Beta-Casein Protein of Bovine Milk and its Possible Effects on Human Health

Beta-Casein Protein of Bovine Milk and its Possible Effects on Human Health

Milk accompanies humans from childhood, first as food from their mother, and then as a product used in the daily diet. It should be noted that humans are almost the only such case in nature who ingest milk in adulthood and who consume the milk of other species. From an evolutionary point of view, this phenomenon is relatively new, as historical data suggest that it has existed for about three thousand years. An increase in consumer interest in milk and dairy products has recently been observed ^[1]. One aspect of this interest is a discussion about the potential influence of the β-casein A1 variant on human health. The hypothesis assuming that the A1 variant of β-casein may have a negative effect on the health of milk consumers was formulated at the beginning of this century. According to this hypothesis, the digestion of cows’ milk with the A1 variant in the gastrointestinal tract may give rise to the opioid peptide β-casomorphin-7, which could be a risk factor in the development of heart disease, insulin-dependent diabetes, or neurological diseases. Unfortunately, there are many conflicting reports on this subject in the scientific literature. The numerous studies and reports that have appeared in recent years do not provide a definitive answer of whether milk containing the A1 variant of β-casein has a negative effect on the human body, and whether there are grounds for avoiding the A1 allele .

The significance and possible negative effects of the β-casein variant A1 on human health have been widely discussed and have aroused interest in the institutions in charge of food safety. The first such report was prepared for the Food Safety Authority of New Zealand . This document concluded that the effects of β-casein A1 consumption on human health are important and require further investigation. Although the report of the European Food Safety Authority (EFSA 2009) “Scientific Report—Review of the potential health impact of β-casomorphins and related peptides” has not supported the hypothesis of the causal relationship between β-casomorphin-7 exposure and the etiology of human diseases, it also does not resolve the credibility hypothesis of the negative impact of the A1 β-casein variant, arguing that there is insufficient evidence and suggests further research in this field .

The hypothesis that a high consumption of A1 β-casein increases the risk of diabetes mellitus type 1 (DM-1), ischemic heart disease (IHD), sudden infant death syndrome (SIDS), schizophrenia, and autism spectrum disorder (ASD) is very intriguing and interesting for basic as well as application studies.

Milk and milk products form a significant source of protein, energy and micronutrients like calcium to most populations, in human diet worldwide. Bovine milk protein comprised of 20% whey and casein contributes more than 80% of the total protein content. Among caseins, beta (β) casein makes second most abundant protein (30%) which accounts one third of total milk protein content (Boro P et al., 2016, Bell S et al., 2006, Sodhi M et al., 2012) .The protein type and proportion vary with species and breed, and with countries (Chitra P, 2021) . In addition to its nutrient supplementation, milk has positive correlation with health conditions when included in diet. Most people prefer milk as a prime source of calcium to combat with osteoporosis, colon cancer; to promote weight loss (Zemel M et al., 2004) . Research showed that A1 milk leads to the production of inflammatory compounds in human digestive system that can induce variety of symptoms from mild stomach pain to much worse. Instead, health advantages of milk consumption over the time, now it has been going through the scrutiny for the progressive relationship with some diseases, chronic conditions and health hazards proposed. In addition, dairy products, along with gluten, appear to be responsible to worsen neurological conditions like autism and schizophrenia. Studies also suggested the other genetic variant A2 could be safe with none of above effects, good for lactose intolerant and better for overall health. We have been gone through the several milk raps in past of sugar content and fat content; antibiotic residues; the alternatives like soy milk and almond milk; now what if we have a question regarding the health safety and milk? (Bell S et al., 2007).

Cow’s milk constitutes a large part of nutrition in the developing world and a liter contains an average of 32g of protein . Beta-casein A1 protein variant might be one of the risk factors in the etiology of human disorders like diabetes and ischemic heart disease. However, from the practical perspective selecting for the A2 allele in dairy cattle requires knowledge of whether the A1/A2 polymorphism is associated with breeding values for production traits.

Composition of Milk

Milk define as a whitish liquid containing milk proteins, fats, lactose, and various vitamins and minerals, produced by the mammary glands of all adult female mammals after childbirth and serves as food for their young . Milk is a considerable resource of products whose composition varies. Four components are dominant in quantitative terms: water, fat, protein and lactose; while the minor components are minerals, enzymes, vitamins, and dissolved gases. It satisfies the demand of the consumer who seeks more and more innovative products with consistent quality. The dairy industry needs to utilize all the riches of this raw material, which is both simple in appearance and complex in composition. In general, goat milk compared to cow milk is less rich in lactose, fat and proteins, but have similar mineral conten.

Milk Protein

Bovine milk contains roughly 3.5% of protein. The natural functions of milk proteins is to supply young mammals with the essential amino acids required for the development of muscular and other protein-containing tissues . In addition, milk proteins also play a very important role in dairy and food products, e.g. during processing, including undesirable behavior such as fouling on heated surfaces, and gelling inside processing equipment .

Proteins are large organic compounds playing a central role in the structure of all living organisms. Besides their structural and functional roles proteins are also a fundamental component of animal and human diets providing a source of energy, nitrogen and essential amino acids . Among milk proteins the major groups are caseins (beta-lactoglobulin) and whey (alpha-lactalbumin) proteins

Caseins

Caseins are phosphoproteins containing approximately 80% of the total protein content of milk .Casein is made up of many components, and the main types are alpha s1-casein, alpha s2-casein, beta-casein, and kappa-casein as defined and validated by analysis of DNA sequences .Bovine milk contains four main casein groups, namely, alpha s1-, alpha s2-, beta- and kappa-casein accounting for 38.10,36, and 13% of total proteins in milk, respectively

Beta-Casein Gene

Beta-casein (CSN2) is the most polymorphic milk protein gene with 13 variants viz. A1, A2, A3, B, C, D, E, F, H1, H2, I, G and mostly observed forms of beta-casein in dairy cattle breeds are A1 and A2 . There are 13 genetic variants of beta-casein: A1, A2, A3, A4 B, C, D, E, F, H1, H2, I, G. The most common forms are A1 and A2, while B is less common, and A3 and C are rare . The composition of beta-casein of milk and milk products has become an important economic trait of dairy animal .

History of A1 and A2 beta-casein

Interest in the distinction between A1 and A2 beta-casein proteins began in the early 1990s  via epidemiological research and animal studies initially conducted by scientists in New Zealand, who found correlations between the prevalence of milk with A1 beta-casein proteins in some countries and the prevalence of various chronic diseases. Investigations on this subject continue to raise interest among the media, the science community and entrepreneurs.

Difference between A1 and A2 beta-casein

A1 and A2 variants are differentiated by the change of one nucleotide in the position 67 of the chain (A1 histidine and A2 proline). Initial studies have indicated that the entire bovine population contained only the A2 allele, and A1 allele was created from a mutation .The nucleotide sequence change in 67th amino acid position of the beta-casein reading frame, from CCT to CAT, causes substitution of proline (A2) by histidine (A1,B) in the amino-acid sequence . This might cause a change in the secondary conformation of the protein structure and affect the physical properties of casein micelle and vulnerability to enzymatic digestion. During this enzymatic process, beta-casein opioid peptide beta-casomorphin-7 (BCM 7) is released exclusively from A1 and B variants .

A2 milk is cow’s milk that contains only the A2 variant of betacasein protein. According to the literature, more than 10,000 years ago, and before they were domesticated, cows produced only the A2 beta-casein protein and not the A1 beta-casein protein. However, some 8,000 years ago a natural single-gene mutation occurred in Holsteins, resulting in production of the A1 beta-casein protein in this breed. This mutation in the beta-casein gene led to 12 genetic variants, of which A1 and A2 are most common. The mutation was passed on to many other breeds, principally because Holsteins are used to genetically improve the production of other breeds. Slowly, the A1 beta-casein variant became dominant in milk. While dairy herds in much of Asia, Africa, and part of Southern Europe remain naturally high in cows producing A2 milk, the A1 version of the protein is common among cattle in the Western world.

In general, milks from Guernsey, Jersey, Asian herds, human milk, and others (sheep, goat, donkeys, yaks, camel, buffalo etc.) contain mostly A2 beta-casein. Milks from Holstein Friesian contain mostly A1 beta-casein. The Holstein breed (the most common dairy cow breed in Australia, Northern Europe, and the United States) comprises A1 and A2 forms of beta-caseins in approximately equal amounts. More than 50 percent of the Jersey breed carries the A2 beta-casein variant, but with considerable variation among the herd, and more than 90 percent of the Guernsey breed carries the A2 beta-casein variant.

Recently, a relationship between disease risk and consumption of a specific bovine beta-casein fraction with either A1 or A2 genetic variants has been identified. BCM7 is suggested to be associated as a risk factor for human health hazards as it can potentially affect numerous opioid receptors in the nervous, endocrine and immune system .

Impact on health of A1 gene

Opioid peptides play a role in various biological processes, including respiration, analgesia, constipation and behavior in the humans . BCM-7, opioid or narcotic as well as being an oxidant it is associated with milk intolerance and play a role in development of some human diseases such as human ischemic heart disease, diabetes mellitus, atherosclerosis, schizophrenia, autism, coronary heart disease, Autistic Spectral Disorder (ASD) and Sudden Infant Death Syndrome (SIDS) . It may also be implicated in an additional range of auto-immune conditions. This protein is also linked to milk intolerance in some people . Genetic polymorphism of bovine milk proteins has great interest in animal breeding, due to its relationships with milk production traits, milk composition and milk quality .

Recent study finds that some large scale milk producing countries which are supplying milk as food throughout the world which is still insufficient to fulfill the public food requirement due to increasing of human population, as well as in bovine milk A1betacasein type of allele present which is proved that by drinking this type of bovine milk causing human health disease, these regions are need to improve more bovine milk production, dairy cattle breeding programs as well as dairy industries by reducing A1 betacasein, which will be search of an efficient way of the improvement of good milk production and the qualitative milk traits. For good milk production and reducing of A1 beta-casein gene in bovine need to analyze the status of beta-casein A1/A2 alleles in these animals to draw a sound breeding policy and minimize the risk of disseminating the A1 allele in Indian cattle. Selection and breeding of animals with desirable genotypes is of crucial importance for the genetic improvement of dairy cows. Study strongly recommend knowing the exact status of a cow in relation to its beta-casein genetic trait i.e. either it’s A1, A2 or homozygous or heterozygous. Many researchers find the A1, A2 beta-casein polymorphism on Indigenous bovine population (Native, cross bred). Exotic cows, buffalos have revealed that A1 beta-casein is more frequent in exotic cattle while Indigenous dairy cattle and buffalo more frequent A2 beta-casein but A1 allele present in some bred.

Possible diseases

1. Human ischemic heart disease
2. Diabetes mellitus
3. Atherosclerosis
4. Schizophrenia
5. Autism
6. Coronary heart disease
7. Autistic spectral disorder (ASD) and
8. Sudden infant death syndrome (SIDS)

Controversy on beta-casein A1 gene

The controversy is that one group of scientists claim BCM a cause of many health problems in human whereas other groups report beneficial effects of BCM and declare both types of milk safe for children and adults. Two international Study Groups comprising of renowned scientists, one in New Zealand and the other by European Union (European Food Safety Authority), have gone through available publications and evidences on the subject and have concluded that fear about A1 milk was unfounded. There are also independent panel reports which have concluded that as yet there is no clinching evidence to ring public warning. The controversy in few developed countries appears to be driven by commercial motives as in New Zealand a company called A2 Corporation has been formed which owns A2 related patents and brand name and has been demanding mandatory testing of animals for purpose of labelling. Obviously, a small group of ‘Activists’ type of researchers is driving this controversy but unless concrete evidence emerges India should not fall prey to this propaganda .

Bovine β Casein variants and Human Health

Beta-Casein Protein

The biological function of the αs1-CN, αs2-CN, and β-CN is to form micelles, which are macromolecular structures responsible for the transfer of calcium to the newborn. The fourth casein (κ-CN) is the stabilizing factor of the micelles and could play an important protective function against Helicobacter pylori infections in infants .

β-casein is the most hydrophobic casein multilaterally bounded in casein micelles in an aqueous solution. As it lacks Cys, β-casein has a flexible and open conformation with a little tertiary structure. The C-terminal region of β-casein is hydrophobic, while the N-terminal domain rich in phosphate groups is highly negatively charged and polar  Phosphoserine residues in the polar part of the molecule interacts electrostatically with colloidal calcium phosphate (CCP) to form nanoclusters. The nonpolar part of the molecule enhances micellar stability by forming hydrophobic bonds with other caseins .

A study by Raynes et al. showed structural differences between bovine A1 and A2 β-casein .As already mentioned, β-casein participates in the formation of the casein micelles, forms oligomeric micelles itself, and functions as a molecular chaperone, preventing the aggregation of a wide range of proteins that also include other caseins. Differences in micelle assembly and chaperone activity may explain differences in the functionality of A1 and A2 milk. The A2 β-casein variant forms smaller micelles than the A1 β-casein. The monomer-micelle form equilibrium of the A2 β-casein is shifted toward the monomer, where the shift comes from the structural differences between the two β-casein variants associated with the adoption of the greater polyproline-II helix in the A2 β-casein, which may lead to enhanced chaperone activity of the A2 β-casein in comparison to the A1 β-casein .To the researchers’ best knowledge, the first study investigating the differences in the protein composition of casein micelles, milk whey, and fat globule membrane (MFGM) in three milk variants (A1A1, A2A2, and A1A2) was presented by Wang et al.  who used the proteomic method with a label-free approach to analyze this aspect .They found different contents of the protein cargo not only in casein micelles, but also in the whey and MFGM fractions. The overall analysis of these three fractions showed that several proteins were significantly associated with each of the milk variants including ceruloplasmin, protein S100-A9, and cathelicidin-2 in A1A1 milk, lactoferrin, protein S100-A8, CD5L, and protein S100-A12 in A2A2 milk, and selenoprotein P, β-glucuronidase, and osteopontin in A1A2 milk. However, the genetic rationale for these findings corresponding to the β-casein variants, its biological meaning, and physiological implications for consumer health remains unclear.

A1 and A2 variants are the most common among 13 known β Casein variants of cow’s milk (Giglioti R et al., 2020) . Recently, researchers have shown a keen interest in A1/A2 variants due to the hypothesis that high A1 β Casein consumption is positively associated with a range of human diseases. Both variants differ at 67th amino acid position with histidine in A1 and proline in A2. This polymorphism derives a key conformation change in β-casein protein’s secondary structure. A1 variant produces a bioactive peptide ‘opiod’ beta casomorphin 7 (BCM-7) during the gastrointestinal digestion of milk, this BCM-7 has been suggested to be linked with human health hazards as it can potentially affect many opiod receptors in nervous, immune and endocrine system (Sodhi M et al., 2012). BCM-7 is also a known oxidant of low dietary lipoproteins (LDL) which is believed to be important in formation of arterial plaque. In A1 variant, histidine has weak bond with BCM-7 and thus releases it into milk, whereas, proline in A2 has a strong bond to BCM-7 to keep it out of the milk (Bell S et al., 2006, Truswell A, 2005). BCM-7 was reported to be absorbed through newborn’s and young infant’s gastrointestinal tract, whereas, adults fail to indicate circulating BCMs. Newborn infant’s gastrointestinal tract have greater permeability enabling the target candidates for the BCM’s opioid effects (Sun Z et al., 2003, Svedberg J et al., 1985). When BCMs get absorbed through gastrointestinal tract, they must then get localized in the nervous tissue to affect endocrine and respiratory system. Evidence of strong relation between A1 milk and health issues are expanding with the explanation of positive correlation with the conditions like type-1 diabetes (Merriman T et al., 2008) [10], cardiovascular diseases (IHD) (Laugesen M and Elliott R., 2003; Truswell A., 2005) , delayed psychomotor development among children, autism, schizophrenia, sudden infant death syndrome (SIDS), auto immune diseases, intolerances and allergies (Şahin et al., 2018) . People with digestive disorders like celiac disease, ulcerative colitis, stomach ulcers, on long term antibiotic treatment are observed to be at higher risk than others (Sodhi M et al., 2012, Jianqin S et al., 2016) . Ischaemic/coronary heart disease (IHD) is one of the major cardiovascular diseases, and A1/A2 hypothesis claims, a high intake of A1 beta-casein is a risk factor for IHD (Kaminski S et al., 2007). A2 milk solely contains A2 beta casein whereas A1 milk has either only A1 or both A1A2 type variant. Source for A1 variant is milk from crossbred and European cattle breeds, whereas A2 has its origin in indigenous cows and buffaloes of India (Asia as a whole). A2 is a milk brand of A2 Milk Company (A2 Corporation) and has its majority market in Australia, New Zealand, United Kingdom and other developed countries (Kaskous S., 2020).

β-Casein Digestion

β-casein is a relatively slowly digestible protein that can be completely degraded, or due to the limited proteolysis, can release bioactive peptides with antioxidant, ACE-inhibitory, or opioid activity in a way that is dependent on the genetic polymorphism . Especially interesting here are morphine-like peptides (β-casomorphins). They are suspected to play an important role in the response to stress,Pain, regulation of food intake, or perform other pathobiological functions as they are able to bind to the opioid μ-receptors (MORs) found principally in the central nervous system, immunological system, and the gastrointestinal tract . Thus, as milk and dairy products are often the main component of the diet, their consumption may predispose hypersensitive individuals to adverse health effects. Peptides can be released from the parent structure in a few ways: hydrolysis by digestive enzymes in the digestive tract, as a consequence of microbiome activity, or as an effect of technological food processing.

World scenario

Milk, all-over and everywhere was A2 type, a genetic mutation thousand years ago in some European cattle gave birth to A1 variant; and subsequent crossbreeding with Asian and African cattle in last few thousand years, might have led A1 genes in cross bred species. A1 variant is common in cattle of the western world, predominant in the North European region (Friesian, Holstein, Ayrshire and British Shorthorn). A2 gene, on the other hand, is found only in a few old species of cows, which haven’t been genetically altered – Channel Island cows, Guernsey and Jersey, Southern French breeds, Charolais and Limousin, Zebu original cattle of Africa and the Gir cow from India. Most dominant cows of today possess A1 genes, while the low yielding Indian Gir cow, on the verge of extinction, possesses the A2 genes. Human milk, goat milk, sheep milk and other species are ‘A2- like’. The only way out for an individual is to look out for organic milk preferably from traditional Indian (Desi) cows.

Indian scenario

India is the world’s largest milk producer. Now, we stand in first row to produce and export more milk than the entire combined EU. This was achieved by an integrated cooperative milk collection system; the white revolution in 1970s and crossbreeding of indigenous cows with that of European. The 1970’s was a beginning that we go on choosing quantity over quality; this was also a beginning of extinction of the indigenous breeds. Venchur, one of the healthiest desi cows from Kerala is near to extinction. Now, more people are tracing back for milk procured from native breeds like Gir from Gujarat, Sahiwal from Punjab, and Tharparkar from Rajasthan. Cow milk is described as ‘Amrit’, The Nectar, in our scriptures. In Indian Vedas, it is referred to improve Ojas, the factor improving body’s immunity. A2 cow milk nourishes body tissues, rejuvenates life expectancy with improved strength and intelligence; it acts as natural aphrodisiac, promote digestion and increases breast milk production in feeding mothers; with assistance in easy intestinal movements, it relieves tiredness, dizziness, excessive thirst and hunger (Reddy P et al., 2016) . Recent demonstrations of the National Bureau of Animal Genetics Research explained the qualitative superiority of milk from cattle breeds; with the scanning of 22 cattle breeds researchers concluded the 100% status of A2 gene in 5 high yielder native cattle breeds i.e. Red Sindhi, Sahiwal, Tharparkar, Rathi and Gir. Whereas exotic breeds was 60% and other Indian breeds was 94%. The pure Indian cow produces A2 milk with less Betacosmophorine-7 (BCM-7), unlikely to the hybrid cows which generally produce A1 milk (Bell S et al., 2007). A study on 15 different Indian cattle breeds screened, only A2A2 and A1A2 genotypes were found and there was absence of A1A1 genotype. Interestingly and fortunately, the India native breeds including Gir, Tharparkar, Rathi, Red Sindhi, Sahiwal and dual purpose breds viz, Kankrej, Hariana had only A2 variant (Mishra B et al., 2009)

Status of Milk Proteins and their Variants among Indian Cattle Breeds

Cow milk has been used globally for its nutritive and medicinal values in all age groups, making it one of the most consumed beverages throughout the world. Its spiritual and nutritional values have been known to our ancestors since time immemorial. There are references on cow milk in ancient Vedas. Milk has special significance in Indian mythology, culture and diets. Our ancient Ayurvedic texts describe the Indian cow-milk as possessing a sweet taste, cooling in nature, highly nutritive, tonic for our vital organs, easily digestible and whose consumption promotes immunity and longevity. Many cow products are used for their health promoting, disease-preventing and therapeutic purposes. Besides its nutritive use, milk and milk products are used in conjunction with medicines to enhance their pharmacokinetic benefits. Bovine milk is a resource of lipids, proteins, amino acids, vitamins and minerals and it also contains several nutrients needed for growth and development not only for calf but also for humans. It contains immunoglobulin, hormones, growth factors, cytokines, nucleotides, peptides, polyamines, enzymes and several others bioactive peptides. Milk contains lactoferrin which is microbicidal. Casein forms about 80% and whey protein forms about 20% of the total protein content in the milk. Thus, casein becomes a major source for supply of all essential amino acids (except sulphur-containing amino acids – methionine and cysteine). Milk protein constitutes 36% a-Casein, 27% b- Casein, 9% k-casein, 27% peptides and amino acids. It also contains immunoglobulins, hormones, growth factors, cytokines, nucleotides, peptides, polyamines, enzymes and several others bioactive peptides. (Priyanka Priyatharsini et al, 2018).

Caseins

The protein fraction composition of ?-casein has become of special interest recently because of a possible relationship between b-casein genotype and the health of consumers. Casein protein is group of proteins coded by four tightly-linked autosomal genes (CSN1S1, CSN1S2, CSN2 and CSN3) linked to alpha s1-casein, alpha s2- casein, beta-casein, and kappa-casein respectively as defined and validated by analysis of DNA sequences.

Beta-Casein Gene and Beta-Casein Polymorphism

There are 12 genetic variants of beta casein: A1, A2, A3, B, C, D, E, F, H1, H2, I and G. Among these A1 and A2 forms are most common, while B is less common and A3 and C are rare. In position 67 of the b-casein chain, proline in A2 is substituted by histidine in A1. Beta casomorphin-7 (BCM7) is produced from A1 b-casein during gastrointestinal digestion. BCM-7 may get absorbed in the infants GIT as compared to adults. In A1 milk, the BCM-7 level is 4 fold higher than in A2 milk (Elliott et al, 1999). The composition of beta-casein of milk and milk products has become an important economic trait of dairy animal. Besides health promoting properties, the A2 variant has also been reported to have positive relationship with milk performance traits especially protein and milk yield. The National Bureau of Animal Genetic Research (NBAGR) demonstrated the superior milk quality of Indian cattle breeds. After scanning 22 cattle breeds, scientists concluded that in five high milk-yielding native breeds – Red Sindhi, Sahiwal, Tharparkar, Rathi and Gir – the status of A2 allele of the beta casein gene was 100 per cent. In other Indian breeds it was around 94 per cent, compared to only 60 per cent in exotic breeds like Jersey and HF. The A2 allele is responsible for making available more Omega-6 fatty acids in milk. The pure Indian breed desi cow produces A2 milk, which contains less Betacosmophorine-7 (BCM-7), as opposed to the hybrid cows which generally produce A1 milk. A1 milk produce conditions like type-1 diabetes, cardiovascular disease, delayed psychomotor development among children, autism, schizophrenia, sudden infant death syndrome (SIDS) auto-immune diseases, intolerances and allergies. (https://www.linkedin.com/pulse/differencebetween-indian-native-a2-cow-milk-cross-breed-osk-reddy) The polymorphic nature and its association with milk, fat and protein yield attracted several efforts in evaluating this locus as a potential dairy trait marker.

A1 and A2 Milk

Milk that contains A1 b-Casein and A2 b-Casein is known as A1 milk and A2 milk, respectively. Demand for A2 milk is growing across the world, especially in countries such as New Zealand, Australia, and the UK, where A2 milk has the fastest growing market share of 8% with increasing sales of 57% in a year (Sharma 2013). Even in India, this milk is gaining popularity. Amul, Desigo milk, Pathmeda and few other Indian milk brands have already launched A2 milk variant in the market (Sharma 2015). With the public becoming more aware about the harmful effects of consuming milk from European breeds of cattle, market for A2 milk is destined to grow in the future.

History of A1 and A2 Milk

A2 beta-casein is the beta-casein from cows that have been produced since before they were first domesticated over 10,000 years ago. It has no known negative effects on human health. In the past few thousand years, a natural mutation occurred in Holsteins, resulting in production of the A1 beta-casein protein in this breed. This mutation in the beta-casein gene led to 12 genetic variants, of which A1 and A2 are most common. (Roginski, 2003) The mutation was passed on to many other breeds, principally because Holsteins are used to cross many other breeds to develop high yielding crossbred cattle. Slowly, the A1 beta-casein variant became dominant in milk, which has resulted in a proportion of cows of European breeds producing a casein variant called A1 beta-casein. Slowly, these protein variant became dominant in milk producing A1 milk. . It seems that in the Western world while intensively selecting bulls for high productivity, this mutation which was low in frequency, also got unintentionally selected and multiplied. But, with advent of selective breeding for high production, better fertility and protein quality, unconsciously A1 allele carrying bulls that were genetically superior were used in breeding programs. Artificial Insemination (AI) of large number of cows with semen of such bulls multiplied A1 gene frequency in the population. The gene encoding beta-casein was changed in such a way that the 67th amino acid in the 209 amino acid betacasein protein was switched from proline to histidine.

Basic Genetics of A1 And A2 Milk and Status of Milk Protein Variants in Cattle

The A1/A2 status of a cow is determined by a gene located on the sixth chromosome. There are two major alleles of the gene i.e. A1 and A2 beta casein alleles. A cow carries two alleles of the beta-casein gene; she can carry either of A2A2 (homozygous), A1A2 (heterozygous) or A1A1 (homozygous) genotype. These alleles are co-dominant in nature. Therefore, an A1A2 cow will produce A1 and A2 beta-casein in equal amounts. An A2A2 cow will only produce A2 beta-casein and an A1A1 cow will only produce A1 beta-casein. Several surveys that A1/A2 frequency is areaspecific rather than breed-specific. Example, A1 gene frequency in Holstein Friesen of North America and North Europe cow is very high (above 90%) but in German Holstein Frisian A2 gene frequency is very high (around 97%). In other countries, frequency of A1 in Holstein Friesian is in the range of 40-65%. Guernsey breed from USA or Europe has high A2 frequency in cows and breeding bulls even more than 98% which is almost equal to Indian breeds. A2 gene frequency is usually higher in Jersey (60-80%) globally (Vet Helpline India, 2015). Genotyping of bulls for A1 and A2 allele is a good method for lowering the risk of A1 allele in human health and increase milk and protein yield (Olenski et al, 2010). The prevalence of A1 and A2 protein varies from one herd of cows to another and also between countries. A1 allele is found mainly in cattle of the western world, predominantly in the North European region (Friesian, Ayrshire, British Shorthorn and Holstein) and their crossbreds. On the other hand, Jersey and Guernsey cows in the Channel Islands, the Charolais and Limousin breeds of Southern France, and the Zebu cattle of Africa and Asia produce A2 milk. Cross breeding of Asian and African cattle with European cattle in last few years, may have led to the presence of A1 genes in the cross bred species. In India, A1 protein variant is commonly found in milk from crossbred and European breeds of cattle. A2 milk is found basically in indigenous cows and buffaloes of India (Asia as a whole). The Northern European breeds of cows such as the Friesian and Holstein carry the A1 and A2 allele at about equal levels. The Southern European breeds and the Jersey carry the A1 allele at about 35% and 65% of A2. Exceptionally, Guernsey breed appears to carry the A1 allele at less than 10% and the Scottish Ayrshire breed appears to be well over 50%. In addition, individual herds may carry the allele at levels that are quite different to the average for the breed. In general, milks from Guernsey, Jersey, Asian herds, human milk, and others (sheep, goat, donkeys, yaks, camel, buffalo etc.) contain mostly A2 beta-casein.

Status of Cattle in India

The world’s largest cattle population resides in India. There are around 192.49 million cattle in India as per the Ministry of Agriculture, India (2019) report. Out of the total, 140.11 million cattle are of indigenous origin belonging to the Bos indicus species and 50.42 million cattle are of exotic/cross bred cattle (Ministry of Agriculture, India 2019). The A2 allele gene in Indian milk breeds of cows and buffaloes are 100% (Red Sindhi, Sahiwal, Tharparkar, Gir and Rathi), other Indian breeds used for farming, is around 94 per cent (Joshi, 2011) and while in foreign breeds (HF and Jersey), it is around 60 per cent (NBAGR, 2011). A1 b-casein is absent in the milk of pure Asian and African Cattle (Ng-Kwai-Hang and Grosclaude, 2002). The A2 allele gene in Indian milch breeds of cows and buffaloes are 100 per cent (Red Sindhi, Sahiwal, Tharparkar, Gir and Rathi), other Indian breeds used for farming is around 94% and while in exotic breeds (HF and Jersey), it is around 60 per cent (NBAGR, 2011). A study conducted by National Bureau of Animal Genetic Resources (NBAGR), Karnal revealed that in 22 indigenous cattle breeds, predominant casein genotype was A2A2, confirming that our indigenous cows and buffaloes produce A2 milk. The frequency of A2 allele was 100 per cent in the five high-yielding milk breeds viz. Red Sindhi, Gir, Rathi, Shahiwal and Tharparkar. In the remaining breeds, the availability of A2 allele was 94%. NBAGR also reported moderate to high frequency of A2 allele in the breeding bulls. Comparatively, in the exotic breeds like Jersey and Holstein Friesian, the availability of A2 allele is very low. NBAGR is implementing a project entitled “Delineating Beta Casein Variants in Indian Cows and potential health implications of A1A2 Milk”. The Bureau is also offering a service for genotyping of A1/A2 allele from milk or blood samples in cattle species at nominal cost. Private companies in India have also ventured into this by offering allele detection kits. In India, the National Bureau of Animal Genetic Resources (NBAGR), the National Dairy Research Institute and the Indian Veterinary Research Institute started research on A1 and A2 milk since 2009.

Branded A2 Milk in India

In the recent past, several dairy units (local units and some organized players) in India have started offering A2 milk at a premium price to consumers. A few of the A2 milk brands are Amul Deshi, Desigo, Haritas, GoShrushti, etc. Considering the lack of conclusive scientific evidence for A1/BCM-7 ill-effects on human health, and predominance of A2 milk in Indian cattle, there is no need for consumers to switch to milk branded as A2. Prevalence of A1 milk protein is not a major problem in India at present, and with increased consumer awareness on A1/A2 milk over time, the A2 branded milk in India may not sustain itself in future.

Compiled  & Shared by- Team, LITD (Livestock Institute of Training & Development)

Image-Courtesy-Google

Reference-On Request.