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Protein Transfer Factors

We have discussed the mechanism by which maternal dietary proteins transfer to human milk as well as the timeframe from ingestion to breast milk to clearance.  While the “how” is fairly well understood, the “why” is still largely a mystery.  If research shows that 50 out of 100 women given a serving of peanuts test positive for peanut protein in their expressed milk [1], what is the difference between those two groups of women?  Even within each group, women report readily transmitting dietary proteins with one baby and not another when both are found to have severe food intolerances.  Mothers find inconsistencies even day to day and meal to meal.  What gives? 

Many Theories

There are many things that could affect mom’s ability to transfer proteins from her digestive system to her mammary glands.  One thing we know for sure is that gastrointestinal permeability is largely genetic [2].  So while each of the below mentioned theories may be important factors to improving maternal GI health, it is critical that we acknowledge that most mothers may never get to the point of not transferring proteins in a form that elicit a response in their child.  Microbiome, hydration, medication, meal composition, protein concentration, stress, food intolerances, and much more.  Here, we will break down the research behind each theory. 

Maternal Food Allergies

Research indicates that food intolerance and allergies could create a perpetual cycle of increased intestinal permeability [3].  This figure, adapted from Perrier and Corthesy, outlines how an increase in allergen/antigen transmission through the gut causes an immune response which damages cell to cell junctions.  This damage, in addition to inflammatory mediators expressed by the body, allows for more allergen/antigen diffusion through the intestine…and around the cycle goes. 


Due to this, many lactation professionals consulting mothers of food reactive children may ask mom about her own health.  Allergenic and intolerance symptoms may be indicative of mom’s intestinal health and increased permeability issues.  The hope is to pinpoint the maternal culprit and therefore reduce the molecules she is transmitting to her mammary tissue.  In other words, your intestinal reaction to cow’s milk protein may contribute to many different ingested food fragments ending up in your breast milk.  Unfortunately, simply eliminating your culprit may not be enough, as research shows patients suffering from food reactions have innately increased permeability [4].


Newborns = Stress.  Lack of sleep, healing body parts, hormonal chaos, breastfeeding learning curves, and so much more contribute to our rising stress levels.  Do not even get me started on maternity and paternity leave (or lack thereof) which compounds this for so many families. 

In one study, researchers induced stress in volunteers via public speech or anticipation of electroshocks.  Compared to controls, those in the speech group saw increase in cortisol levels as well as intestinal permeability.  They determined that mast cells were vital to this response by pretreating another public speech group with an agent that prevents mast cell activation [5].  Essentially, increased stress could lead to higher concentrations of molecules making their way to your breast milk.  Medication that impacts this response could also be a factor.

Dehydration & Exercise

Exercise, particularly endurance exercise, without proper hydration causes reduced intestinal blood flow [6].  This circulatory redistribution changes nutrient absorption, motility, and mucosal integrity [7].  A combination of eating the trigger food then exercising while dehydrated is a recipe for protein transmission to your circulatory system and breast tissue. 

Maternal Environmental Allergies


One study compared nonhuman protein molecules in breast milk from 10 allergic mothers and 10 nonallergic mothers.  Allergic mothers were identified as having either high specific IgE against house dust mites, asthma, hay fever, or pet allergy.  This is important because asthma is associated with increased intestinal barrier permeability and house dust mites can often be found in the gut disrupting the intestinal epithelial barrier.  Researchers also recorded cow’s milk protein consumption in the study.

In the 20 samples, 78 non-human proteins were identified.  Notably, 62 of the 78 originated from cow’s milk protein.  Out of this data, scientists found 9 total proteins which were significantly different between the two groups.  Beta-lactoglobulin was found at significantly higher levels while bovine albumin was significantly lower levels in allergic mother’s breast milk [8].  This suggests there is a difference between molecular transmission through maternal intestinal barrier, but unfortunately does not pinpoint the difference. 


Protease Inhibitors


A very similarly structured study to the one described above found that allergic mothers expressed higher levels of protease inhibitors (aka: molecules that stop the breakdown of protein) [9].  These inhibitors would end up in baby’s digestive system, possibly slowing down the much-needed breakdown of proteins like beta-lactoglobulin in the gut.

Debunking Myths


Since there are so many possible contributors, let’s discuss what is not.  A recent study found no correlation between percent body fat and intestinal barrier dysfunction [10].  Your weight, BMI, body fat percentage, or other similar metrics show no impact on whether you transmit proteins while lactating. 



Why do some ingested proteins enter human milk while others do not?  What is different about mothers who seemingly never transfer their dietary molecules to breast tissue?  Our team is actively working on better understanding these complex questions.  There are so many biological mysteries when it comes to this system.  Luckily, the above foundational work can give us direction to focus our efforts in ways that will best impact breastfeeding mothers and infants.  Specific actions which are already highly beneficial for mom such as meditation to relieve stress, proper hydration, and reactive food avoidance can also reduce intestinal permeability while understanding that how we transfer proteins is largely genetic and therefore out of our control. 



  1. Vadas, Peter, et al. "Detection of peanut allergens in breast milk of lactating women." Jama 285.13 (2001): 1746-1748.

  2. Michielan, Andrea, and Renata D’Incà. "Intestinal permeability in inflammatory bowel disease: pathogenesis, clinical evaluation, and therapy of leaky gut." Mediators of inflammation 2015 (2015).

  3. Perrier, C., and B. Corthesy. "Gut permeability and food allergies." Clinical & Experimental Allergy 41.1 (2011): 20-28.

  4. Ventura, M. T., et al. "Intestinal permeability in patients with adverse reactions to food." Digestive and liver disease 38.10 (2006): 732-736.

  5. Vanuytsel, Tim, et al. "Psychological stress and corticotropin-releasing hormone increase intestinal permeability in humans by a mast cell-dependent mechanism." Gut 63.8 (2014): 1293-1299.

  6. Kim, Kwi-Baek, and Yi-Sub Kwak. "Dehydration affects exercise-induced asthma and anaphylaxis." Journal of exercise rehabilitation 15.5 (2019): 647.

  7. Lambert, G. Patrick. "Gastrointestinal and metabolic responses to body fluid imbalance during exercise." Fluid Balance, Hydration, and Athletic Performance 1 (2016): 97-108.

  8. Dekker, Pieter M., et al. "Maternal allergy and the presence of nonhuman proteinaceous molecules in human milk." Nutrients 12.4 (2020): 1169.

  9. Hettinga, Kasper A., et al. "Difference in the breast milk proteome between allergic and non-allergic mothers." PloS one 10.3 (2015).

  10. Lambert, G. Patrick, et al. "Effect of Percent Body Fat on Intestinal Permeability." Journal of Exercise Physiology Online 22.5 (2019).

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