Can You Mix Breast Milk with Baby Formula?

According to the CDC, breast milk is the best nutritional option for infants because of the bioavailability of nutrients, meaning in their most absorptive and active form, along with the reduction in various risks for both mother and baby, from diabetes to female related cancers. Live cells, called leukocytes, contribute to the immune-boosting qualities babies need as they develop their immune system, which does not mature until closer to their second birthday. We know this, and have plenty of evidence to support these claims, but there are also very different lifestyles, family and health situations, along with breastfeeding obstacles that may prevent the ability to exclusively breastfeed or provide breast milk exclusively one way or another, through pumping or donor milk. Luckily, we have the science to provide the next best thing, which is infant formula, designed to provide the right amount of calories per ounce, along with the proper breakdown of proteins for digestion. This brings up the question: can we mix formula into breast milk?

In short, the answer is, "Yes." Just like with breastfeeding, formula feeding does not have to be all-or-nothing. The term "supplementing" is appropriately applied for this concept, as you are essentially filling in whatever gaps needed to provide the adequate amount of calories. This can look very different from baby-to-baby, as one may be predominantly on breast milk, but need the occasional bottle of formula, while on the flip side, a baby may be completely bottle feeding and on infant formula, with whatever breast milk can be provided. We can also refer to this as "combination feeding."

The idea to mix the two when formula is needed is not only efficient, but when done appropriately, quite beneficial. Formula is synthetically put together, and while its the next best thing, it does not contain the advantage of enzymes, like non-processed human breast milk, to breakdown proteins, fats, and lactose, the naturally-occurring milk sugar found in ALL mammalian milk, regardless of species. So to have any available breast milk mixed in will actually contain some of those enzymes to break down these macronutrients, along with the immune factors. In other words, every drop truly does count!

How To Mix

What is important to take away from this is that the handling and mixing of the two, the order especially, is what needs to be focused on. Whether you are using powdered formula or concentrated liquid formula, those need to be mixed according to instruction FIRST, prior to adding breast milk. This keeps the integrity and the concentration of electrolytes, calories, and macronutrients in check and avoids unnecessary issues. If using a read-to-feed formula, already premixed and sterilized, breast milk can be added to the bottle with it as-is. 

Regardless of how much is needed, there are steps recommended to take with formula use. Powdered and concentrated infant formulas are not considered sterile, and just like raw flour and other foods, need "cooking." Taking these extra steps is especially important for babies 3 months and younger, and those with compromised immune systems.

The amount of water we mix with it, even "nursery water," which is essentially filtered tap water, is also not sterile, unless it is labeled as so. Sterile is the lack of microbes that may cause diarrhea in the baby, since their gut is still maturing. Filtered water, nursery water, bottled water, and distilled water do not mean "sterile." This means, in order to properly mix formula in its safest state, we need to take steps to decrease the amount of microbe activity in both the formula and the water.

Preparation Steps for Powdered and Concentrate

Remember to wash hands, and use sterilized bottles and nipples!

  1. Boil water for 5 minutes.
  2. Measure the amount of formula needed. For powdered, level off the scoop. Use liquid measuring tools for concentrate.
  3. Take water off the heat.
  4. Measure the amount of water needed for mixing.
  5. Stir the infant formula into the water while it is still hot.
  6. Allow to cool thoroughly before feeding or adding breastmilk. 

The heat from the water helps to minimize microbe activity in the formula, as close to sterilization as possible. 

Formula bottles, with or without breast milk mixed in, can stay at room temperature up to 2 hours. Refrigerate in between uses and discard after 24 hours. Larger batches and unused bottles can be kept up to 3 days in the refrigerator. 

Developing Your Personal "Formula"

Working with a lactation consultant can help find the right feeding plan for you, and you do not have to be a candidate for exclusive breastfeeding! In fact, an IBCLC can accurately help develop what is best for you and your baby(s) based on individual goals, current situation, and pediatrician's recommendations, and they must provide information that is evidence-based. 

Through weighted feeds and other assessments for milk supply, they can precisely determine the amount of formula needed for supplementation, how to communicate these plans with the caregiver, along with other guidance needed, such as how to properly and safely mix infant formula, weaning, starting solids, and more. 

These care plans may change with the changing situation(s), which may evolve based on breastfeeding and pumping plans, whether or not we achieve enough milk for exclusive breastfeeding, changes in desires and goals, but they also may look like long-term maintenance plans. 



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