A Nutritional View of Breastfeeding

This is the nineteenth entry in a six-week series of topics following the syllabus of my Human Nutrition & Obesity graduate summer course. Each entry offers a snapshot or principal take home message from that lecture. This lecture was entitled: A Nutritional View of Breastfeeding. @DrTomSherman or @GUFoodStudies

There are few practices a mother can engage in that contributes more towards the long-term health and intellectual growth of her children than breastfeeding. Proper immune system function, gastrointestinal health, and brain and cognitive development all depend on nutrients provided by human breast milk, and many of these nutrients, either in terms of specific molecular forms or quantities, are unique to human milk. Not to be described here, but also important, is that many additional benefits of breastfeeding extend to the mother, including decreased postpartum bleeding, more rapid uterine involution, the return to pre-pregnancy weight is much quicker, risks of premenopausal breast and ovarian cancers are reduced, and increased child spacing. This is not even taking into account the positive psychological and emotional impact on the mother-child bond.

Current recommendations are for six months of exclusive breastfeeding, and a combination of foods with breastfeeding for as long as possible; but any amount of breastfeeding is better than none at all for two reasons: a critical first stage of breastfeeding that includes colostrum lasts for only a few days, and many of the brain and cognitive benefits of breastfeeding increase almost linearly with breastfeeding duration, so that each month of breastfeeding adds significant additional benefits.

Milk is a complex mixture of many molecular and cellular elements contained within several phases. Technically, milk is an emulsion of fat globules in a plasma phase (skim milk), which itself contains casein micelles (protein particles) in a colloidal suspension in a serum phase (whey). Perhaps more recognizably stated: if you remove the fat you obtain skim milk; if you remove the fat and protein particles – as happens during cheese production – you are left with the whey, a thin solution of soluble proteins, milk sugars, and miscellaneous nitrogen compounds and vitamins. Not included in this list are the significant number of immune cells found in milk, such as macrophages, neutrophils and lymphocytes.

Beyond being a complex mixture, human breast milk is also a dynamic mixture, with important nutritional components changing as a function of breastfeeding duration. This applies to both the maturation of human milk from colostrum (3–5 days), to transitional milk (~14 days), to mature milk, over the days, weeks and months of breastfeeding, and as a function of the time course of an individual feeding. For example, as illustrated in the figure below, the foremilk, the first portion of milk that a suckling infant receives, contains much less fat than does the latter hindmilk delivered as the breast empties. Addressing first the thirst and then the hunger of a feeding infant, this pattern underlies the important recommendation that a mother encourage her nursing infant to empty a breast before proceeding to the other during a feeding session. Not surprisingly, the energy density of milk increases significantly during a feed: from 15–17 kcal/ounce early to 25–27 kcal/ounce late. Protein and lactose concentrations remain constant throughout a feeding.

Given the many interesting and valuable aspects of breastfeeding worth discussing, it was difficult deciding how to limit this blog entry to just a few. But with our emphasis on nutrition, I thought it most useful to focus on what it is about human breast milk that makes it such a perfect first food and to discuss what significant differences there are between human milk, cows’ milk, and many infant formulas. The figure at left lists what I believe are the six most significant differences, and where they have the greatest impact for the growing newborn:

Colostrum: if breast milk is the perfect first food, than colostrum is the perfect appetizer. Low in fat, high in carbohydrate, protein and antibodies, colostrum is easily digested and provides a laxative effect to help the infant pass early stools, aids the excretion of excess bilirubin, and helps prevent jaundice. Of great significance, colostrum contains large quantities of secretory immunoglobulin A (IgA) proteins that protect mucous membranes in the throat, lungs, and intestines. Unlike other mammals, such as cows, newborns already possess maternal circulatory IgG antibodies via placental blood transfer; these are the antibodies that convey passive immunity against invading pathogens. Thus, human breast milk can focus on delivering IgA antibodies to prevent colonization of bacteria and other pathogens in mucous membranes. The figure at above right illustrates the relative distribution of IgG, IgA and IgM antibodies in colostrum (outer circle) and in milk (inner circle) between cows’ and breast milk. The relatively large IgA fraction offers significant protection against gastrointestinal and respiratory infections. Accordingly, breastfed infants experience dramatically decreased incidence of diarrhea, and respiratory tract and ear infections. Such protections are lifesavers in areas with poor water quality where diarrhea is often the number one cause of infant mortality. Even in the U.S., at our current rates of breastfeeding, it is estimated that postneonatal mortality is reduced by 21% by simply initiating breastfeeding.

Oligosaccharides: carbohydrates comprised of 3-9 sugar units are called oligosaccharides (glucose is a monosaccharide, sucrose and lactose are disaccharides, and starches are polysaccharides). We are capable of digesting only a few forms of carbohydrate, such as many plant starches, glycogen from animal sources, and several disaccharide sugars; those polysaccharides we are not able to digest are called fiber, and fiber comes in many, many forms, including the two broad categories of soluble and insoluble fiber. An important subset of soluble fiber are the oligosaccharides found in vegetables. Among their many nutritional roles is that oligosaccharides are digested not by us, but by the bacteria (microbiome) that colonize our colon. These beneficial bacteria digest (ferment) oligosaccharides into short chain carboxylic acids that are, in part, absorbed and metabolized for energy by cells lining our intestines, and in part absorbed and metabolized by other bacteria. Soluble fiber serves an invaluable role in the maintenance and health of our gastrointestinal microbiome, and its presence at high concentrations in breast milk is, in a real sense, our first prebiotic. Prebiotics are a category of functional food defined as a non-digestible ingredient that beneficially affects the host by selectively stimulating the growth and/or activity of one or a limited number of bacteria in the colon, and thus improving host health. Quantitatively, oligosaccharides are the third highest component of human milk, after lactose and fat, and reach the highest concentration in colostrum (more than 20 g/L), decreasing over the first several weeks to approximately 12 to 14 g/L in mature milk. Cows’ milk, and many infant formulas, contain less than 1 g/L oligosaccharides. Not only does human milk contain significantly more oligosaccharides, it is a type of oligosaccharide that is unique to humans. In fact, there is evidence indicating a coevolution of human milk oligosaccharides and the genetic capability of select intestinal bacteria to utilize them.

Proteins: one of the mysteries of human breast milk is why its protein content is so low. In years past, human breast milk has been labeled deficient in nutrients, based in part, on its low protein content. This conundrum has confounded infant formula manufacturers, who are simply not able to bring themselves to keep proteins concentrations in formula that low, arguing that growing infants required substrates for growth. Cows’ milk has 35 grams of protein per liter, most infant formulas over the years have slowly lowered their protein levels to 17–20 g/L, and human breast milk has half that at 9 g/L. Part of the reason for this difference in protein concentrations is that the nature of the protein is different. As highlighted in the table below, the ratio of whey and casein proteins range from 80/20 – 60/40 as breast milk progresses from colostrum to mature milk, whereas cows’ milk is completely opposite at a 20/80 ratio. Although infant formulas typically mimic the whey/casein ratio of human breast milk, the dominant whey protein in human milk is α-lactalbumin, whereas, the dominant whey protein in cows’ milk and formula is β-lactoglobulin, a protein not naturally found in human milk. β-lactoglobulin is relatively deficient in the amino acid tryptophan, however, a fact that forces infant formula manufacturers to keep protein levels high in order to meet the minimum dietary requirements of tryptophan. As some formulas adopt α-lactalbumin enriched recipes, protein concentrations can drop, relieving infants of the high nitrogen burden that too much protein requires.

It is important to note that β-lactoglobulin can be found in the breast milk of many mothers who drink cows’ milk, indicating that this protein can be absorbed intact from the GI tract and transported into breast milk. This is particularly important for those infants with cows’ milk allergies.

Lactose: milk sugar is a disaccharide of galactose and glucose (sucrose is a disaccharide of fructose and glucose). Interestingly, lactose is synthesized in the breast by an enzyme, lactose synthase, a heterodimer consisting of two proteins, one of which is α-lactalbumin; therefore, α-lactalbumin has both a nutritional role as a major dietary protein and a biochemical role as an enzyme making milk sugar. Lactose is the major carbohydrate in breast milk, accounting for approximately 40% of the calories of breast milk, and as shown in the table above, is found at much higher concentrations in breast milk compared to cows’ milk. Lactose production is an important determinant of breast milk volume: as lactose is produced and its osmotic concentration in secretory cells of the breast increases, water follows, increasing the volume of milk.

As mentioned, lactose levels are not only high in human milk, but are higher than in the milk of any other mammal. Accordingly, some anthropologists note a rough correlation between lactose concentrations in milk and the intelligence of a species. This may be due to the galactose half of the lactose disaccharide, which contributes to the formation of galactolipids important in central nervous system membrane formation.

Lactose intolerance in infants is very, very rare, a fact that calls into question the many lactose-free formulas on the market that needlessly market to the fears of new parents that colicky behaviors or GI discomfort are due to the lactose in breast milk. Lactose is an important milk component for both nutrition and brain development. Adults should not project their lactose intolerance symptoms or suspicions on their young, and should instead focus their attention on other potential allergens and sensitizing proteins, perhaps in cows’ milk.

Taurine: a small, amino acid-like molecular, taurine is chemically defined as an amino sulfonic acid. Found exclusively in foods derived from animals, taurine is considered a conditionally essential nutrient because we synthesize it poorly. It is found at high levels in human breast milk, at 40–80 mg/L, compared to cows’ milk, which contains only trace amounts, <1 mg/L.

Long considered important for proper skeletal muscle function, taurine is now recognized as a critical component in cardiovascular function, bile acid synthesis and function, and the retina. The contributions of taurine to central nervous system development were accidently discovered with the analysis of a clinical study in which preterm babies, randomly assigned to a standard formula designed for full-term babies, subsequently (at 7.5-8 years of age) had lower developmental scores than those receiving a multi-nutrient enriched preterm formula. A significant difference between the two formulas was the trace amount of taurine in the term formula compared to 51 mg/L of taurine in the preterm formula [British Medical Journal 317: 1481 (1998)]. Accordingly, many studies now demonstrate a positive association between taurine consumption and mental development.

Very interestingly, the authors of the 1998 BMJ study noted: “Yet, paradoxically, infants randomly assigned donated breast milk or the same preterm formula had similar [verbal and overall] scores, despite the lower macronutrient content of human milk.” Shockingly, perceptions that human breast milk is nutritionally deficient continue to persist even at the turn of the 21st century!

Fats: the final highlighted nutritional difference between cows’ milk, many formulas, and human breast milk is fat composition. Total fat content is only slightly higher in human breast milk compared to cows’ milk and formula, yet the types of fat are very different. Much of these distinctions reflect differences in diet, in particular the consumption of the long chain omega-3 fatty acids so critical for brain development and intelligence (see earlier post), such as EPA and DHA, the omega-3 fatty acids enriched in seafood and fish oil.

In the table above, the dietary impact of switching to a Mediterranean diet on milk fat composition is well illustrated. The frequent intake of seafood increases EPA and DHA levels in breast milk 3–6-fold, a significant increase in a significant contributory component to central nervous system membrane biosynthesis and function. This is because the fat in milk is not synthesized in the breast like much of its carbohydrates and protein, but is transported directly from dietary fats and fat stores, as shown below. Although women synthesize EPA and DHA much more efficiently from the essential omega-3 fatty acid, α-linolenic acid (ALA), found in plants, walnuts, and flax seed, than do men, women still produce EPA and DHA poorly, thus, explaining the recommendations that pregnant and nursing mothers eat seafood or take fish oil supplements that contain preformed EPA and DHA.

The associations between breastfeeding duration and IQ are complicated, and perhaps diminished, by the confounding contributions of parental IQ and education levels, environment, and other dietary and genetic factors, but these associations are significant. Despite overwhelming evidence, it was still surprisingly difficult to convince manufacturers to supplement formulas with DHA. Clearly, however, given all the significant differences between human breast milk and formula described in this post, breastfeeding is the optimal choice. Similarly clear is that breastfeeding need not be limited to the classic paintings of old:

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