Who doesn't love a nice, hot meal? I mean, you've probably not had one since having kids, but you remember what they were like, right? And now you're providing your baby with several hot (body temperature) meals a day, but what about pumped milk? Can you heat up breast milk? Does it do anything to precious milk you've labored over expressing?
Babies are fickle little beasts. One day, they're quite content with a cool, refreshing hit of breast milk straight from the refrigerator. You can practically hear the "ahhh" happening inside their adorable little heads as they get a taste of the good stuff. (I think it sounds like me drinking a martini after the kids go to bed.) The next day, they spit that cold milk back out — cold will no longer cut the mustard. They want it warmed precisely to breast temperature, and swirled six times, counter clockwise, preferably while you remark upon the vintage of that particular bottle. (May 2, I remember it well. I ate four Twinkies that day. I think you'll find this vintage smells a bit of vanilla, high fructose corn syrup, and gastrointestinal regret.)
Turns out, you can warm it for your baby, you just need to follow some guidelines. La Leche League International suggested that you heat your breast milk, either from the fridge or from the freezer, slowly, under warm, running water until it comes to the desired temperature. You should not allow the milk to come to a boil, and instead, stop the heating process when it approaches body temperature.
The reason you want to heat it slowly, and only until room temperature, is because exposure to extreme heat deteriorates the microbial goodness present in the milk, according to The Breastfeeding Medicine Journal. Another study, this one in the Journal of Pediatric Gastroenterology and Nutrition, linked high-heat methods of heating breast milk with lowering the fats and proteins that naturally occur in milk.
Breast pump manufacturer, Medela, noted that it's best to thaw frozen milk overnight in the fridge, and heat slowly with warm water. You should also discard any thawed milk within 24 hours. The Mayo Clinic agreed that you can heat up breast milk, but added that milk should never be thawed at room temperature, which would encourage bacteria growth. It's also imperative to not re-heat previously warmed breast milk, as each heating detracts from the original nutritive content and puts baby in danger of unwanted bacteria.
When I warmed breast milk, I'd take the bag of milk, place it in a ceramic bowl, and stick it under a faucet with running water that was just slightly warmer than body temperature. Every so often, I'd knead the bag to break up the frozen bits, and resettle the cream throughout the milk. (Human milk separates just like cow's milk, which is actually super cool.) After I couldn't register a difference between the temperature of the water and the temperature of the bag of milk, I poured it into a bottle and fed my baby. La Leche League insisted this was the best and safest method, and it's also really simple. Hopefully, after you warm it, your baby won't want the cold version again.