Picture this: A hose.

A bottle is a hose that is uncapped. Water turned on? Water comes out. There’s very little that can be done to control it. It comes out fast. Take most bottles and turn them over. The milk streams out of the nipple. with some bottles you have to apply slight compression to the nipple, but not usually.

A breast is like a hose that has one of those sprayers attached. The water’s there. It requires that the water be released. And there are multiple ways in which the water can be released. Baby learns to control the flow by nursing differently for different purposes. Take your shirt/bra off. Lean over. Does milk pour out until your breasts are empty? Chances are.. No. At most you might leak a bit.

The reason why you can overfeed a baby with a bottle very easily but can’t easily overfeed a baby at the breast have two components:

1- Baby-controlled flow.

2- The difference between how “supply” functions within the breast and how “supply” functions with a bottle.

Babies are programmed to suck for a variety of reasons because their instincts evolved alongside breasts and not bottles. Breasts require stimulation to produce milk and to maintain supply.

In the early days when baby’s just learning to nurse, isn’t very efficient, is at risk of starving, etc. mom’s breasts release milk VERY easily and baby needs this. As baby gets more efficient mom’s supply slows down, baby learns to suck harder to get more milk, suck more shallowly to get less milk, and baby learns non-nutritive suckling that provides comfort, communicates with mom’s immune system to get antibodies for pathogens baby’s exposed to, and helps maintain supply where baby needs the supply to be.

Breasts and babies are also in a delicate dance. It’s a conversation. Baby needs to work for food otherwise there will be no food. So baby constantly works to keep supply up to meet baby’s growth requirements and to increase supply during growth spurts. Mom’s body on the other hand has a vested interest in lowering supply. Too much milk means mom’s at risk for mastitis, plugged ducts, etc. It also means that mom’s nutritional reserves are basically leaking out into her bra instead of feeding her baby or nourishing her body. Nature doesn’t like this and works to lower mom’s supply while baby works to increase it. The human breast creates a protein that inhibits lactation. It creates this protein when mom’s breasts have mik in them. This means that if mom goes for a longer period without nursing, her supply will drop. Bottles have no “decrease in supply” and adults are habit driven and tend to fill bottles with the same amount all the time always, or increase it.

Since baby has to work for the food that baby consumes, baby’s sucks are going to be harder and stronger when baby’s hungry or upset. As baby calms down and is no longer hungry baby’s sucks will become more relaxed and turn into non-nutritive comfort sucking.

Again- turn a bottle upside down. Poke at the nipple. Chances are you’re gonna get milk streaming out like there’s no tomorrow. Take baby’s calmer non-nutritive comfort sucking that is biologically engineered into baby and instinct driven and you’re still pouring milk into baby.

Combine that with a caregiver that doesn’t understand a breastfed baby’s hunger cues, that believes in large bottles, or that feeds baby at every given opportunity and suddenly you have a baby that is eating 6oz bottles every 3 hours which essentially is double baby’s needed intake. Many caregivers also feel that baby needs to “finish the bottle” and will continue to offer the bottle to the baby even if the baby is turning away.

It IS possible to over-feed a breastfed baby at the breast. It’s just uncommon and it requires that both mother and baby have something out of the norm. Ie: A baby with reflux and a mother who is taking a medication that increases supply and that has hyperactive letdown. A baby who is in this situation is not going to be asymptomatic though. And this type of situation is pretty rare, whereas a situation where baby’s over-eating from a bottle is more the “norm” than the exception.

Another issue is that bottle feeds tend to be constant across a 24 hour period. Mom’s supply tends to be variable across a 24 hour period. Breastmilk is very easy to digest. Baby will take what baby needs from the milk and poop or pee out the rest. So if baby’s eating 6 oz bottles of pumped milk every 3 hours, baby’s going to poop a ton, gain weight very rapidly, have a stretched out belly, and will be eating 48oz of breastmilk per day that mom has to pump vs. if baby was eating directly from the breast where the average intake for meeting the needs of a breastfed baby is 25oz/24 hours (varying from 19 oz-30 oz.) Drastic over-feeding. Baby doesn’t need it, baby will poop it out, and baby will learn to over-eat and associate “very full” with “done eating” rather than “no longer hungry”. This perpetuates the cycle of drastic over-feeding and often makes mom feel that she “isn’t making enough” because baby learns to prefer the fast flow and easy food of the bottle.

Best practice for mom’s supply and for baby’s health is this: When a bottle feed is necessary,  limit bottle feeds and provide smaller more frequent bottle feeds rather than large feeds. This allows baby’s system to use everything in the milk rather than just flushing it out quickly. It allows mom’s supply to function at more normal levels, and it enforces healthy eating habits in baby.

References: http://www.kellymom.com/bf/pumping/milkcalc.html