Unravelling the secrets of milk
Cow’s milk has a significant place in human nutrition, especially children, because of the biological value of the basic nutrients related to protein, fat and water-mineral elements, and an essential energy source for the body.
It is particularly worth noting the wealth of biologically active compounds, in the form of peptides, that arise from the enzymatic digestion of the milk mother proteins. Cow milk generally contains beta-casein (CSN2), although there are 13 genetic variants: A1, A2, A3, A4, B, C, D, E, F, H1, H2, I and G [Singh et al., 2024]. Only seven (A1, A2, A3, B, C, I and E) have been detected in European breeds. The A2 variant is considered to be the oldest variant, which gave rise to others through mutation.
Around 10,000 years ago, the first domesticated cows produced milk with beta-casein exclusively in the A2A2 variant. However, sometime later, a natural mutation arose that caused cows to produce the A1 variant of beta-casein. The mutation spread through the cow population. This probably happened as a result of using Holstein-Friesian cows (the globally dominant breed in milk production) to improve other breeds through the so-called Holsteinisation process.
A2 milk contains a specific type of protein – the A2 beta-casein version. As consumers, we are not able to feel or see the difference in taste, smell or texture between milk containing different polymorphic proteins e.g. A1 and A2 beta-casein. Significant amounts of BCM7 are detected in the blood of humans and animals drinking ‘standard’ milk (which contains a mixture of A1 and A2 casein). Meanwhile, in the population that consumes A2 milk, it is not present.
Beta-casein is one of the milk casein protein fractions consisting of 229 amino acids. The sole difference is that a proline occurs at position 67 in the chain of amino acids that make up the A2 beta-casein, while in A1 beta-casein, a histidine occurs at that position. The A1 beta-casein protein breaks down into a peptide called BCM-7. It is a part of beta-casein with opioid properties, i.e. similar to morphine in its action. It has adverse effects on both animal and human health. When proline bonds to it, the connection is so strong that it is difficult to separate this harmful fragment. When bound by histidine, it can be easily separated. This is very important in the digestive process.
Animal studies have shown that milk containing A1 β-casein increases digestive transit time and enhances myeloperoxidase activity. Individuals with lactose intolerance prefer A2 milk to conventional A1 milk, as BCM-7 in A1 milk can lead to inflammation and discomfort in sensitive individuals. Important parts of human health involving the potential activity of BCM-7 are still poorly defined and absent in some studies.
Studies on digestive diseases are important because of their link to other aspects of potential health problems for dairy consumers, i.e. lactose intolerance or hypersensitivity to non-casein cow’s milk proteins, e.g. β-lactoglobulin, which is not present in human milk.
A2 milk, which contains A2 β-casein, is believed to be more easily digestible than A1 β-casein. Its popularity has grown owing to reports linking A1 casein to diseases such as type 1 diabetes, heart disease, and autism. A2 milk has gained popularity as an alternative to A1 milk, primarily because of its potential benefits for individuals with certain diseases. This review aims to provide an updated understanding of A2 milk consumption and its health benefits. This review aims to provide an updated understanding of A2 milk consumption and its health benefits. Consumption of A2 milk significantly changed gut microbiota and increased the content of short-chain fatty acids in the gut. Lijun [2021] examined the effect of variant β-casein A2 on intestinal epithelia. Lijun observed that bowel movements and stool structure were improved compared to the consumption of commercially available milk. Moreover, the amount of Bifidobacterium spp. in the human gut increased with the consumption of A2 milk. However, one limitation of this study is that the characteristics of the milk tested are unknown, and therefore it is impossible to draw any good conclusions from the comparison of casein types.
Li’s latest study [2024] was to investigate the immunomodulatory effects of A2 β-casein (β-CN) in cyclophosphamide-induced immunosuppressed BALB/c mice. Experiments conducted in vitro revealed that A2 β-CN digestive products have potent immunostimulatory activities. The effect of A2 β-CN significantly increased the proliferation of splenic lymphocytes, the phagocytosis of macrophages, and NK cell activity. A2 β-CN can improve the concentration of SCFAs, adjust the diversity and composition of the gut microbiota in CTX-induced immunosuppressed BALB/c mice, enhance the intestinal mucosal immune function, including recovering the immune organ index, enhancing immune cell functions, and reducing spleen injury. This result further suggests that A2 β-CN has the potential to act as a gut microbiota and immune modulator.
Nowadays, most milk contains mainly beta-casein protein in the A1 form, and although some cows produce milk with the A2 variant, store-bought milk contains both the A1 and the A2 variant of beta-casein. 100% of A2 milk is produced by the so-called, primitive Indus and Masal breeds, as well as yaks, donkeys and camels. All over the world, A2 milk has been known for several decades. The A2 Company, founded in 2000 in New Zealand, is a prime example. Its share of the Australian market grew from 0% in 2007 to as much as 9% in 2014. In 2020, A2A2’s global milk market was worth $4574.6 million, and by 2026 it is forecast to reach $15610 million, a 19% growth rate. A2-labelled milk from genotyped cows is now entering the shops in Poland.
Developed by: Prof. dr hab. Beata Kuczyńska, Katedra Hodowli Zwierząt, Instytut Nauk o Zwierzętach
Dr inż. Alicja Ponder, Katedra Żywności Funkcjonalnej i Ekologicznej, Instytut Nauk o Żywieniu Człowieka
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