yourself, as it takes a great deal of time and dedication to make pumping work!