The main reason to heat is to ease digestion. I'm a feeding therapist and there are actually times we tell parents NOT to heat bottles. This is especially true for babies that are having a hard time swallowing. Cold milk flows slower so it's easier to swallow, but it goes slower through the digestive tract as well. As long as she's not gassy or constipated I'd say is fine. Is it formula or beat milk? With breast milk you just need to make sure it's warm enough to mix the fat that separated back in.
Warm food (any food) is easier to digest. Chilled milk could cause some gastro issues (gas, burping, cramps) and there may be some nutrients baby is not absorbing. However, if the baby seems happy to drink the cold stuff and is thriving, I wouldn't worry too much. Perhaps you could mix it up and only give her/him the very cold stuff 1/3 of the time?
We give our son cold milk every evening. Even on our breast milk how to guide it said it could be served straight from the fridge. I was told warming it is a preference thing. He just wants a nipple in his mouth. Haha!
We stared out room temp and then went to fridge so we could make bigger batches to make it easier during the day, our LO took both cold and room temp, but her poo became small and hard with the cold formula. within days of just using room temp her poo went back to normal. I asked my pedi and they said it's fine but they prefer older babies to do this, because sometimes cold formula is hard on their digestive system.
Re: Cold Bottles