Maybe they gulp down so much, because they're drinking it from a bottle and using the strong suction they would use when they're drinking from a breast. Since it comes out of a bottle easier, they drink it faster and therefore can drink more?
I remember when mine were exactly at that age, 6 weeks. I was very uninformed and new to this and felt like no milk was coming out of the breast, so I would feed them with expressed breastmilk from a bottle and they would gulp down the whole thing, so I don't think it's necessarily the formula that's causing them to do that.
I remember when mine were exactly at that age, 6 weeks. I was very uninformed and new to this and felt like no milk was coming out of the breast, so I would feed them with expressed breastmilk from a bottle and they would gulp down the whole thing, so I don't think it's necessarily the formula that's causing them to do that.