Lactose: your breastmilk and your baby depend upon it
It's not true that the first part of your milk is weak
Sometimes you might hear it said that the first part of your milk is sugary or weak or not strong enough. This isn’t true!
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You don’t need to be trying to get the creamy portion of milk into your baby.
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You also don't need to be worrying about 'emptying' the breast or always waiting until your newborn or baby falls off the breast himself - although sometimes you and your baby might enjoy this.
The amount of cream or fat your baby receives from breastfeeds looks after itself, if you offer the breast frequently and flexibly.
Lactose is a powerhouse of energy for your baby
Your baby does, however, need lots of lactose!
Most of the carbohydrate in your milk is lactose, which is essential for your baby’s body and brain development. Lactose is the mother lode of breastmilk nutrition.
Lactose is a molecule of glucose joined up with a molecule of galactose. A lot of glucose is required to make your milk, but glucose can't be made in your breast. It’s brought in by the bloodstream to the tiny capillaries which wrap around your milk glands.
For your baby, lactose is
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The main source of rapid-release energy (glucose) for your baby’s brain and body
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The main source of slow-release energy (glycogen, made in your baby's liver from the galactose portion of your milk's lactose)
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A vital influence on the gut microbiome.
Lactose has kept mammalian milk volumes stable for the past 250 million years
Lactose is the ingredient in your milk which ensures molecular stability and volume regulation. The concentration of lactose in your milk, once your milk is mature, is much the same as the concentration of lactose in your breastfeeding friend’s milk. From an evolutionary point of view, this lack of variability (this stability) indicates just how important lactose is for your milk's stability and for baby’s development!
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Lactose is not very soluble, and contributes 60-80% of the osmolality of your milk, drawing water in and stabilising your milk's volume
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Lactose concentrations regulate your milk's uptake of sodium and calcium.
Osmotic pressure determines whether molecules defuse out into the milk, which is what we want, or back into the blood plasma (which is what happens when your breasts are making colostrum in pregnancy or in the first few days after giving birth). Because of lactose, human milk has the same osmotic pressure as your blood plasma. By keeping your milk osmotically stable, lactose ensures that water content is stable in your milk and also in your baby’s body.
By joining up the two carbohydrate molecules of glucose and galactose, your milk has a much lower solute load than it would if both were being transported separately. This means that the lactose is less likely to draw too much water out of your own cells once it is formed and secreted into the alveolar lumen, and less likely to draw too much water from your baby into her gut.
To read about the concepts of osmotic pressure and osmotic defusion, please go to Wickipedia here.
What happens when lactose is swallowed into your baby’s gut?
Lactase is an enzyme created by the lining of your baby’s small intestine. Once your baby starts swallowing colostrum, there is a rapid increase in lactase activity. After about five days, your baby’s lactase is at 98% efficiency.
Lactase concentrations are highest in the first part of your baby’s small intestine and is in low concentrations towards the last part of your small intestine. Lactase breaks lactose down into glucose and galactose in your baby’s gut. Glucose and galactose then pass into your baby's gut's lining cells, and go on to pass into the blood stream capillaries which surround your baby's gut.
Your baby’s large intestine microbiome also adapts to digest any lactose that was missed in the small intestine. Any lactose that remains undigested, it is fermented by bacteria in the large intestine into beneficial short chain fatty acids. It’s also possible that lactose which makes it through to the large intestine is taken in by the cells lining the large intestine. Usually, lactose can’t be detected in your newborn’s stool, because it has all been broken down.
Digestion of lactose affects the osmotic properties of milk more than the breakdown of any other milk component. When lactose is digested, the solute load in the baby’s gut increases and this draws out water into the gut. A baby's ability to cope with increased solute loads is less than that of an adult due to immaturity of the baby's kidney.
What else might be interesting to know about lactose?
Lactose is
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A factor which helps shapes your baby’s gut microbiome
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The building block for the more complex human milk oligosaccharides in your milk
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A weak taste which doesn’t particularly stimulate the brain’s reward centres for sweetness
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The least likely of all the carbohydrates to promote dental caries
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Not affected by your diet.
Recommended resources
Possums Breastfeeding & Lactation Foundations
Is your baby unsettled or having trouble sleeping because of lactose?
Possums Breastfeeding & Lactation Advanced
What is the difference between lactose malabsorption, lactose intolerance, and lactose overload?
Lactose: mammary gland synthesis and secretion + digestion in the infant gut
Selected references
Forsgard RA. Lactose digestion in humans: intestinal lactase appears to be constitutive whereas the colonic microbiome is adaptable. American Journal of Clinical Nutrition. 2019;110:273-279.
Misselwitz B, Butter M, Verbeke K, Fox MR. Update on lactose malabsorption and intolerance: pathogenesis, diagnosis and clinical management. Gut. 2019;68:2080-2091.
Sadovnikova A, Garcia SC, Hovey RC. A comparative review of the extrinsic and intrinsic factors regulating lactose synthesis. Journal of Mammary Gland Biology and Neoplasia. 2021;26(197-215).
Sadovnikova A, Garcia SC, Hovey RC. Comparative review of the cell biology, biochemistry, and genetics of lactose synthesis. Journal of Mammary Gland Biology and Neoplasia. 2021;26(181-196).
Toca MdC, Fernandez A, Orsi M, Tabacco O, Vinderola G. Lactose intolerance: myths and facts. An update. Archivos Argentinos de Pediatria. 2022;120(1):59-66.