Cow milk occupies a special position among foods in being an animal food that has a vegetarian connotation. Milk carries many nutrients that the infant needs for growth and development. For children, adolescent, elderly people pregnant and nursing mothers, milk plays an important role in meeting the requirements of many essential nutrients, and hence milk is considered as a protective food. Milk helps to balance human diet by supplementing good quality protein, calcium and vitamins particularly, vitamin A, riboflavin, niacin and folic acid. In addition milk contains several bio-protective molecule that ensure health security to humans. 

Nutritional quality of milk proteins

Milk proteins are rich in essential amino acids. The Protein Efficiency Ratio (3.1), Biological Value (91) and Net Protein Utilization (82) of milk protein are very close to that of egg proteins (3.8, 100 and 94, respectively). Lactalbumin is superior to casein having Biological Value (BV), Net Protein Utilization (NPU) and Protein Efficiency Ratio (PER) 100, 92 and 3.6, respectively. The corresponding value for casein are 77, 76 and 2.5, respectively. Only 14.5g of lactalbumin or 28.5g milk proteins is sufficient to meet the daily requirement of essential amino acids for adult humans. 

Supplementary value and digestibility of milk proteins

Because milk proteins contain a surplus of certain essential amino acids (lysine and threonine), they can raise the BV of vegetable proteins. Milk proteins can be added to cereal based products to increase their lysine and threonine content. Whey proteins have even better supplementary value and can raise the BV of soy proteins because of their high concentration of sulphur amino acids. Whey obtained as a by- product in cheese manufacture has thus a great potential for incorporation in cereal based products.

The digestibility of milk proteins is rated higher (96%) then that of plant proteins (74-78%). Because of their high BV, the milk proteins are useful in the diet of patients suffering from liver and gall bladder diseases, hyperlipidaemia and diabetes. Patients with impaired kidney functions rely on protein with high BV for relieving strain on the excretory function of the kidney. The milk proteins are also used in slimming diets. 

Milk fat and misconceptions about its role in coronary heart diseases (CHD)

Milk fat has often been implicated in CHD because of its cholesterol content and composition of its fatty acids. It is however, not correct to judge the implication of milk fat in development of CHD solely on the basis of its fatty acid composition and cholesterol content. The average cholesterol content in cow and buffalo milk is only 2.8 and 1.9 mg/g fat respectively. Moreover, humans absorb 10-14% of dietary cholesterol, thus Only 20-40 mg cholesterol will be absorbed from 50g of dietary milk fat. On the other hand, the body itself synthesizes cholesterol (1-4g daily) in much higher amounts than what is absorbed from the diet.

The so called "Lipid hypothesis" states that there is a connection between fatty acid composition of the diet and cholesterol content of serum in that the saturated fatty acids (SFA) increase cholesterol and polyunsaturated fatty acids (PUFA) decrease it. Since increased cholesterol levels are believed to play a role in the development of CHD, the demand is often made that the dietary fat having low proportion of PUFA be replaced with oil that are rich in PUFA. The question that naturally arises is should one avoid milk or milk fat because it contains high content of saturated fat. The major saturated fatty acids in milk fat are palmitic acid (24-28%), myristic acid (13-14%) and stearic acid (11-12%), and the major unsaturated fatty acid is oleic acid (23-28%). Milk fat has high proportion of short and medium-chain saturated fatty acids, which do not raise serum cholesterol levels, nor does oleic and stearic acid. Only palmitic and myristic acid has some effect. The idea that excess of SFA and/or cholesterol were associated with the development of CHD arose from epidemiological association between high total and animal fat intake, high serum cholesterol levels and incidences of CHD, in some countries.  The conclusion made from such empirical studies have been criticized by many researchers in that the population that consumed fat with higher PUFA/ SFA ratio also consumed less calories from sugars and total fat.

Regarding the role of PUFA in decreasing cholesterol levels and preventing heart diseases, it has not been proven unambiguously. On the other hand, the recommendations advocating for increasing intake of PUFA have been questioned. There is an increasing evidence that excessive intake of PUFA is harmful in that it may lead to (1) formation of gall stones due to increased production of bile acids (2) increased requirements for vitamin E, (3) increased formation of peroxides which may cause alteration in membrane of blood corpuscles, (4) oxidation of PUFA enriched LDL increasing susceptibility to atherosclerosis and (5) higher incidences of colon tumours due to colonisation of the intestine with bile degrading bacteria, since the excessive intake of PUFA increases the excretion of cholesterol in the form of bile acids. It is, therefore, unjustified to raise the hope that cardiac infracts could be prevented by a diet rich in PUFA. An excessive intake of energy, resulting in excess weight is one of the major reasons of altered cholesterol metabolism and atherosclerosis. Indeed, a diet containing optimum amounts of calorie and essential nutrients, wherein the type of fat has no significance, is a real safeguard against high mortality from atherosclerosis. 

Nutritional benefits of milk fat

Compared to other fats and oils, milk fat is easily digestible. The digestibility of milk fat is 99%, while that of natural palm oil is 91%. The excellent digestibility of milk fat is due to dispersion of fat globules in the aqueous phase of milk forming an emulsion. They are absorbed directly unlike other dietary fats that have to be emulsified by bile, pancreatic enzymes and intestinal lipases before they can pass through intestinal well. Also, milk fat is rich in short and medium-chain fatty acids, which are more easily absorbed than long chain fatty acids. The ester bonds involving short-chain fatty acids are more easily cleaved by lipases. The easy digestibility of milk fat makes it a valuable dietary constituent in diseases of stomach, intestine, liver, gall bladder, kidney and disorders of fat digestion.

In infant and child nutrition, milk fat is of immense benefit. It helps them in meeting their energy requirements by increasing energy density of the diet. A sufficient fat supply is essential for thriving babies, a rosy and smooth skin and also resistance to bacterial infections.

Milk fat has a low content of essential fatty acids (EFA), linoleic and linolenic acid. The EFA requirement is only 3% of total calories, two-third of which is met from invisible fat present in dietary cereals, pulses and vegetables. Therefore there is no justification to replace milk fat with another fat having a higher linoleic acid content.  

Milk is a rich source of vitamins

Milk is a rich source of vitamins not only in terms of their contents but also their better bioavailability. Milk is one of the richest natural sources of riboflavin (vitamin B₂). A 250 ml serving of cow milk contains riboflavin equivalent to 50% of the daily requirement of a pre-school child. Although milk contains only small amounts of preformed niacin (Vitamin B₃), nevertheless, it is a very good source of this vitamin. Niacin can be synthesized in the body from tryptophan, which is present in milk proteins in good amount (480 mg/L). Sixty mg tryptophan is metabolised in the body to give rise to one mg niacin. Indeed, milk is used as dietary ingredient for patient suffering from pellagra, a niacin deficiency disease. For vegetarian, milk is sole natural source of vitamin B₁₂, as this vitamin is present only in animal foods. Milk is also a good source of folic acid. Vitamin A deficiency is a major cause of widespread blindness among children in India. A 250 ml serving of cow milk contain vitamin A sufficient to meet 75% daily vitamin A requirement of pre-school child. 

Milk is a richest natural source of calcium

Recent research has shown that poor nutritional status with respect to calcium is related to diseases like osteoporosis, hypertension and colon cancer. The hypertensive patients have shown significant reduction in blood pressure in response to increased calcium intake. The effect of calcium on blood pressure is mediated by (1) increase in urinary excretion of sodium, (2)) preventing the rise in vitamin D hormone which increases blood vessel resistance (3) relaxing smooth muscle cells which lines the blood vessels (4) suppressing the renin-angiotensin system and (5) increasing production of endothelial relaxing factors.

Introduction of increased dietary calcium through dairy products has been shown to reduce incidences in colon cancer and hyper-proliferation in the colonic mucosa in rodents. Calcium produces these effects by neutralizing deconjugated bile acids and free acids, thereby removing their mitogenic/ toxic influence.

Milk and dairy products are the most important source of calcium in readily available form. A 250 ml serving of cow milk contains calcium equivalent to 60% of ICMR’s Recommended Dietary Allowance (RDA) for adults. Incorporation of milk in the diet also improves the bioavailability of calcium from vegetable foods. The factors that contribute to better availability of calcium from milk include lactose, protein and phosphorus.

A part of calcium in milk is associated with phosphorylated casein. The casein phosphopeptides, released in the gastrointestinal tract during digestion, form soluble complex with calcium phosphate salts and improves the diffusion of calcium across the intestine. Milk is rich in phosphorus that reduces urinary calcium excretion, and counter balances, at least in part, the calciuric effect of dietary proteins. Milk and most dairy products, except some processed cheese, have a near 1:1 calcium to phosphorus ratio considered to be ideal for retention of calcium in the body. 

Therapeutic properties and extra-nutritional role of milk constituents

Milk proteins have high buffering power; therefore it is useful in the treatment of inflammation of mucous lining of stomach and of stomach ulcers, preventing hyperacidity. Milk and milk products are used as a source of proteins in hyperuricaemia and goat disease. In contrast to other foods, these do not contain purines, which are precursors in the synthesis of uric acid that causes gout when deposited in the joints or may lead to formation of urinary calculi.

Short and medium-chain fatty acids with 4-12 carbon atoms, which occur in a relatively high concentration in milk fat are reported to have antibacterial and fungicidal activity against gram negative bacteria and certain moulds. Milk fat has a protective effect against human tooth decay. This effect has been ascribed in part to adsorption of milk fat onto the enamel surface and in part to antimicrobial effect of milk fatty acids.

Protective effect of milk fat against some types of cancer (colon, breast and skin) has recently been reported. Fat in general is regarded as increasing cancer risk, although not for all types of cancer. However, a specific fatty acid (a cis-trans isomer of linoleic acid) has been identified in milk fat, which appears to be an inhibitor of cancerous growth. 

Immunological aspects of proteins

The substances in milk which have an antimicrobial effect are immunoglobulins, lactoferrin, lysozyme, lactoperoxidase and vitamin B₁₂-binding protein. The immunoglobulins, mainly 1gA are not broken down by the digestive enzymes. Thus, they not only act against the microorganisms in the intestine but also prevent the absorption of foreign proteins.

Lactoferrin is an iron binding glycoprotein that occurs in cow milk at a level of 0.2 mg/ml. Mother's milk contains large amounts of lactoferrin (0.1-0.2g/100 ml). Lactoferrin plays an important role in the resistance against intestinal infection, particularly Escherichia coli. Lactoferrin has both bacteriostatic and bactericidal properties. The bacteriostatic effect is of lactoferrin is due to its iron binding ability making iron unavailable for iron requiring bacteria. Mother's milk also contains a large amount of an unsaturated vitamin B₁₂-binding protein, which competes with bacteria that have a vitamin B₁₂.

A number of enzymes are also involved in the milk immune system. These are lactoperoxidase, xanthin oxidase and lysozyme. Lysozyme has a direct effect by breaking down the cell wall of gram-positive bacteria. The lactoperoxidase-thiocyanate-H₂O₂ system is an antibacterial system. Lactoperoxidase and thiocyanate are found in milk and other tissue secretions, and H₂O₂ is produced by lactic acid bacteria or by the action of xanthin oxidase. Thiocyanate is oxidized by H₂O₂ and lactoperoxidase to an intermediate product that destroys the microorganisms. 

Functional peptides

Many milk-derived peptides possess functional properties. Several peptides with opium like (sleep inducing) activity have been extracted from the degradation products of milk proteins. These include ß-casomorphins (from ß-casein), exorphin (from µ_S1 casin), ß-lactostensin (from lactoglobulin) and serorphin (from serum albumin). These opium-like     peptides have been shown to prolong gastrointestinal transit time exerting anti-diarrhoeal effect. They also stimulate secretion of insulin and somatostatin.

 Glycomacro peptide (GMP) derived from K-casein induces production of cholecystokinin, a hormone associated with satiety. GMP and other fractions of K-casein digest inhibit the adhesion of oral actinomycetes and streptococci to erythrocytes, and binding of cholera toxins to its receptor.

 Angiotensin-converting enzyme (ACE) located in different tissue, splits two amino acids from C-terminal end of angiotensinogen I converting it into angiotensinogen II, which is a highly hypertensive octa-peptide. Peptides with anti-hypertensive activity that act through inhibition of ACE have been identified in the sequence of bovine and human ß-and µ_S1 casein. Recently  µ-lactalbumin and ß-lactglobulin fragments that inhibit ACE have also been characterized.

Several immune-stimulatory peptides have been identified from both bovine and human casein and whey proteins. These peptides have been shown to stimulate the phagocytic activities of murine and human macrophages and enhance resistance against certain bacteria. Certain peptides from casein stimulate the production of immunoglobulins.

            Casecidin, a chymosin digest of casein in vitro, inhibits Sarcina, Bacillus subtitis, Diplococcus pneumoniae and Streptococcus pyrogenes. Similarly, fragments of human ß-casein have a protective effect against Klebsiella pneumoniae. Iracidin, an µ_S1 -casein fragment has both therapeutic and prophylatic effect. Lactoferricin, an acid-pepsin digestion product of lactoferrin, has stronger bactericidal activity compared to the native molecule.    

Milk has cholesterol lowering factors

Several studies have shown that milk reduces serum cholesterol levels of the consumers. Experiments with volunteers have shown that cholesterol levels do not rise when as much as 2 litres of milk is consumed daily. On the contrary, the cholesterol level is reduced. Both decreased formation and increased breakdown of cholesterol are responsible for the cholesterol lowering effect of milk. Orotic acid and another nucleotide associated with proteose-peptone fraction of milk proteins and calcium are suggested to have cholesterol reducing properties.  Milk slows down the biochemical processes leading to atherogenesis in rabbits fed on atherogenic diet. It has been suggested that the regular intake of milk keeps blood vessels healthy. 

Therapeutic role of milk sugar

Lactose, the principal milk sugar, is slowly metabolised and therefore, a considerable portion of it passes into the large intestine where it promotes the growth of lactic acid producing bacteria. Lactic acid creates a desirable condition that inhibits the growth of proteolytic and putrefying bacteria in the intestine and replaces them gradually with acidophilic bacteria.

Several investigations have shown that lactose promotes the utilization of calcium, magnesium and phosphorus. The acidic condition created by fermentation of lactose by intestinal bacteria may increase the solubilization and absorption of calcium. Part of the effect may also be due to the ability of lactose to form soluble complex with calcium. Since lactose is slowly absorbed, it has slight laxative effect. It is due to lowering of the pH that increases the peristalsis of the intestine. The blood glucose does not rise rapidly on lactose diet. Milk consumption, therefore, enables the diabetic person to obtain the biologically highly valuable milk proteins without running the risk of rise in blood glucose levels. 

Milk production and per capita availability

 India is the largest producer of milk in the world. Milk production in the country was 22.5 million tons in 1970-71, and is now projected to reach 88.5 million tons in 2001-02. Despite increase in population, per capita availability of milk in the country has increased from 114 g per day in 1970-71 to 231 g per day. Milk production enhancement programmes like Operation Flood, artificial insemination, cross breeding, better animal health care and improved feeding practices has led to white revolution in the country. ICMR recommends inclusion of 150 g milk in a balanced Indian diet. The per capita availability of milk is above ICMR recommendations in most of the states, except in north-eastern states, Orrisa and Bihar, and marginally low in Andhra Pradesh. 

                         Vinod K. Kansal

Division of Animal Biochemistry

National Dairy Research Institute, Karnal - 132001, Haryana