Taste of Human Milk
“. . . too full o’ th’ milk of human kindness to catch the nearest way.”
Shakespeare’s Macbeth, Act I, Scene V
This line from Shakespeare reflects the centuries-old belief that a breastfeeding woman’s diet influences the composi-tion of her milk and has a long-lasting influence on the child. The flavor of human milk is an important taste ex-perience for newborn infants, but flavor of human milk is often ignored when the benefits of human milk or its composition is considered. Human milk is slightly sweet61 and it carries the flavors of compounds ingested, such as mint, garlic, vanilla, and alcohol.62
The transfer of flavor compounds appears to occur se-lectively and in relatively low amounts.63 Infant responses to flavors in milk seem to depend on the length of time since the mother consumed the food, and the amount and frequency of the flavor that the mother consumed (new versus repeated exposure). Infants seem more interested in their mother’s milk when flavors are new to them. Re-searchers found that infants nursed at the breast longer if a flavor (garlic) was new to them than if the mother had taken garlic tablets for several days.64 Infants who were exposed to carrot juice flavor in their mother’s milk ate less of a carrot-flavored cereal and spent less time feed-ing at the breast than infants who had not been exposed to the carrot flavor. Thus, exposing infants to a variety of flavors in human milk may contribute to their interest in and consumption of human milk as well as their accept-ance of new flavors in solid foods.65
(e.g., complement, bifidus factor), carrier proteins (lactoferrin, transferring, vitamin B12-binding pro-tein, and corticoid-binding protein), and enzymes (lys-ozyme, lipoprotein lipase, leukocyte enzymes) are com-ponents of milk that confer immunological benefits.
Cellular components in human milk (macro-phages, neutrophils, T- and B-lymphocytes, and epi-thelial cells) are especially high in colostrum but are also present for months in mature human milk in lower concentrations. The function of macrophages in human milk includes phagocytosis of fungi and bacteria, killing of bacteria, and production of the com-plement proteins, lysozyme, and lactoferrin and immu-noglobin A and G.13
Leukocyte function appears to offer more protection to the breast than to immunocompetence of the infant.
Neutrophils, however, appear to be activated and contrib-ute to phagocytosis at the mucosa of the infant’s gastroin-testinal tract.73 Both T- and B-lymphocytes provide the infant with protection against organisms in the gastroin-testinal tract. This protection may extend beyond acute infection to allergy, necrotizing enterocolitis, tuberculosis, and neonatal meningitis.13
Immunoglobins are thought to be transported from maternal plasma across secretory epithelium to create secretory immunoglobins.74 The predominant (90%) im-munoglobulin in human milk, secretory immunoglobin A(sIgA), also appears to be most important in terms of the protection conferred to the infant. sIgA and sIgM protect the infant by blocking colonization with pathogens and limiting the number of antigens that cross the mucosal bar-rier. sIgA protects against enteroviruses, cytomegalovirus, herpes simplex virus, respiratory syncytial virus, rubella, retrovirus, and rotavirus,13 and sIgM protects against cy-tomegalovirus, respiratory syncytial virus, and rubella.
Bifidus factor is a growth factor (probably a carbo-hydrate) that supports growth of Lactobacillus bifidus.
Lactobacillus is a probiotic bacterium that stimulates antibody production and enhances phagocytosis of antigens.13
Lysozyme protects against enterobacteria and other gram-positive bacteria. Lysozyme is secreted by neu-trophils and macrophages.
benefits, women who nurse at a younger age and for longer duration have lower risk of breast and ovarian cancer69-70 and rheumatoid arthritis.71
Breastfeeding Benefits for Infants
“Breastfeeding—the main source of active and passive immunity in the vulnerable early months and years of life—
is considered to be the most effective preventive means of reducing the death rate of children under five.”72
Lubbock, M. H., Clark, D., and Goldman, A. S. Breastfeeding:
maintaining an irreplaceable immunological resource.
Nature Reviews Immunology. 2004; 4(Jul):565–572.
Nutritional Benefits The value of the composition of human milk is widely recognized. Companies that make HMS often use human milk as the standard, recognizing the many unique properties of human milk:
With its dynamic composition and the appropriate
●
balance of nutrients, human milk provides optimal nutrition to the infant.13,17
The balance of nutrients in human milk matches
●
human infant requirements for growth and de-velopment closely; no other animal milk or HMS meets infant needs as well.
Human milk is isosmotic (of similar ion
concentra-●
tion; in this case human milk and plasma are of similar ion concentration) and therefore meets the requirements for infants without other forms of food or water.
The relatively low protein content of breast milk
●
compared to cow’s milk meets the infant’s needs with-out overloading the immature kidneys with nitrogen.
Whey protein in human milk forms a soft, easily
●
digestible curd.
Human milk provides generous amounts of lipids
●
in the form of essential fatty acids, saturated fatty acids, medium-chain triglycerides, and cholesterol.
Long-chain polyunsaturated fatty acids, especially
●
docosahexaenoic acid (DHA), which promotes op-timal development of the central nervous system, are present in human milk and are present in only some of the HMS marketed in the United States.
Minerals in breast milk are largely protein-bound
●
and balanced to enhance their availability and meet infant needs with minimal demand on maternal reserves.
Immunological Benefits One of the most impor-tant realizations about breastfeeding in the last few decades is the ability of human milk to protect against infections. Cells (T- and B-lymphocytes), secretory im-munoglobins (sIgA, sIgG, sIgM, sIgE, sIgD), histocompat-ibility antigens, T-cell products, many nonspecific factors
Immunoglobin A specific protein that is produced by blood cells to fight infection.
Macrophages A white blood cell that acts mainly through phagocytosis.
Neutrophils Class of white blood cells that are involved in the protection against infection.
T-lymphocyte A white blood cell that is active in fighting infection. (May also be called T-cell; the t in T-cell stands for thymus.) These cells coordinate the immune system by secreting hormones that act on other cells.
B-lymphocytes White blood cells that are responsible for producing immunoglobulins.
Epithelial Cells Cells that line the surface of the body.
Secretory Immunoglobin A A protein found in secretions that protect the body’s mucosal surfaces from infections. The mode of action may be by reducing the binding of a microorganism with cells lining the digestive tract. It is present in human colostrum but not transferred across the placenta.
33411_06_Ch06_p159-192.indd 169
33411_06_Ch06_p159-192.indd 169 5/1/10 1:27:51 AM5/1/10 1:27:51 AM
Copyright 2011 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part.
50% lower in exclusively breastfed infants.80 Internation-ally, gastrointestinal infection was lower among infants ex-clusively breastfed for 6 months when compared to those exclusively breastfed for only 3 months.80,81 Ear infections are 19% lower, and the number of prolonged episodes of ear infection was 80% lower among breastfed infants than among infants fed HMS. In a U.S. population study, breast-fed infants experienced 17% less coughing and wheezing and 29% less vomiting than did infants fed HMS.82 Reductions in Chronic Illness In addition to the lower rate of acute illnesses in breastfed children, breastfeeding also seems to protect against chronic childhood diseases.
Breastfeeding may reduce the risk of celiac disease,83 in-flammatory bowel disease,84 and neuroblastoma.85 HMS feeding results in an increase in the risk of allergy (30%) and asthmatic disease (25%).86 These reductions in acute and chronic infant illness increase with greater use of hu-man milk.87 For example, infants who receive some hu-man milk and some HMS are at 60% greater risk of ear infection than those fed exclusively human milk. The risks, particularly for allergy and asthmatic disease, are reduced for the duration of breastfeeding and for months to years after weaning.86
Breastfeeding and Childhood Overweight Consid-erable attention has been paid to the role of breastfeeding in preventing obesity, but this relationship is still a topic of controversy. Breastfed infants typically are leaner than HMS-fed infants at 1 year of age without any difference in activity level or development.88 A large body of liter-ature suggests that there is a small reduction in risk of overweight in children older than 3 years of age who were breastfed.89 The effect of breastfeeding on the incidence of overweight was greater with longer duration of breast-feeding in some studies,90 but not all.91 Several potential mechanisms have been identified for the modest reduc-tion in obesity in children who were breastfed, including metabolic programming, possibly related to leptin, ghre-lin, and other neurometabolic messengers delivered in hu-man milk.92, 93 Other contributors could include learned self-regulation of energy intake and other characteristics of the families or parents, such as healthy lifestyle. Given the epidemic of obesity in the United States and beyond, it is likely that the research in this area will continue.
Cognitive Benefits Several reports have linked breast-feeding, and especially duration of breastbreast-feeding, with cognitive benefits, assessed by IQ.30,94 The increases in cognitive ability associated with breastfeeding are sig-nificant even after adjusting for family environment.95 Cognitive development gains increase with the duration of breastfeeding.94 In addition, higher intelligence quo-tients (IQ) of infants breastfed for 6 months appear to be greater among infants born small for gestational age (11 points) than among infants born appropriate weight Binding proteins in human milk bind iron and
vita-min B12, making the nutrients unavailable for pathogens to grow in the infant’s gastrointestinal tract. Such factors are also responsible for some of the differences in intesti-nal flora (natural bacteria of the gastrointestiintesti-nal tract) of breastfed infants versus HMS-fed infants.
Individual fatty acids and other milk components (ol-igosaccharides, gangliosides, and glycoconjugates) result-ing from digestion of human milk are antimicrobial.75 The digestive products of triacylglycerides and the lipid glob-ule appear to protect against Escherichia coli 0157:H7, Campylobacter jejuni, Listeria monocytogenes, and Clostridium perfringens.76 Monoacylglycerides are able to lyse enveloped viruses, bacteria, and protozoa. Glyco-conjugates (glycoproteins, glycolipids, glycoaminoglycans, and oligosaccharides) may bind pathogens directly, thus preventing infection. Nucleotides are reported to increase resistance to Staphylococcus aureus and Candida albicans and may increase response to vaccine antigens.77
Growth factors and hormones in human milk, such as insulin, enhance the maturation of the infant’s gas-trointestinal tract. These substances also help to protect the infant, especially neonates, against viral and bacterial pathogens.
Lower Infant Mortality in Developing Countries In the developing world, 10 million children die each year, and 60% are believed to be preventable deaths.78 Im-proving breastfeeding practices could save approximately 1.3 million lives annually, and continuing breastfeed-ing with complementary foods could save an additional 600,000. This protection of lives is at the center of the World Health Organization (WHO) and UNICEF’s joint efforts called the Global Strategy for Infant and Young Child Feeding to remind the international community of the impact of feeding practices (including breastfeeding) on children’s health outcomes.78
Breastfeeding may also play a role in reducing the risk of sudden infant death syndrome (SIDS), but this is still under investigation. Researchers disagree about whether breastfeeding has a primary effect in reducing risk of SIDS.
An analysis of available studies found that bottle-feeding increases the risk of SIDS, but other factors related to feeding choice may be responsible for this finding.79 Fewer Acute Illnesses Reduced infant illness is evi-dent in countries with high infant illness (morbidity) and death (mortality) rates, poor sanitation, and questionable water supplies. Even in the United States and other de-veloped nations, where modern health care systems, safe water, and proper sanitation are commonplace, there is a clear relationship between breastfeeding and reduced rates of illness in infants. In U.S. samples, the incidence of diarrhea is estimated to be Morbidity The rate of illnesses in a
population.
Mortality Rate of death.
33411_06_Ch06_p159-192.indd 170
33411_06_Ch06_p159-192.indd 170 5/1/10 1:27:51 AM5/1/10 1:27:51 AM
Copyright 2011 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part.
An average of 24% of milk is left in the breast after feeding.106 Thus, the short-term milk storage of the
breast does not seem to be a limiting factor to infant milk intake. The average rate of synthesis in a day is only 64%
of the highest rate of milk synthesis, suggesting that milk synthesis could be increased considerably. Comparisons of milk production between mothers of singletons and twins shows that the breasts have the capacity to synthesize much more than a singleton infant usually drinks.107
Does the Size of the Breast Limit a Woman’s Ability to Nurse Her Infant?
The size of a woman’s breast does not determine the amount of milk production tissue (clusters of alveoli con-taining secretory cells that produce the milk).108 Much of the variation in breast size is due to the amount of fat in the breast. The size of the breast does limit storage be-cause of limitations in the expansion of the ducts. Daily milk production is not related to the total milk storage capacity within the breast, however.106 This means that women with small breasts can produce the same amount of milk as women with large breasts, although the latter woman may be able to feed her infant less frequently to deliver the same volume of milk compared with a woman with smaller breasts.
Is Feeding Frequency Related to the Amount of Milk a Woman Can Make?
Feeding frequency is not consistently related to milk production. The rate of milk synthesis is highly variable between breasts and between feedings.109 However, the amount of milk produced in 24 hours and the total milk withdrawn in that 24-hour period are highly related.106 Milk synthesis is able to quickly respond to infant demand.
The breast responds to the degree of emptying dur-ing a feeddur-ing, and this response is a link between mater-nal milk supply and infant demand. Daly proposed that the breast responds to the infant’s need by measuring how completely the infant empties the breast.102,106,109
For example, if a lot of milk is left in the breast, then milk synthesis will be low to prevent engorgement; if the breast is fully emptied, synthesis will be high to replenish the milk supply.
Exact mechanisms of milk supply and demand are not well understood, but they seem to be related to a pro-tein called feedback inhibitor of lactation (FIL).109 FIL is an active whey protein that inhibits milk secretion. This protein inhibits all milk components equally according to their concentration in milk. Therefore, this protein seems to affect milk quantity only, not milk composition.
for age (3 points).96 The differences in cognitive func-tion are also greater in premature infants fed human milk than in those fed HMS.97 Recognition that the fatty acid composition of milk plays an important role in neuropsy-chological development bolsters the credibility of psycho-logical or cognitive benefits from breastfeeding.
Analgesic Effects Breastfeeding seems to work as an analgesic in infants. Breastfeeding during venipuncture seems to reduce infant pain as well as a 30% glucose so-lution followed by pacifier use.98 However, breastfeeding before a heel prick99 did not seem to reduce infant pain response. Breastfeeding may be used to reduce infant dis-comfort during minor invasive procedures.100
Socioeconomic Benefits A decrease in medical care for breastfed infants is the primary socioeconomic benefit of breastfeeding. Medicaid costs for breastfed WIC in-fants in Colorado were $175 lower than for inin-fants who were fed HMS.101 Never-breastfed infants have an excess of care for lower respiratory tract illness, otitis media, and gastrointestinal disease compared to infants breastfed for at least 3 months.102 Each 1000 never-breastfed infants had 2033 more sick care visits, 212 days of hospitaliza-tion, and 609 more prescriptions. In addihospitaliza-tion, in one study of two companies with established lactation programs, the one-day maternal absenteeism from work due to infant illness was approximately two-thirds lower in breastfeed-ing women than nonbreastfeedbreastfeed-ing women.103 Companies benefit through lower medical costs and greater employee productivity.