Breast milk is always doctor recommended and the best way to feed your baby. However, there may be certain circumstances that you will want to mix breast milk and formula. Since breast milk is so perfect and valuable to your baby, if you mix the two in one bottle and your baby doesn’t take the entire bottle, then you lose some of your most valuable milk, the breast milk. By keeping the two separate, you can give the breast milk priority and use formula only as needed. If you are sure you will need to supplement with formula more than one feeding a day, you could alternate the feedings. Because formula is more difficult for your baby to digest, by interspersing them with the easier-to-digest breast milk, it will put less stress on your baby's system.
Bottle-feeding is very different from breastfeeding. Breastfeeding requires a completely different mouth and tongue movement than sucking from a bottle. You can always switch from breastfeeding to bottle-feeding, but it’s difficult to switch back the other direction. With all the benefits of breastfeeding and breast milk, for both your baby and you, it’s worth trying your best to make it work.
Another thing to consider is when you give your baby formula, you are telling your body to make less breast milk. Many mothers have unknowingly jeopardized their breastfeeding relationship with their babies by giving formula, not knowing that this will reduce their milk supplies. It is a vicious cycle: baby gets some formula, so mom's breasts make less milk....so baby gets more formula, so mom's breasts make even less milk, and so on and so on, until baby is weaned prematurely! If you find yourself needing to give formula, be sure to pump your breasts to make up for the formula the baby receives. Any time you must miss a feeding, pumping is essential to keep up your milk supply. Pump... pump... pump...
Good luck!