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A1 and A2 ß-Casein: The Two Phases of Milk Proteins
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Tuesday, 31 December, 2019, 14 : 00 PM [IST]
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Ashoka S
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Milk is defined 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 ones. (Ahmet Fatih D et al., 2018)
Milk is reflected as one of the crucial foods all over the world, providing an imperative source of nutrients including high quality protein, carbohydrates and particular micronutrients. Being rich in these constituents, milk has been gazed as nature’s flawless food. Milk is an emulsion of oil in water, comprising of 87.7% water, 3.3% protein, 4.9% lactose, 3.4% fat, 0.70% minerals and 3.36% minor components. Milk contains various types of protein, out of which predominantly two are considered imperative, Whey protein (14%) (Roginski. 2003) and Casein (80%) (Niki et al., 1994; Groenen, et al., 1994)
A1 and A2 milk
Milk from breeds of cows that originated in Northern Europe is generally high in A1 ß-casein and it comes from breeds like the Holstein, Friesian, Ayrshire and British Shorthorn. Milk that is high in A2 ß-casein is mainly found in breeds that originated in India, Channel Islands and so on. This includes breeds like the Gir, Sahiwal, Tharparkar and Ongul.
A1 and A2 beta-casein protein differs from each other in single amino acid sequencing at “67th” position. For A2 beta-casein protein, at 67th place “Proline” is present, whereas for A1 beta casein protein in 67th place, “Histidine” is found. This polymorphism is responsible to cause changes in digestion patterns. A1 beta-casein digestion by digestive enzymes develops bioactive seven-amino-acid peptide called as “Beta-Casomorphin-7” (BCM-7) and, on the other hand, A2 beta casein digestion results in minimal development and least release of BCM-7.
A2 milk history and production aspect
- Researchers became interested to understand the effects of proteins and peptides, and research was initiated to find the effects of peptides in human digestion and overall health.
- In 1990s, RB Elliott, McLachlan and collaborators reported that consumption of milk containing a particular class of protein may increase the chances of coronary heart diseases, type-1diabetes and other diseases.
- A2 Corporation was established in New Zealand by Dr Corran McLachlan, which initially started with the breeding of cows for A2 milk production, checking A2 protein and launched A2 milk.
Milk proteins
- It contains all nine essential amino acids, making milk protein suitable essential food for human.
- Total milk protein is categorised under two heads: casein and whey protein.
- Casein has the unique property of coagulation at or below pH 4.6, whereas the whey protein part remains in soluble form.
Based on this acid coagulation property, the protein is classified as casein and whey protein.
- The protein is 80% casein and 20% whey. Beta casein is 30% of the total protein content in the milk, or about 30% of the total protein content.
Casein
Casein is a composite of several components and is the predominant class of proteins in milk. There are four chief sub-groups of Casein, aS1 Casein (39–46% of total caseins), aS2 Casein (8–11%), ß Casein (25–35%), ? Casein (8–15%) (Roginski. 2003), which are all heterogeneous and consist of several genetic variants. Among Caseins, ß Casein is the second copious protein fraction in cow milk.
Beta-casein is the most polymorphic milk protein gene with 13 variants viz. A1, A2, A3, B, C, D, E, F, H1, H2, I, G and the most common forms are A1 and A2, while B is less common, A3 and C are rare. The composition of beta-casein of milk and milk products has become an important economic trait of dairy animal.
Biological function of caseins is to carry calcium and phosphate and to form a clot in the stomach for efficient digestion.
A1 ß-Casein
Digestive enzymes act differently upon A1 and A2 beta-casein proteins during digestion process. Beta-casomorphin-7 (BCM-7) is a bioactive seven-amino peptide is released by digestive enzymes from the A1-beta-casein protein but these enzymes cannot split the A2 protein due to presence of proline at that location. So BCM-7 is not released from A2 proteins digestion. BCM-7 interacts with the human gastrointestinal tract, internal organs, brainstem and is regarded as the “devil” in A1 milk (snowvillecreamery.com, Truswell, 2005).
A2 ß-Casein
- About 39% of the protein in cow’s milk is ß-casein.
- Transports essential minerals and nutrients such as calcium and phosphorous.
- Nowadays mostly 25-30% of people are lactose-intolerant. But the indigestion of milk in the stomach occur not only by the lactose but also by the A1 type protein. So, consumption of A2 milk has less chances of causing lactose intolerance.
Benefits of A2 Milk
- A2 milk is free from BCM-7 and hence is safe for consumption (Pattanayak, 2013).
- The populations consuming milk having more beta-casein A2 exhibit a lower occurrence of cardiovascular disease and Type 1 diabetes (Sodhi et al., 2012).
- People consuming A2 milk had better stool consistency, less incidence of bloating and less abdominal pain (Ho et al., 2014).
- Regular milk contains A1 ß-casein, which is partly broken down into fragments of proteins and amino acids like ß-casomorphin-7 (BCM-7) in the stomach. ß-casomorphin-7 a peptide fragment and µ-opioid receptor agonist, is formed as a result of the incomplete digestion of A1 ß-casein.
- In contrast to the A1 beta casein, there is no evidence of opioid effects from the A2 beta casein in relation to either food transit times or pro inflammation effects. (Bandana, et al., 2015).
Conclusion
A1 and A2 beta casein are overall similar, but only differing in amino acid sequencing at 67th position, resulting in release of BCM- 7 during gastrointestinal proteolytic digestion of A1 beta casein, but not in A2 beta casein. A1 beta casein has been related to various disease risk factors. However, some pieces of evidence in relation to human illnesses are not strong enough and should be verified. Therefore, it is necessary to continue research into the role of BCM-7 (originating from both raw and processed milk) for human health.
(The author is Sr MSc, FSN Dept)
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