Calories and Hind Milk. Exposing the Myth with the Numbers

Most people are familiar with the fact that in the very first days after birth small
A few months ago I wrote a blog on Foremilk and Hindmilk.  That blog was inspired by the fact that I was frequently visiting mums of low weight gain babies who had been advised to keep baby on one side for various lengths of time in order to get the "hind milk".  There was a big focus on the hind milk being fatty and therefore important for weight gain.  I wrote the Foremilk and Hindmilk blog in order to explain how this was not actually helpful, and in fact was generally detrimental for those low weight gain babies.  I explained how our bodies do not make 2 types of milk, nor is there any mechanism to switch to some different kind of milk at some arbitrary time into the feed and how milk gets squeezed down the ducts during the muscular contractions of a letdown.  If you haven't read that blog, please have a read through it now, as this blog is devised to be an addition or part 2 to that one.

The hindmilk=weight gain myth is a very pervasive one, and I am certainly hearing of mums on a weekly basis being advised to get the hind milk to boost calories and weight gain.  Often really judgmental terms are used like "the good milk", as if the milk in the first part of the feed is somehow not good!  I want to completely expose this myth, by looking at the composition of a feed, by looking at fat variability in milk and by looking at the calories available in milk carbohydrate, protein and fat

Fat Content

So, lets start by considering how much fat we have in human milk.  If you google, or look up a breastfeeding textbook you will probably find the figure of 3-4%, but in fact fat content is extremely variable.  It's variable depending on the time of day (lower in the morning and higher in the evening), but also extremely variable from mother to mother and depending on age of baby.  Fat content can range from 22g/L to 62g/L depending on the mother (Kent et al 2006).  To give those figures some meaning lets look at 1oz or 30ml of milk.  One mother may have 0.66g of fat in that 1 oz of milk, but another mother may have 1.86g of fat in that milk.  If we convert that to calories the mother with the lower fat content provides around 6 calories from milk fat in that 1 oz of milk.  The mother with the higher fat content provides around around 17 calories from milk fat in that 1 oz of milk.  That's a massive difference.  A few years ago I remember watching a presentation of a study looking at milk fat difference over the course of the day.  It was quite astonishing.  From memory it showed that for some women fat content might range from 3% at their lowest concentration to perhaps 7% at their highest.  Another woman in contrast might have 9% at her lowest concentration and 20% at their highest.  So one women will have more fat at their lowest concentration time of day than another at their highest. 
I can't find the study and wish I could so if anyone is aware of it, please do let me know. 
Fat also varies by sex, with boy babies getting fattier milk than girl babies.  Milk is individual to each woman and baby and that is why each finds their own feeding and switching sides rhythm.  In fact a 2002 study (Mitoulas 2002) looking at fat content in breastmilk and weight gain found that the growth rate of children in the study was not related to the percentage of fat in the milk - it was related to the overall 24 hr milk volume, not the individual composition of the milk.

The breakdown of a milk ejection

In the previous blog I discussed how a letdown or milk ejection happens within the first 2 minutes of a breastfeed.  This ejection is triggered by a release of oxytocin in response to the baby stimulating nerves around the areola.  We know from studies that on average the amount of milk which is ejected in a letdown is approximately 1oz / 30ml and we know that milk ejection (or letdown) is vital to the transfer of milk.  It is ONLY during a letdown /milk ejection that a significant amount of milk is transferred to the baby.  Research also suggests that between letdowns a baby may get very small amounts of milk - perhaps only 10ml. (Ramsay et al 2004, Ramsey et al 2006).  This means that the letdown is key.  A baby attached to the breast who never triggers a letdown, or who stays on for a long time never getting a second letdown is likely getting very minimal amounts of milk.

So, lets look at how a letdown or milk ejection works.  For the purposes of simplicity, lets think of the breast like a cluster of grapes or berries on a plant (much like the picture here).  The grapes are like the alveoli in our breasts.  The alveoli are little balloon like sacs which make and then contain the milk until it is removed by breastfeeding or expressing.  Coming out of each alveoli is a little ductule tube (like the stalk on the grape) and these small ductules join together into larger ducts (like the grape stalks join into a larger main stalk) and these larger ducts lead to the nipple.  Around each of the alveoli /grapes in the breast we have a tiny network of muscle cells latticed over it - almost like a spider web over each grape, and these muscle cells are key to the letdown. 

When a baby is at the breast the suckling causes us to release a surge of oxytocin.  Most people who are familiar with pregnancy associate oxytocin with the muscular contractions in the uterus, but oxytocin does the same thing with the muscle cells around the milk alveoli.  It causes them to contract and push the milk out of the alveoli into the duct.  In addition to this the ducts dilate (or get wider) in order to  facilitate a fast flow of milk.  It is these changes that women normally feel when they feel warmth or tingling during a letdown.  It is also this surge of oxytocin which causes those afterpains when breastfeeding as the uterus is contracting back down after birth.  A milk ejection lasts a very short time, often just a couple of minutes and then it's over.  So the baby gets a good quantity of milk during this 2 minutes and then is back to very little milk transfer.

Up to 45% of the milk available in the breast is released during the first milk let-down (Ramsay et al 2006).  If we include the time it takes to trigger the letdown and the time that the contractions are happening, this generally will take less than 5 minutes.  So take that in - up to 45% of the milk available is likely released to the baby in the first 5 mins.  As letdown/milk ejection is hormonal it affects both breasts at the same time.  This means that the breast the baby is not latched to also has milk squeezed down the ducts.  Many women will notice leaking in this other breast, and this is also how passive collection bottles that attach to the other breast work to collect milk - the little bit of vacuum they create works in combination with the letdown/milk ejection pushing milk to the nipple.  Even when leaking doesn't happen the milk will still have been squeezed down the ducts and be sitting close to the nipple area ready for feeding.  Over time, if this milk is not removed it will migrate back up the ducts into the alveoli.  This is a crucial point when we talk about why switch nursing works to increase milk intake!

Once breastfeeding is established and a baby is drinking a full supply of milk (around 6 weeks), on average a woman will have 3-4 milk ejections at each feed.  If we think that a full supply is around 800ml of breastmilk, and a baby will feed approximately 8-12 times a day we can see that the numbers fit very nicely together.  If we assume that a milk ejection is around 30ml and it happens 3 times in a feed  (90ml), then over 8 feeds the baby would get 720ml.  These are obviously approximations and women can have different milk production, different capacity for milk storage and different patterns of milk ejection but it helps to illustrate the point about the importance of the milk ejections.

We only get a milk ejection in alveoli which are full.  Alveoli which are full, will eject their milk when we get that oxytocin burst.  Think of a full squeezy bottle, and how a little bit of pressure causes the contents to squirt out.  Now think of that squueezy bottle only a third full and how that little bit of pressure no longer has much of an effect.  We need to squeeze much much harder to have the same ejection effect.  Similarly in the breast, we don't get a milk ejection in partially full alveoli. 

Low Milk Supply & Milk Ejections

That takes us on to the issue of low milk supply, milk ejections and swapping sides.  Usually I find that this advice about staying on one side is given to mums around 2-3 weeks if their baby is not back to birthweight.  The fact that baby is not back to birthweight at that stage tells us that something is already slightly slow about the amount of milk the baby is drinking, and if a baby isn't drinking enough or stimulating the breasts well, then milk production doesn't rise sufficiently.  We can assume that these mums need a little help in getting more milk production underway.  At 2-3 weeks a baby is, on average, drinking around 60-90ml per feed  (600-750ml per day) in order to gain appropriately.  Otherwise healthy babies who are not at birthweight at this age are not drinking this amount of milk and that is why the gain is slower than average.

What I typically see in practice is that those babies are going to the breast, perhaps taking a little longer than average to trigger a letdown, the letdown is shorter than average, and a lot longer between letdowns.  This is simply a function of how much milk is in the breast (how many alveoli are full of milk) and how quickly milk is being created.  The baby simply falls asleep after the first letdown, or stays attached and sucks on and off but never gets a 2nd letdown.  If they fall asleep and unlatch they typically wake a very short time later -maybe 10-15 mins later and want to relatch as they are still hungry - but if put back to the same breast they don't trigger a letdown and suck for a short time before falling asleep again.  These babies have a constant cycle of feeding, but never seeming totally satisfied.  They may be feeding for what seems like all day.  Even more worrying are the babies that fall asleep and are very hard to rouse for another feed.

So does sticking to one side help or hinder calorie intake in these babies?  Well, again, it's all about the letdowns.  Let's take 2 scenarios  - the 1st is where mum sticks to one side to get the hind milk.  The 2nd is where we switch nurse - as I've suggested on this blog on low weight gain.  For the purposes of the scenarios we'll assume that in both cases the amount of milk the baby drinks before the 1st letdown is 10ml, and that in each letdown the baby gets 30ml.  We'll assume that between letdowns the baby transfers only 10ml.  We'll work with the average fat content of 4% in the first letdown on either breast, and we'll assume a fat content of 6% on the 2nd letdown on either breast.  We'll assume that 10ml "foremilk" before the first letdown has a fat content of 3%.  Yes these numbers are a bit contrived and it wouldn't work like this in practice where fat would gradually increase through the feed, but it will allow us to illustrate a point. 

SCENARIO 1 - Staying on 1 Side
10mls of "foremilk" at 3% = 0.3g of milk fat = 2.7 calories
30mls from 1st MER at 4% = 1.2g of milk fat = 10.8 calories
10mls between MER at 5% = 0.5g of milk fat = 4.5 calories
Baby then falls asleep during this period of drinking (so may in fact not get 10mls but we'll work with that figure)
Total calories from milk fat here - 18 calories
Total quantity of milk = 50ml
7% carbohydrate = 3.5g = 14 kcal
1% protein =0.5g = 2kcal
TOTAL calories = 34 kcal


SCENARIO 2 - Switching sides once
10 mls of "foremilk" at 3% = 0.3g of milk fat = 2.7 calories
30mls from 1st MER at 4% = 1.2g of milk fat = 10.8 calories
Mum swaps sides.  Milk is already near the ducts and so baby starts drinking straight away
30mls from 1st MER (2nd side) at 4% = 1.2g of milk fat = 10.8 calories
Total calories from milk fat - 24.3 calories
Total quantity of milk = 70ml
7% carbohydrate = 4.9g = 19.6 kcal
1% protein = 0.7g = 2.8kcal
TOTAL calories = 46.7kcal


So swapping sides just once has provided over extra 30% of milk volume and around the same percentage in calories.

In the previous blog I actually suggest swapping a 2nd time to try to get a 3rd letdown.  This is because swapping once the flow slows on that first breast often keeps the baby awake because he/she gets instant flow on the other side.  Young babies really respond to the flow of milk.  If milk is flowing they drink if they are hungry.  If the milk slows down they fall asleep.  After 2 letdowns a baby may be full and go to sleep full and contented, sleeping for a full sleep cycle rather than a few minutes, or if not yet full they have the energy reserves to work for a short time to trigger that 3rd letdown - which will now be much higher in fat.  A young baby may not want the 3rd letdown or may take a small amount and come off full, but just to illustrate let's look at how that 3rd letdown increases the figures in addition to the 46.7kcal above.

Mum swaps sides for the 2nd time
10mls between MER at 5% = 0.5g of milk fat = 4.5 calories
30mls from 2nd MER at 6% = 1.8g of milk fat = 16.2 calories
Total calories from  milk fat now - 45
Total quantity of milk = 110ml
7% carbohydrate = 7.7g = 30.8 kcal
1% protein = 1.1g = 4.4 kcal
TOTAL calories = 80.2 kcal

As this demonstrates - yes, fat content increases as the feed goes on.  Yes, if a baby drinks more from one side they will get more calories BUT a low gaining baby doesn't actually do this.  A low gaining baby falls asleep after the first letdown so simply doesn't get these extra calories.  Babies will stay latched when sleeping.  They will suck lightly and may look like they are feeding, but they are not because they are not swallowing, or at least not in any volume.  Unless they trigger a letdown they just aren't getting much volume of milk.
A mum's milk supply is based entirely on how much milk is removed so the more letdowns a baby drinks, the more the mum's milk supply is stimulated and the more milk she produces.  This is why switch nursing moves a low gaining baby very quickly from that low gain pattern into a normal gain pattern, and once they are gaining appropriately (so we know mum's milk supply is now ok) she can stop actively managing the switch nursing and instead allow the baby to determine when it's time to stop. 

I hope this has made sense.  I don't think I can make it more clear than this  - the idea of staying on one side longer to get the more calorie rich milk simply isn't appropriate for a baby who isn't gaining weight well.  It will work for a baby who is already gaining appropriately and a mum with a good supply (although the baby may complain as he/she wants faster flowing milk), but it won't work for a baby who is already having difficulties with getting enough milk.  In practice it doesn't help the weight gain and creates a never ending cycle of feeding, which frankly isn't sustainable for either.

Please let's stop telling mums to stick to one side to increase weight gain. 

Further Reading

1.  Kent et al 2006 Volume and frequency of breastfeeds and fat content of breastmilk throughout the day PEDIATRICS 117(3):e387-95 https://www.researchgate.net/publication/7266502_Volume_and_frequency_of_breastfeeds_and_fat_content_of_breastmilk_throughout_the_day

3.  https://www.scientificamerican.com/article/boys-and-girls-may-get-different-breast-milk/

4.  Ramsay et al 2004 Ultrasound imaging of milk ejection in the breast of lactating women.  Pediatrics. 2004 Feb;113(2):361-7.https://www.ncbi.nlm.nih.gov/pubmed/14754950