Lesson 45. HUMAN MILK AND COW MILK
HUMAN MILK AND COW MILK
Milk is nature’s ideal food f or infants and growing children, except in rare cases of lactose intolerance. After arrival on this earth and during the first phase of growth, a human baby thrives on milk. In fact, mother's milk is not only desirable but essential from the time the child is born and hence the baby is recommended to be breast-fed until it is normal. It has always been an accepted fact that the best food for babies younger than six months is mother's milk due to the composition and content of various vitamins and essential amino acids. Human milk not only provides the infant with essential nutrients required during a critical growth period but also protect the new born from disease .
The requirements for an infant for proper growth include carbohydrate, fat, protein, vitamins, minerals and water. The concentration of these nutrients varies according to the needs of the off-spring, which depends upon maturity of birth, rate of growth, digestive system and environment it is born in to. In each species, the milk meets the specific needs of the off-spring. The milk of one animal can not be a real substitute for that of another. Hence, the most ideal situation for infant feeding would be to feed the child with human milk during the early stage of his growth. However, situation may arise when a human baby may be required to be given milk other than from his mother.
Many babies are fed with ordinary cow's milk to which some water and sugar are added. The widespread availability and commercial promotion of prepared infant formulas has led to a progressive decline in breast feeding throughout the world. For this reason, the WHO not only has sought to encourage breast feeding in the developing countries, but also has passed an International Code of Marketing of Breast Milk Substitutes.
Infant formula as per CODEX standards is a breast-milk substitute specially manufactured to satisfy, by itself, the nutritional requirements of infants during the first months of life up to the introduction of appropriate complementary feeding. The product is so processed by physical means only and so packaged as to prevent spoilage and contamination under all normal conditions of handling, storage and distribution in the country where the product is sold.
45.2 Human Milk
Chemical composition of human milk differs from that of cow or buffalo milk as shown in Table 45. 1 & 45.2 below:
Table 45.1 Chemical composition of human, cow and buffalo milks
Table 45.2 Major difference in the make up of different constituents of human, cow and buffalo milks
Human milk contains higher carbohydrates (lactose) than cow or buffalo milk and hence 40% of the total energy of human milk is supplied by this constituent of milk which is only 29% in case of cow milk. Moreover, presence of non-lactose oligosaccharides such as bifidus stimulating factors in human milk enables the growth of healthy microflora in the intenstines.
Fat present in human milk provides essential fatty acids (required for nervous system development and prostaglandin synthesis), fat soluble vitamins and sterols. A unique feature of human milk is that increase in level of fat during feeding to infant serves as an appetite control mechanism for the infant. Almost an equal amount of fat is present in cow and human milks; however, qualitatively they differ widely in their fatty acid make-up as shown in Table 45.3.
Table 45.3 Fatty acid composition (% wt.) of human and cow milks
- Human milk fat is rich in polyunsaturated fatty acids (PUFA) (notably linoleic acid- an essential fatty acid- about 7 to 8 times higher) which play an important role in brain development.
- Moreover, two important saturated fatty acids - myristic and palmitic - along with stearic acid are preferentially esterified at the beta position in human milk fat.
- The presence of comparatively high content of palmi¬tic acid in position Sn-2 increases the digestibility of human milk fat. As a conseq¬uence of this, lipids from human milk are more easily absorbed and excretion of fat by infants fed with breast milk is only about half to that of bottle-fed infants.
- In addition, human milk fat contains easily digestible oleic acid in higher proportion while cow milk fat contains excess of butyric acid which may give rise to fatty diarrhea (steatorrhea).
- Higher content of 18:1 in human milk also enables improved retention of calcium in breastfed infants.
- Absence of orotic acid in human milk makes it less prone to occurrence of atherosclerotic plaques in breast-fed infants.
- Over and above these differences, human milk has been shown to have higher content of unsaponifiable matter, higher ratio of phosphatidyl ethanolamine to sphingomylein and predominancy of phosphatidyl inositol.
Because of the differences in fatty acid make up of cow or buffalo milk from that of human milk, it has been proposed that all infant foods should contain PUFA rich vegetable oils so as to provide a minimum of 300 mg/k cal of linoleic acid in the formulas.
The characteristics of the human milk as compared to cow milk are shown in Table 45.4
Table 45.4 Protein components of cow and human milk
- The whey proteins are richer in essential amino acid content (PER = 3.11) compared to caseins (PER=2.50), hence nutritional requirements of infant could be met from relatively lower amount of good quality proteins, typical of human milk .
- Human milk has a higher proportion of α-lactalbumin but no β - lactoglobulin. α-lactalbumin, the most abundant protein of human milk , is of major importance to the infant from a nutritional view as it serves as a source of essential amino acids for the infant as well as acting as a crucial component of an enzyme system that biosynthesi¬zes lactose.
- The antimicrobial proteins of human milk (lactoferrin, lysozyme and IgA) account for 75% of the protein in human colostrum, versus 39% in mature human milk and less than 0.1% in cow milk.
- The amino acid profile of human milk has unique ratio (0.76) between cystine and methionine as against 3.1 in case of cow milk.
- The addition of demineralized whey to adjust the casein to whey protein ratio of cow milk to that of human milk changes the ratio to 1.4.
- Human milk contains 13 g free amino acids of which 4 g are essential amino acids for the human infant. Cow milk contains 33 g free amino acids of which 16 g are essential.
i. Glutamic acid accounts for ~ 40% of the free amino acid pool in human milk.
ii. Taurine is the next major constituent of the free amino acid pool in human milk, but is only in trace amounts in cow milk.
iii. Taurine has been postulated to play a major role in fat utilization and brain development in the infant as well as in stimulating "in vitro" activity of pancreatic lipases.
- The proteins most concerned with the nutritional activities are the caseins, α-lactalbumin and the folate and vitamin B12- binding proteins. There are also fat globule membrane proteins, β- microglobulin and corticosteroid - binding proteins.
In several brands of infant foods presently available, the protein content is adjusted to around 1.6% (similar to human milk) but still the casein content is 75% of the total protein. Use of demineralized whey is helpful, however, in adjustment of casein to whey protein ratio (40:60) similar to that in human milk. However, yet there is an ample scope for making the infant foods similar to human milk with respect to various metabolic responses.
Human milk possesses a mineral content which is less than one third than in cow/buffalo milk and calcium to phosphorus ratio of 2.1 as compared to about 1.5 in cow and 1.6 in buffalo milk. The low mineral content results in a low renal solute load.
45.3 Striking Features of Human Milk
The protein and non-protein nitrogen fractions of human and cow milk are shown in Table 45.5. The striking features of human milk when fed to infants are as under:
1. Digestion of milk by a baby is speedy - due to formation of soft curd.
2. Quite less load on the kidney - due to low calcium content.
3. Easy digestion and faster absorption of casein-due to smaller size of casein micelles.
4. Proper development of brain constituents - due to higher lactose content and presence of relatively more amount of PUFA.
Bactericidal and anti - infective properties - due to presence of lysozyme, secretory IgA, bifidus factor and lactoferrin
Table 45.5 Protein and no protein N components in human and cow milk
45.3.1 Host – Resistant Factors in Human Milk
1. Immunoglobulins (Ig) (milk antibodies): The most abundant Ig of human milk is IgA versus Ig G in cow milk. It provides the infant with protection against enteric infection and serves as an "intestinal paint" to prevent the passage of various foreign proteins and bacteria from the intestinal tract into circulation. Milk antibodies function as specific host resistance factors and provide crucial immunological protection until the new born infant’s defense system can be established.
2. Lactoferrin (LF): Human milk contains from 3 to 100 times as much iron binding protein LF as cow milk . LF is active "in vitro" against enteropathogenic E. coli , Vibrio cholerae, St. mutans and Candida albicans , presumably by chelating iron and making it unavailable for microbial growth.
3. Lactoperoxidase (LP): In cow milk, LP serves as a major antimicrobial agent; it catalyzes the oxidation of thiocyanate by H202 to hypothiocyanate. It is active against several microorganisms. Although human milk - LP survives gastric digestion "in vitro" its low concentration in milk may preclude any significant antibacterial role in infant intestine.
4. Lysozyme: It cleaves the cell wall peptoglycan of a number of Gram positive and Gram negative microorganisms and appears to act with complement to potentiate the activity of IgA against E. coli, and with peroxide and ascorbate to lyse E. coli and salmonella. It also plays a role in protecting against various viruses. Human milk contains ~ 3000 times as much lysozyme as cow milk .
5. Bifidobacteria: Protect the infant against disease by producing volatile acids which inhibit the proliferation of pathogenic microorganisms in the gut. Within 3-4 days after birth, the intestinal tract of breast fed infant contains up to 99% Bifidobacterium bifidum Type IV whereas the formula -fed infants do not contain Type IV but rather 30-40% B. bifidum Type II. A number of substances in human milk have been reported to stimulate the proliferation of the bifidobacteria:
a. Buffering capacity: Because of low buffering capacity of human milk, the pH of intestine drops, making growth conditions favourable for bifidobacteria and unfavourable for others.
b. Lactulose: Not present in human milk or cow milk , but when milk is sterilized (as in case of sterilized formulas) ~ 1.5% of the total lactose is converted to lactulose during heating. It encourages the growth of bifidobacteria.
c. Lactoferrin: Indirectly promotes the growth of bifidobacteria by inhibiting the growth of competing E. coli.
d. Pantothenic acid : Derivatives of this acid stimulate one strain of B. infantis .
e. Oligosaccharides and glycoproteins: Stimulate the growth of B. bifidum var: Pennsylvanicus . Human milk contains ~ 30 to 100 fold more stimulatory activity than cow milk.
f. Others: Human milk contains fatty acids, monoglycerides, triglycerides, free fatty acids, copper, orosomucoid, influenze virus hemagglutination inhibitor, etc. which help an infant to get protection from several pathogenic organisms and viruses.
Thus the presence of bifidus factor in human milk is important as it helps in establishment of bifidobacteria in the intestine and in maintaining lower pH of the stool, better absorption of minerals, synthesis of certain B-complex vitamins, resistance to enteropathogenic bacteria, detoxification in chronic liver diseases, etc.