Cow’s milk contains various proteins, including beta-casein, which exists in two common forms: A1 and A2. There is growing interest in how these protein types might influence human digestion, leading to questions about specific dairy breeds like Jersey cows.
What Are A1 and A2 Milk?
Beta-casein makes up about 30% of the protein in cow’s milk, with A1 and A2 being the most prevalent genetic variants. The difference lies in a single amino acid at position 67: A1 beta-casein has a histidine, while A2 beta-casein has a proline.
This subtle structural variation influences how the proteins break down during digestion. When A1 beta-casein is digested, it can release a peptide called beta-casomorphin-7 (BCM-7). In contrast, A2 beta-casein breaks down differently and does not produce significant amounts of BCM-7. Historically, it is believed that all cows originally produced milk containing only the A2 beta-casein protein.
The A1 variant is thought to have emerged from a natural genetic mutation in European dairy herds several thousand years ago. As a result, most conventional milk available today typically contains a mixture of both A1 and A2 beta-casein proteins.
Genetic Makeup of Jersey Cows
The type of beta-casein a cow produces is determined by its genetic makeup. Each cow inherits two copies of the beta-casein gene, meaning it can be A1/A1 (producing only A1 protein), A1/A2 (producing both), or A2/A2 (producing only A2 protein). Only cows with the A2/A2 genotype will produce milk that contains solely the A2 beta-casein.
Jersey cows, along with other heritage breeds like Guernsey and Normande, have a higher natural prevalence of the A2 gene. For instance, more than 50% of Jersey cows are reported to carry the A2 beta-casein gene, with some sources indicating around 60% or even 66% in certain populations. While Jerseys show a higher likelihood of producing A2 milk compared to breeds like Holsteins, it is important to note that not all Jersey cows are A2/A2.
To confirm whether an individual cow produces only A2 milk, genetic testing is necessary. This can be done through various methods, including hair, blood, or milk samples. Hair and blood tests offer about 99% accuracy, while milk sample testing is approximately 94% accurate. This genetic screening allows dairy farmers to identify and selectively breed cows to establish herds that consistently produce A2 milk.
Why A2 Milk Matters
The interest in A2 milk stems largely from anecdotal reports and ongoing research regarding its potential impact on digestive comfort. Many individuals experience digestive issues after consuming conventional milk, which can sometimes be mistaken for lactose intolerance. However, some research suggests these symptoms might be linked to the A1 beta-casein protein rather than lactose itself.
The peptide BCM-7, released during the digestion of A1 beta-casein, is thought to interact with the digestive system, potentially affecting gut motility and contributing to inflammation. Because A2 milk does not produce significant amounts of this peptide, it is often perceived as gentler on the stomach for some individuals.
Studies have indicated that switching to A2 milk may reduce gastrointestinal discomfort for some people. It is important to understand that A2 milk still contains lactose, so it is not a solution for individuals with diagnosed lactose intolerance. While the scientific community continues to explore the full extent of A1 and A2 milk’s effects, the focus remains on its potential to offer a more comfortable dairy experience for certain consumers.