My lactation consultant confirmed that it’s nothing against mixing breast milk with formula. However if you think about it from the practical standpoint giving them separate is a much better approach simply to avoid wasting your precious breast milk that has all the goodies.
Because you are putting breast milk and formula in the same bottle you may risk that your baby won’t finish it during the feed and you have to dispose what’s left of that bottle all together.
The reason is that formula from a bottle that your baby has drunk from must be discarded within an hour of preparation. Bacteria will grow from the saliva of the baby.
On the other hand breast milk can be kept refrigerated for a number of hours in a bottle that’s been fed.
simply because of the difference in flavors and for not wasting the breastmilk if the baby does not finish the bottle.
So the short answer is: YES, YOU CAN mix the two BUT our suggestion is to feed your baby breast milk first and then if she/he needs a top-up take a separate bottle and prepare the formula. This way you are not wasting breast milk.