Your milk is rich in healthy fats - you don't have to worry about 'emptying' your breasts when baby feeds!
What are some myths you might hear about the fat in your milk?
There are a lot of myths about the cream or fats in your milk. For example, you might have heard that
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You need to make sure your baby drains your breast to get all of the available fat and to help your baby gain weight or sleep well
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Fat globules are sticky and adhere to the walls of the ducts, or block the ducts, or stick on the gland and duct walls until the breast empties when they are released into the milk
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It helps to shake up your breasts with your hands before breastfeeding to release the fats into your milk
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Fat and fatty acid composition changes as the breast fills.
None of this is true.
Your baby needs the fats in your milk (but your breasts look after this and you don’t have to do anything)
There is a lot of worry in our society about bodies and fat. But your milk contains healthy, protective fats, which provide about half of the calories your baby takes from your milk.
Fats can’t dissolve into milk or blood or tissue spaces. So the globules of fat in your milk have to be transported in their own special, edible, little carry-bag. This carry-bag is made from the lipid-rich cell membrane which surrounds your milk-making lactocytes.
How does this happen?
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When your milk glands don’t have much milk left in them, the lactocytes squash down snugly together and become plump and triangular. Their rounded, pointy end looks into the lumen. The part of the lactocyte facing into the lumen becomes elongated, even like a finger, and to my eye - looking at microscope images - rather frayed. Once the lumen empties and the lactocytes take on this shape, the gland is now in filling mode.
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Fat globules are made inside the lactocyte. Some fat globules join together inside the cell to make a bigger globule. The fat globules then migrate to the cell membrane at the rounded, pointy end of the lactocyte and lean into the lactocyte’s lipid-rich cell membrane.
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The lactocyte cell membrane wraps around the fat globules, and pinches them off into round globules of variable sizes, each wrapped in that membranous carry-bag. This is how fat globules set off on their independent adventures within your milk – though there is not that much milk in the lumen when this happens.
As the inside of the gland fills with milk, the lactocytes stretch out thinner and thinner, into flat rectangles. The more stretched and thinned the lactocytes are, the less their cell membrane can wrap around and pinch off fat globules. This is how the wall of the alveolus is protected from leaking or breaking. junctions, to prevent them leaking. Lactocytes absorb the backpressure. They absorb the the growing pressure of the milk.
The amount of fat in your milk is highly variable (but you don’t need to worry about this)
If milk is removed when the lumens are small, the milk is high in fat content. If the milk is removed as the lumens fill, the milk contains a lower concentration of fat. Fat concentration depends on the average volumes of milk in the lumens across the breast at any time the baby is removing the milk. So if your baby hasn’t fed for quite some time, there will be a lower proportion of fat in the milk that your baby next drinks from you. This, by the way, doesn’t mean that your milk becomes ‘weak’ or ‘low in calories’.
Milk production in the glands is variable across the breast, with some alveoli and some lobules fuller and others emptier at any given time. After longer breastfeeds, many of the glands are likely to be closer to empty, although the breast itself is never completely emptied since every little gland is constantly generating more milk! But it’s true that after a longer breastfeed, any milk your baby takes out will have a higher fat concentration.
However, the research also shows that no matter how frequently or infrequently a baby is fed (in babies who are gaining weight well from exclusive breastfeeding), the amount of fat a baby takes in over a 24 hour period is about the same. (Of course, if the baby isn’t feeding frequently, we can expect a woman’s supply to decrease over time.) Babies consume highly variable amounts of milk over a 24-hour period, and also highly variable amounts of milk each time they are at the breast, so the lipid content of milk is highly variable. Also, fat concentration in the morning is less than during the day and afternoon.
This variability of fat content in your milk is another reason why it’s so important to think about your baby’s breastfeeds as patterns over a day or two, without feeling under pressure to get a lot of milk in at any one feed.
Globules of fat are broken down into tiny droplets in your baby’s stomach by bile salts. This prepares the fat for digestion further down your baby’s gut.
If you have a preterm baby, hand expressing combined with pumping, and breast compressions during pumping, have been shown to result in a higher milk fat content than pumping. But there's no evidence to prove that this will improve baby's total cream or caloric intake from your milk, overall.
Recommended resources
Selected references
Daly SE, Di Rosso A, Owens RA, Hartmann PE. Degree of breast empting explains changes in the fat content, but not fatty acid composition, of human milk. Experimental Physiology. 1993;78:741-755.
George A, Gay MCL, Muhlhausler B, Wlodek M, Geddes DT. Human milk sampling protocols affect estimation of infant lipid intake. The Journal of Nutrition. 2020:doi:org/10.1093/jn/nxaa1246.
Kent JC, Mitoulas LR, Cregan MD, Ramsay DT, Doherty DA, Hartmann PE. Volume and frequency of breastfeedings and fat content of breast milk throughout the day. Pediatics. 2006;117(3):e387-e395.
Lee H, Padhi E, Hasegawa Y, Larke J. Compositional dynamics of the milk fat globule and its role in infant development. Frontiers in Pediatrics. 2018;6(313):doi:10.3389/fped.2018.00313.
Ramsay DT, Kent JC, Hartmann RA, Hartmann PE. Anatomy of the lactating human breast redefined with ultrasound imaging. Journal of Anatomy. 2005;206:525-534.