What are A1 and A2 proteins and how do they differ?

November 16, 2025 •

3 min read

Beta-casein constitutes about 30% of the total protein in cow's milk, and its two primary variants, A1 and A2, have sparked significant debate. Understanding what are A1 and A2 proteins is crucial, as they differ by a single amino acid, which can influence how the milk is digested and processed by the body.

Quick Summary

A1 and A2 are genetic variants of beta-casein protein found in cow's milk that differ due to a single amino acid substitution. The A1 variant's digestion releases the peptide BCM-7, which some research links to digestive discomfort, while the A2 variant does not release this peptide. This variation may explain why some individuals experience issues with conventional milk but tolerate A2 milk better.

Key Points

Beta-Casein Variants: A1 and A2 are different genetic variants of beta-casein protein in cow's milk.
Single Amino Acid Difference: The primary distinction is the 67th amino acid: histidine in A1 and proline in A2.
BCM-7 Release: A1 protein's digestion can release the bioactive peptide BCM-7, which has been linked to digestive issues in sensitive individuals.
Potential for Improved Digestion: Some people who experience digestive discomfort with conventional milk report feeling better when they consume A2-only milk.
Scientific Evidence Varies: While evidence for digestive effects is moderate, stronger, human-based evidence is still needed to confirm links between A1 milk and other chronic health conditions.
Source of Variants: A2 milk comes from specific cow breeds selected to produce milk with only the A2 protein, whereas most conventional milk contains a mix of A1 and A2 proteins.

The Genetic Origin of A1 and A2 Proteins

A minor genetic mutation that occurred in cattle thousands of years ago is the fundamental difference between A1 and A2 beta-casein proteins. Initially, all cows produced milk with only the A2 beta-casein protein, similar to human, goat, and sheep milk. A natural mutation in some European dairy herds approximately 5,000 to 10,000 years ago changed the 67th amino acid from proline to histidine, resulting in the A1 beta-casein variant. Many modern European dairy breeds, like Holstein-Friesian, now produce milk containing both A1 and A2 proteins.

The Digestive Difference: BCM-7

This single amino acid change significantly impacts protein breakdown during digestion. Digestion of A1 beta-casein releases beta-casomorphin-7 (BCM-7), a bioactive opioid peptide. Some studies suggest BCM-7 can bind to gut receptors, potentially slowing digestion and contributing to symptoms such as bloating and discomfort in some individuals.

The A2 beta-casein variant, with proline at position 67, maintains a tighter protein structure, preventing the significant release of BCM-7 during digestion. This digestive difference is the primary basis for claims that A2 milk may be easier to digest for some people, potentially alleviating gastrointestinal issues often confused with lactose intolerance.

How A1 and A2 Affect Digestion

A1 protein: Releases BCM-7, which may affect gut motility and potentially cause inflammation.
A2 protein: Minimal to no BCM-7 release, potentially leading to smoother digestion.
Self-reported improvement: Individuals with digestive sensitivity to regular milk sometimes report improved symptoms with A2 milk.
Lactose content: Both A1 and A2 milk contain lactose, so A2 milk does not alleviate true lactose intolerance symptoms.

A1 vs. A2 Milk: A Comparison


Feature	A1 Milk (Conventional Milk)	A2 Milk	Potential Health Implications
Beta-Casein Variants	Contains both A1 and A2; A1 is common in European breeds.	Contains only A2; found in specific breeds and indigenous cattle.	May cause more digestive discomfort for sensitive individuals due to BCM-7 release.
Digestive By-products	Releases BCM-7 during digestion.	Releases minimal BCM-7.	BCM-7 is linked to potential digestive issues in some people.
Gastrointestinal Effects	Linked to potential inflammation and symptoms similar to lactose intolerance in sensitive people.	Associated with improved comfort for some with dairy sensitivity.	Individuals sensitive to BCM-7 may find A2 milk easier to digest.
Historical Origin	A newer variant from a mutation in European cows.	The original beta-casein form found in all mammals and older breeds.	A1 is a relatively new protein in the human diet.
Nutritional Profile	Same essential nutrients, vitamins, and minerals as A2 milk.	Same essential nutrients, vitamins, and minerals as A1 milk.	Overall nutritional content is similar, but digestion may differ.

The Scientific Debate and Market Growth

The extent of BCM-7's impact on human health beyond digestive comfort is debated. Early studies hinted at links between A1 milk and conditions like heart disease, but human trials have been inconclusive.

Some human studies show A2 milk reduces digestive symptoms in those who report milk intolerance.
Other research on health markers like cardiovascular indicators found no significant difference between A1 and A2 milk.
The market for A2 milk is growing as consumers seek potentially gentler dairy options.

Switching to A2 milk for digestive issues is a personal decision that may offer relief, even while broader health claims are still being researched.

A Note on Other Health Claims

Some advocates suggest other benefits for A2 milk, though these claims lack strong human clinical evidence. Research on potential links between A1 milk and inflammation, cognitive effects, or infant feeding concerns is ongoing but not conclusive.

Conclusion: Making an Informed Choice

Understanding what are A1 and A2 proteins allows for informed dairy choices. The key difference, a single amino acid, affects digestion by potentially causing BCM-7 release from A1 milk, which can cause discomfort for some. While broader health impacts are still being studied, A2 milk may be an option for those with digestive issues from conventional dairy, provided it's not a true lactose intolerance or allergy. Monitoring your body's response is key. For more on scientific findings, a review of clinical studies is available from Nutrients.

Trial A2 milk: If you have post-dairy discomfort but aren't lactose intolerant, try A2 milk.
Seek professional advice: Consult a healthcare provider for suspected allergies or persistent digestive problems.
Distinguish sensitivity: Understand the difference between lactose intolerance and potential A1 protein sensitivity.

Both A1 and A2 milk are nutritious. The choice depends on individual digestive sensitivity to the beta-casein variant and the potential effects of BCM-7 from A1 milk.

Frequently Asked Questions

The main difference is the type of beta-casein protein they contain. A1 milk contains both A1 and A2 beta-casein, while A2 milk contains only the A2 beta-casein variant.

BCM-7 (beta-casomorphin-7) is a peptide released during the digestion of A1 protein. Some research suggests BCM-7 can cause digestive discomfort and inflammation, though more studies are needed for conclusive evidence in humans.

A2 milk is not a solution for lactose intolerance. It contains the same amount of lactose as conventional milk. A2 milk may help those sensitive to the A1 beta-casein protein, but those with true lactose malabsorption will still experience symptoms.

Historically, all cows produced A2 milk. Today, breeds like Guernsey and Jersey, along with many indigenous Indian breeds, are known to produce milk higher in A2 protein, while European breeds like Holstein-Friesian typically produce milk with a higher A1 content.

No, A2 milk has the same nutritional content, including protein, vitamins, and minerals, as conventional A1 milk. The difference lies solely in the composition of the beta-casein protein.

A2 milk is produced by specially selected herds of cows that are genotyped to ensure they only produce the A2 protein variant. This is not a technological process but rather a selection of animals that naturally produce the desired protein.

Yes, A2 milk can be suitable for children, especially if they have digestive issues with conventional milk. However, it's important to consult a pediatrician, particularly for children under 12 months or those with a diagnosed milk allergy.