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Breast Milk



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What Goes In, May Come Out

What goes into your mouth may come out in your milk.  This is because breast milk is made from blood.  The water, nutrients, and chemicals that are found in breast milk originated in the blood stream.  For that reason, anything that is floating around in your circulatory system may then be passed on to your infant.  While that means essential vitamins and minerals from your diet are going to your baby, it also means that any contaminants could as well.  Let’s dig into the science and implications behind this intricate biological function.



Breast milk is up to 90% water!  That water comes directly from the mother’s blood stream through capillaries in the breast.  This is why hydration is one of the main factors in milk supply.  Without proper water intake(dehydration), there will not be enough to continuously produce more milk.  Ensure you are drinking enough water each day (at least half your body weight in ounces) and beware of medications that list dehydration as a side effect.




Many scientific articles have shown that proteins ingested by the mother can enter breast milk completely unchanged [1].  In other words, the exact almond protein in its full form that you ate is what your child may then receive. This was an interesting discovery, since prior to these studies it was believed that the process of digestion and absorption would always alter the protein structure.  The majority of human milk proteins are created by mammary cells, transported through the secretory pathway, packaged in vesicles, and released by what is known as exocytotic fusion [2]. Additional proteins are transported to the mammary gland by transcytosis from blood plasma.  Many nonhuman proteins transferred through this mechanism have been detected via immunoassays, including those originating from peanuts, wheat, eggs, cow’s milk, and more [3]!


Having the same or similar protein structure means that infants with food sensitivities are likely to react to certain proteins passed on through the mother’s diet.  Some proteins, such as dairy and soy, can be difficult for the infant’s digestive system to breakdown [4].  The inability to fully digest breast milk can cause a plethora of symptoms.  Other proteins, such as peanuts, may lead to an allergic immune response in the form of rash, bloody stool, anaphylaxis, and more [5].




“Consult your physician before taking this medication if you are pregnant or breastfeeding” is a common statement to find on medication and supplements.  This is because research has found that chemicals will cross the blood-breast barrier similar to that of the blood-brain barrier [6].  The concentration in which this occurs is only significant in select medications.  That is because the dosage decreases with each stage from mother to infant.  Typically, the chemical is absorbed into the mother’s bloodstream, crosses into her breast milk, becomes orally available to the infant, is absorbed by the infant’s digestive system, and lastly enters their bloodstream at a fraction of the original dosage.  Many medications do not even make it completely through this process to reach the infant’s bloodstream [7]. 


While this is the case for a majority of medication, it is not true for all.  Chemotherapy agents can be especially harmful if passed through the mother’s milk.  Discuss any prescription and over the counter medications with your Pharmacist.  Alternatively, a quick search can give you a list of medication which can be harmful to either your supply or your infant. 




This is a point of contention that causes a lot of unnecessary mom-shaming.  Dr. Jack Newman states, “Reasonable alcohol intake should not be discouraged at all. As is the case with most drugs, very little alcohol comes out in the milk. The mother can take some alcohol and continue breastfeeding as she normally does. Prohibiting alcohol is another way we make life unnecessarily restrictive for nursing mothers” [8].  It has been shown that less than 2% of the alcohol consumed by the mother will reach her milk [9].


Alcohol enters and leaves breast milk in the same fashion that it does the bloodstream.  This means that you should NOT “Pump and Dump”.  The alcohol in breast milk is not “trapped” there in need of being removed, it will simply dissipate with time as the metabolism does its job [9]. 



  • Stay hydrated

  • Examine protein sources if your infant shows signs of food allergies

  • Look up medications prior to use

  • Only a small percentage of alcohol can transfer to breast milk and will dissipate with time

  • Do not pump and dump.  Anything that is in your milk can and will be removed naturally by cellular metabolism



  1. Kilshaw PJ, Cant AJ. The passage of maternal dietary proteins into human breast milk. International Archives of Allergy and Immunology. 1984;75(1):8-15.

  2. Franke, W. W., Lüder, M. R., Kartenbeck, J., Zerban, H., & Keenan, T. W. (1976). Involvement of vesicle coat material in casein secretion and surface regeneration. Journal of Cell Biology.

  3. Ollivier-Bousquet, M. (1998). Transferrin and prolactin transcytosis in the lactating mammary epithelial cell. Journal of Mammary Gland Biology and Neoplasia.

  4. Bock SA. The natural history of food sensitivity. Journal of Allergy and Clinical Immunology. 1982 Feb 1;69(2):173-7.

  5. Des Roches A, Paradis L, Singer S, Seidman E. An allergic reaction to peanut in an exclusively breastfed infant. Allergy. 2005 Feb;60(2):266-7.

  6. Sachs HC. The transfer of drugs and therapeutics into human breast milk: an update on selected topics. Pediatrics. 2013 Aug 1:peds-2013.

  7. Ito S, Lee A. Drug excretion into breast milk—overview. Advanced drug delivery reviews. 2003 Apr 29;55(5):617-27.

  8. Newman J, Pitman T. Dr. Jack Newman's guide to breastfeeding. HarperCollins Publishers; 2000 Nov.

  9. Haastrup MB, Pottegård A, Damkier P. Alcohol and breastfeeding. Basic & clinical pharmacology & toxicology. 2014 Feb;114(2):168-73.

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