The Hygiene Hypothesis / Microbial-Deprivation Hypothesis,

as Related to Breastfeeding

As is often pointed out, breastfeeding transmits immune cells/antibodies from the mother to the baby, which help protect the infant from microbes.  But there is an important reverse side to that coin, related to the major recent reduction in stimuli of the kind needed for a child’s own immune system to develop properly.  It is well established, in the underlying principle of most vaccinations, that the immune system develops its own capacities in response to challenges.  It has recently been learned that some microbial challenges need to occur very soon after birth in order to properly promote development of the child’s immune system (see below).  As explained in detail in Section 1.2.p.2  of and in, there is considerable evidence indicating worse long-term health outcomes among people who have had the most exposure to breastfeeding.


When Homo Sapiens was evolving, sanitation did not exist, and our forebears until late in the 19th Century knew nothing about protecting infants from bacteria.  For well over 99% of our evolutionary history, before modern sanitation, clean water, pasteurized milk, food inspection, vaccinations, and refrigeration came into existence, transmission of immune cells via breastfeeding promoted survival and increase of our species.   There can be no doubt that most infants’ exposures to microbes in developed countries have already been reduced to extremely low levels in relation to what our species became adapted to over almost all of our evolutionary history.  Given what has been observed about harm that may result from too little exposure to microbes (see below for specific evidence), the time has come to re-consider the desirability of a kind of infant feeding that further combats microbes. 


Of great relevance here is the “hygiene hypothesis” described in a web page of the U.S. Food and Drug Administration, according to which proper infant development depends on the necessary exposure to germs required to “educate” the immune system so it can learn to launch its defense responses to infectious organisms….   In the period immediately after birth the child’s own immune system must take over and learn how to fend for itself.”(1) (emphasis added)   A study published in mid-2014 found that “reduced exposure to bacterial richness” in infancy was associated with increases in childhood allergies; and it helped reaffirm that, with regard to the apparent importance of bacterial exposure to proper immune development, “the first few months of life is a critical time period.”(2)  The reader should try to figure out where activity of microbe-destroying cells from outside the infant’s body, via breast milk beginning right after birth, fits into this picture of necessary exposure of the infant to germs right after birth.   The FDA continues, “The “hygiene hypothesis” is supported by epidemiologic studies….(1)   A study found on the NIH’s website discusses “the microbial exposure which may be critical for immune priming” and suggests it would be helpful to re-name the “hygiene hypothesis” as “microbial deprivation hypothesis.”(3)   "Just as a baby's brain needs stimulation, input, and interaction to develop normally, the young immune system is strengthened by exposure to everyday germs so that it can learn, adapt, and regulate itself," according to the director of the Laboratory for Human Biology Research at Northwestern University.  A recent study found that children who were exposed to more animal feces and had more cases of diarrhea before age 2 had less incidence of inflammation in the body as they grew into adulthood. (Inflammation has been linked to many chronic adulthood illnesses, such as heart disease, diabetes, and Alzheimer's.)(4)  A 2009 study “suggests that exposure to infectious microbes early in life may actually protect individuals from cardiovascular diseases that can lead to death as an adult.”(5)  Another study (in the website of the National Center for Biotechnology Information, U.S. National Library of Medicine, NIH) also refers to studies that help verify the long-term beneficial effects of early exposure to microorganisms; specifically applicable studies found apparent benefits of infant exposure to microbes on farms (with animals) and benefits to younger children of exposure to microbes from older siblings.  A key question with regard to the hygiene hypothesis, which remains to be addressed, is how big the critical microbial exposure needs to be…..(6)  The question that remains open appears to be not whether development of the immune system depends on exposure to microbes, but "Does further reduction in exposure to microbes (below the already historically low levels) by means of externally-provided immune cells do anything other than harm an infant’s long-term health?"


Various studies have recently added to the evidence along the above lines.  "These studies show the critical importance of proper immune conditioning by microbes during the earliest periods of life" (emphasis added), according to Richard Blumberg, MD, chief of the Brigham and Women’s Hospital’s Division of Gastroenterology, Hepatology and Endoscopy.(7)    It appears that the hygiene hypothesis (or microbial deprivation hypothesis) is advancing past the stage of being merely supported by epidemiological evidence, if a highly-placed medical official flatly declares that early conditioning by microbes is of “critical importance.”  By implication, the apparently logical next step is to look with serious skepticism at the notion that it is beneficial in developed countries to recommend infant feeding that contains externally-created microbe-destroying components; that is especially the case if that infant feeding is recognized by the EPA to include very hazardous concentrations of developmental toxins and carcinogens, in many-times higher amounts than alternative feedings.  


In many ways, living organisms become healthier as a result of being subjected to challenges.  Muscles of all kinds are only the most conspicuous of many verifications of this principle.  Lung capacity is known to increase among people whose pulmonary systems are challenged through exercise.  Bones become stronger in response to mechanical stress.(8)   The circulatory system functions better in response to strenuous challenges.  It is well established that challenges to the brain strengthen mental powers, most notably in older people but also in children.  Immunities of all kinds are produced by vaccinations that subject the immune system to moderate challenges, thereby activating the person's own immune system, not by pumping in externally-sourced immune cells and alleviating the need for a person's own immune system to develop.  


In addition to the apparent importance of not shielding people in general from challenges, there is good reason to actually encourage the exposure of children to microbial challenges.  In earlier generations, measles and mumps were normal childhood illnesses, illnesses that could be described as temporary discomforts for the child, which were important to go through as a child because that exposure built immunity against what would be a serious disease if incurred during adulthood.  A child's body is apparently better able to tolerate at least some, if not most, illnesses than an adult's body.  And childhood is probably a better time to have those illnesses than adulthood, considering the relative costs of time spent in an incapacitated state, and especially considering the sometimes very serious consequences of incurring that disease as an adult.  A mature reproductive system as well as actual reproduction are aspects of adulthood that are very much subject to serious harm by certain diseases (e.g. German measles); development of immunity to those diseases as a result of going through them while still a child (as opposed to being shielded from them at that stage) is clearly beneficial.  Before the days of the MMR vaccination, no knowledgeable person would have recommended that children be protected from mumps and measles viruses.  Yet that kind of thing is being done every day when infants are breastfed and thereby are shielded from microbes by an external substance that essentially prevents the child's own immune system from being stimulated to develop.


Obviously, excessive bacterial exposure is bad, just as excessive amounts of many ordinary things (dietary fat, iron, Vitamin A, salt, sunshine, stress, etc.) are bad.  But too little of each of these exposures is also unhealthy.  Warding off most of the remaining relatively small amount of microbial exposure in today's developed countries, by means of externally-provided immune cells, is equivalent to always carrying a toddler wherever he wants to go, or providing him with a motorized cart as soon as he can operate it; development of his own capacities will suffer.


Some real-world verification of the importance of avoiding excessive shielding of an infant from microbes can be seen in the facts that

(a) compared with people over age 15 in higher-breastfeeding countries, 8% more over-age-15 residents in high-breastfeeding European countries report a long-standing illness or health problem than do those in low-breastfeeding European countries (Section 1.2.p.2  of; and

(b) childhood cancer and many other often-fatal childhood diseases are 23% to 5000% more prevalent in Europe's highest-breastfeeding countries than in the low-breastfeeding countries (above linked location and also; and

(c) other serious disorders including asthma and allergies, although reduced by breastfeeding in early childhood, become much higher among the more-breastfed after the years of early childhood. (

To read summaries of scores of peer-reviewed scientific studies that have found higher rates of various diseases among breastfed than among bottle-fed children, see  

The above may seem hard to believe, considering how popularly-held the idea is that breastfeeding is beneficial.  But the above points were presented to the U.S. Surgeon General, to high officials in the U.S. Department of Health and Human Services (including the CDC), and to high officials of the American Academy of Pediatrics in letters in May, June, September and mid-October of 2012, pointing out that this content is being widely publicized (in opposition to their promotion of breastfeeding) but is open to criticism; and the only responses received as of the following May said absolutely nothing to contradict these points.  Moreover, anybody can go to the sources indicated for anything presented in this paper and see that the contents presented here are accurately drawn from major, trustworthy sources.


To summarize:  It is entirely open to question whether, considering lifetime health, there is validity to the idea that infants in developed countries benefit by receiving maternal immune cells in breast milk.   There are excellent grounds to believe that, in the hygienic modern conditions in developed countries, and given the typically-high concentrations of dioxins, PCBs, PBDEs, and other toxins in breast milk in those countries, the child's development is likely to be harmed by breastfeeding.  The reader should remember from above (with more detail in and in Section 1.2.p.2  of the considerable data indicating worse long-term outcomes among those who are most likely to have been breastfed.



(2)  S.V. Lynch et al., Effects of early-life exposure to allergens and bacteria on recurrent wheeze and atopy in urban childrenJournal of Allergy and Clinical Immunology, June, 2014  at     Published Online: June 04, 2014

(3)  Clin Exp Allergy. 2006 April; 36(4): 402–425.  Blackwell Publishing Ltd  Too clean, or not too clean: the Hygiene Hypothesis and home hygiene  SF Bloomfield et al.

(4) (


(6)  also Development and Maturation of the Immune System: Vulnerability to Toxicants   at, p. 8   

(7) Cell Research advance online publication 24 April 2012; doi: 10.1038/cr.2012.65  Early exposure to germs and the Hygiene Hypothesis  Dale T Umetsu  Division of Immunology, Karp Laboratories, Children's Hospital Boston, Harvard Medical School, Boston, MA

(8)  (Bone Strength: Current Concepts  Charles H. Turner  Orthopaedic Research Laboratories and Biomechanics and Biomaterials  Research Center, Indiana University Purdue University at (






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