What is the difference between casein 1 and 2?

November 26, 2025 •

4 min read

Casein is the primary protein found in milk, comprising about 80% of its total protein content. The core of the debate surrounding A1 and A2 milk is a subtle but significant distinction in their beta-casein protein structure, which influences how the body digests milk and its potential health effects.

Quick Summary

This article explores the genetic mutation that created the A1 variant of beta-casein and its impact on digestion compared to the original A2 version. It details the release of the peptide BCM-7 from A1 protein and its potential effects on the gut and overall health, based on current research.

Key Points

Genetic Difference: The distinction between A1 and A2 casein is a single amino acid at position 67 of the beta-casein protein chain: histidine in A1 and proline in A2.
Digestion Byproduct: A1 beta-casein releases the peptide beta-casomorphin-7 (BCM-7) during digestion, while A2 beta-casein does not produce this compound.
Digestive Comfort: Some individuals with perceived dairy sensitivity find A2 milk easier to digest than milk containing A1 protein, experiencing less bloating, gas, and pain.
Sources of Casein: Milk from modern cow breeds like Holstein is often high in A1 protein, while older breeds, as well as milk from goats and sheep, contain mostly A2 protein.
Controversial Health Claims: Claims linking A1 casein to chronic diseases like type 1 diabetes and heart disease are not definitively proven, and the evidence remains mixed and debated.
Personal Choice: The best way to determine personal tolerance is to test if your body feels a difference when switching from conventional milk to A2 milk or other A2-containing dairy sources.
Allergy vs. Intolerance: A1 casein sensitivity is different from lactose intolerance and a true milk protein allergy; A2 milk still contains lactose and milk protein.

What Exactly is Beta-Casein?

To understand the difference between casein 1 (A1) and casein 2 (A2), it's essential to look at beta-casein, a major casein protein found in cow's milk. All beta-casein proteins consist of a chain of 209 amino acids. Historically, all cows produced milk containing only the A2 beta-casein variant. However, a genetic mutation occurred thousands of years ago, altering a single amino acid in the chain, which led to the creation of the A1 variant. This small structural difference is the basis for the distinction between A1 and A2 milk.

The Genetic Mutation: Proline vs. Histidine

The key to this difference is the amino acid at position 67 of the beta-casein chain. In A2 beta-casein, this position is occupied by the amino acid proline. In A1 beta-casein, a genetic mutation caused this amino acid to change to histidine. This seemingly minor substitution has a major consequence during digestion.

BCM-7: The Digestive Byproduct of A1 Casein

The histidine at position 67 in A1 beta-casein holds onto its neighboring amino acids less strongly than the proline in A2 beta-casein. As a result, during digestion, the A1 beta-casein chain can be broken down to release a specific peptide called beta-casomorphin-7 (BCM-7). BCM-7 is an opioid-like compound that some studies have linked to digestive issues and other health concerns. A2 beta-casein, due to the stronger proline bond, does not release significant amounts of BCM-7 upon digestion.

Potential Health Implications and Research Findings

The debate over A1 versus A2 milk centers on the effects of BCM-7. While many studies have been conducted, the scientific community's conclusions are not unanimous, and more research in humans is needed.

Digestive Comfort: A primary claim from A2 milk producers is that their milk is easier to digest and causes less stomach discomfort, such as bloating and gas, for sensitive individuals. Some human studies have found that A1 milk is associated with increased gastrointestinal inflammation and discomfort in some people, while A2 milk caused no such effects. It's worth noting that some symptoms attributed to lactose intolerance might actually be related to A1 casein sensitivity in some cases, although A2 milk still contains lactose.
Other Health Concerns: Some initial observational studies suggested potential links between A1 beta-casein and type 1 diabetes, heart disease, and neurological disorders like autism. However, subsequent, more controlled human studies have largely failed to find a definitive link, and the evidence remains inconclusive. Major food safety authorities, including the European Food Safety Authority (EFSA), have stated that there is no established cause-and-effect relationship between A1 protein and these chronic diseases.

The Casein Content of Different Milks

Not all cow's milk is the same. The type of beta-casein produced depends on the breed of cow. Dairy cow breeds from northern Europe, such as Holstein and Friesian, are more likely to produce milk with a high proportion of A1 beta-casein. Conversely, older cow breeds and those from Africa and Asia, like Guernsey, Jersey, and indigenous Indian breeds, typically produce milk with only the A2 protein. Milk from goats, sheep, and buffalo also naturally contains A2 beta-casein. Today, some dairy companies use DNA testing to identify and separate cows that only produce the A2 protein, marketing the resulting product as A2 milk.

What is in Your Glass?

Here is a general breakdown of beta-casein types found in various milks:

Standard Cow's Milk (Western): Contains a mixture of both A1 and A2 beta-casein.
A2 Milk: Specially sourced milk from cows that produce only the A2 variant.
Goat's Milk: Primarily contains A2 beta-casein.
Sheep's Milk: Primarily contains A2 beta-casein.
Human Milk: Contains A2 beta-casein.

Comparison Table: Casein 1 (A1) vs. Casein 2 (A2)


Feature	Casein 1 (A1)	Casein 2 (A2)
Genetic Variant	A mutation that arose several thousand years ago in European cattle.	The original, older beta-casein gene variant.
Key Amino Acid	Histidine at position 67 of the protein chain.	Proline at position 67 of the protein chain.
Digestion Byproduct	Releases beta-casomorphin-7 (BCM-7) during digestion.	Prevents or limits the release of BCM-7 during digestion.
Common Sources	Found in milk from modern breeds like Holstein, Friesian, and Ayrshire.	Found in milk from older breeds like Guernsey and Jersey, and non-cow milk.
Digestive Impact	Some individuals report experiencing digestive discomfort, such as bloating and gas.	Often perceived as easier to digest and gentler on the gut for sensitive people.
Health Implications	Observational studies have raised questions about links to certain chronic diseases, though evidence is not conclusive.	Not associated with the same theoretical health concerns linked to BCM-7.

How to Choose the Right Milk for You

For most people, the difference between A1 and A2 milk is negligible, and regular milk is perfectly fine. However, if you experience digestive discomfort after drinking milk and have ruled out lactose intolerance, experimenting with A2 milk might be worthwhile. The effects can be highly individual, and what works for one person may not work for another. The debate continues among health experts, and robust, independent human trials are still ongoing to provide more definitive answers. Making an informed choice involves considering your personal tolerance and any digestive symptoms you experience.

Conclusion

The difference between casein 1 and 2 is fundamentally a single amino acid change in the beta-casein protein chain. This seemingly minor genetic variation can impact how milk is digested, with A1 beta-casein potentially releasing the peptide BCM-7, which has been linked to digestive discomfort in some individuals. A2 beta-casein, lacking this specific amino acid substitution, is thought to be more easily digestible for those with sensitivity to A1 protein. While compelling for some with digestive issues, the evidence connecting A1 milk to more serious long-term health problems remains inconclusive. Ultimately, individual experience is the best guide for determining which milk is the most comfortable and beneficial for your body.

Frequently Asked Questions

The primary difference lies in a single amino acid at position 67 of the beta-casein protein. A1 casein has histidine, while A2 has proline. This seemingly small change affects how the milk is digested.

BCM-7 (beta-casomorphin-7) is an opioid peptide released during the digestion of A1 beta-casein. Some research suggests it may be responsible for digestive discomfort in sensitive individuals, as it affects gut transit time.

A2 milk is not a substitute for lactose-free milk, as it still contains lactose. However, some people who experience lactose intolerance-like symptoms may find A2 milk easier to digest because their symptoms were actually triggered by the A1 protein and not the lactose.

Naturally, milk from goats, sheep, buffalo, and human breast milk contains only A2 beta-casein. Among cows, older breeds like Guernsey and Jersey are more likely to produce milk with a high proportion of A2 protein. Special A2 milk products are also widely available from tested A2-only herds.

While initial studies correlated A1 milk with certain chronic diseases, later and more controlled research has not conclusively established a cause-and-effect relationship. The health effects remain a topic of debate and further research.

There is no widely available blood test for A1 casein sensitivity. The best way to determine if you react to A1 protein is through a self-conducted elimination diet, switching to A2 milk and observing any changes in your digestive comfort.

From a nutritional standpoint, A1 and A2 milk are nearly identical in terms of fat, calcium, and protein content. The only difference is the protein variant.