The Canadian and American Pediatric Associations recommend exclusive breastfeeding for the first six months of life and then the slow introduction of complementary foods while continuing to breastfeed for a year or longer. Exclusive breastfeeding means nothing but breastmilk; no water, no juice and no formula. Many mothers are happy to know that their breastmilk is all baby needs in the first six months but other mothers may consider supplementing with formula for a variety of reasons. Those mothers may have questions about whether it is okay to mix formula into their breastmilk.
Here are some science based answers to this question:
It's best not to mix breastmilk and formula in the same bottle because the storage guidelines for the two substances are very different. If you mix your milk with formula, and the baby doesn't finish the bottle, then some of your milk goes down the drain. Formula has to be discarded after the feeding (Alberta Health) whereas human milk left in the bottle can be refrigerated for another feed see:https://www.llli.org/breastfeeding-info/storingmilk/
Mixing formula into the breastmilk will affect some of the protective components of human milk. The following quotes are from the Human Milk Banking Association of North America (HMBANA), in their 2005 publication "Best Practice for Expressing, Storing and Handling Human Milk in Hospitals, Homes and Child Care Settings": "Feed human milk, with or without fortifiers, separate from formula to maximise the benefits of human milk. If the mother is not producing sufficient milk for each feeding, collect as many pumpings as necessary to make one exclusive human milk feeding and use formula at the other feedings.” The rationale: Quan et al found that when bovine milk based formula is mixed with human milk prior to feeding, there is a significant (41-74%) decrease in lysozyme activity. Additionally, there is significant increase in the growth of E coli, resulting from the decrease in anti-infective properties in the milk. "Adding human milk fortifier causes a 19% decrease in lysozyme but no corresponding effect on the anti-infective properties or increased growth of E coli."
It makes sense that human milk, which is alive, would not be in its ideal state by adding a dead substance, possibly containing agents that might bind up some of the live constituents in the human milk, making them hard to absorb by the baby, or maybe not available at all.
The aim for most mothers is to exclusively human milk feed their babies. Therefore, if formula is used because the mother has not enough milk, but might in the future, it is best to treat the formula like a medicine, used temporarily and separately, to 'top up' her milk until such time as she has enough of her own.
Mothers with adequate milk supplies who may choose to use formula for some feedings rather than expressed breastmilk will also want to ensure that their babies get the full benefits of the breastmilk by using the formula separately rather than mixing it with breastmilk.
If you have questions about how to increase your milk supply or any other breastfeeding questions please contact a LLLC Leader.