Understanding the Health Controversy: What is the Disease Caused by A1 Milk?

January 2, 2026 •

3 min read

According to a 2017 review of human studies, consumption of milk containing A1 β-casein may negatively impact gut health by altering microbial composition and reducing intestinal motility. For decades, the dairy industry and scientific community have debated the specific health risks and what is the disease caused by A1 milk in some individuals, focusing on a protein called A1 beta-casein.

Quick Summary

The health debate around A1 milk centers on the protein A1 beta-casein, which releases a peptide called BCM-7 upon digestion. While some studies have suggested links to digestive discomfort and chronic conditions like type 1 diabetes and heart disease, evidence remains inconclusive, and much research is observational.

Key Points

BCM-7 Peptide: Digestion of the A1 beta-casein protein in regular milk releases a bioactive peptide, BCM-7, which is a key player in the health controversy.
No Proven Causal Disease: While controversial claims have existed, no specific disease has been conclusively proven to be caused by A1 milk in the general population.
Gastrointestinal Discomfort: The strongest evidence links A1 milk consumption to increased digestive issues like bloating, gas, and abdominal pain in some sensitive individuals.
A2 Milk as an Alternative: A2 milk, which lacks the A1 protein, does not produce BCM-7 during digestion and is often a better-tolerated option for those with sensitivities.
Conflicting and Inconclusive Research: Studies on A1 milk's link to major health problems like type 1 diabetes and heart disease are often observational and have yielded conflicting results, with no clear consensus.
Individual Sensitivity is Key: The effects of A1 milk appear to be highly dependent on individual tolerance and genetic makeup, not affecting everyone equally.
Further Research Needed: Major regulatory bodies and researchers agree that more high-quality human trials are needed to fully understand the long-term health implications.

The dairy industry and the scientific community have long been at odds regarding the potential health effects of A1 milk. Unlike standard lactose intolerance, which is related to milk sugars, the A1 milk controversy revolves around a specific protein variant called beta-casein. The central claim is that during digestion, the A1 protein releases a bioactive peptide, beta-casomorphin-7 (BCM-7), which has been linked to various adverse health outcomes, particularly in sensitive individuals. This article explores the science behind the A1 milk debate and addresses the critical question: What is the disease caused by A1 milk, and what does the evidence truly suggest?

The A1 vs. A2 Milk Distinction

To understand the controversy, one must first grasp the fundamental difference between A1 and A2 milk. This distinction originates from a genetic mutation that occurred in European cow breeds centuries ago.

The Genetic Mutation and Protein Variation

The difference lies in the beta-casein protein structure. A2 milk contains a proline amino acid at position 67, which is the original form found in indigenous Indian cows and other animals like goats and sheep. A genetic mutation resulted in a histidine amino acid at this position in A1 beta-casein, common in breeds like Holstein and Friesian.

The BCM-7 Connection

During digestion, the histidine in A1 beta-casein allows enzymes to release the BCM-7 peptide. The proline in A2 beta-casein prevents this. BCM-7 is an opioid peptide that may interact with the body's systems, including digestive health.

The Health Concerns Linked to A1 Milk

While direct causation is unproven for many claims, A1 milk has been linked to potential health issues, primarily as a risk factor in susceptible individuals.

Gastrointestinal Discomfort

The most supported link is to digestive issues like bloating, gas, and pain in some individuals. BCM-7 may slow digestion and increase gut inflammation. Switching to A2 milk has been reported to help alleviate these symptoms.

Potential Links to Chronic Diseases

Early studies suggested links between A1 milk and chronic conditions, but later research has been inconclusive.

Type 1 Diabetes: Some studies correlated A1 milk intake with a higher risk, potentially through immune system interactions. However, animal and human trials haven't confirmed a causal link.
Heart Disease: Some epidemiological data showed a correlation, and animal studies suggested a link to arterial plaque. However, human relevance is debated, and regulatory bodies haven't found sufficient evidence.
Neurological Disorders: The opioid nature of BCM-7 led to speculation about its role in conditions like autism and schizophrenia. Animal studies showed BCM-7 crossing the blood-brain barrier, but human data is lacking.

A Comparative Look: A1 vs. A2 Milk


Feature	A1 Milk	A2 Milk
Protein Type	Contains both A1 and A2 beta-casein	Contains only A2 beta-casein
Origin	Common in modern European breeds (e.g., Holstein, Friesian)	Common in older breeds and indigenous cows (e.g., Jersey, Guernsey)
Digestion	Can release BCM-7 peptide during digestion	Does not release BCM-7 during digestion
Digestion Speed	May slow down food digestion in sensitive individuals	May be easier to digest for some people
Gastrointestinal Effects	Linked to symptoms like bloating, gas, and discomfort in some	Often associated with fewer digestive issues and reduced inflammation
Chronic Disease Links	Some studies suggest association with type 1 diabetes and heart disease, but not proven	No strong links found to chronic diseases

Scientific Consensus and Controversy

The scientific view is mixed, with regulatory bodies taking a cautious stance due to inconclusive evidence. Much of the research on chronic diseases is observational, while evidence for digestive symptom relief with A2 milk is stronger. There are calls for more independent human clinical trials to resolve the debate.

Conclusion: A Balanced Perspective on A1 Milk Health Risks

No single disease is definitively caused by A1 milk for everyone. The science suggests that A1 beta-casein can trigger uncomfortable gastrointestinal symptoms in a subset of individuals with digestive sensitivities, linked to BCM-7 release. Links to more severe conditions like type 1 diabetes and heart disease are not proven in large human trials and are often based on limited data. For those experiencing digestive issues with regular milk, A2 milk may be a better tolerated option. Dietary choices should be based on personal experience and, if needed, medical advice.

Optional Outbound Link

For more in-depth scientific reviews on the topic, the National Institutes of Health (NIH) provides access to various peer-reviewed studies on A1/A2 beta-casein research through their database, accessible via PubMed Central.

NIH PubMed Central - Health Effects of A1 and A2 β-Casein

Frequently Asked Questions

A1 and A2 milk differ based on a single amino acid in their beta-casein protein structure. A1 milk has a histidine at position 67, which leads to the release of the BCM-7 peptide during digestion. A2 milk has a proline at that position, which prevents BCM-7 release.

BCM-7, or beta-casomorphin-7, is an opioid peptide released during the digestion of A1 milk's beta-casein. It is a concern because it has been linked to potential issues like slowed digestion, inflammation, and potential immune system interactions in sensitive individuals.

No, a definitive causal link has not been proven. While some observational and animal studies have suggested an association between A1 milk consumption and type 1 diabetes, other studies have found no connection. Much of the evidence is considered inconclusive.

The evidence is inconclusive. Some epidemiological and animal studies have suggested an association with an increased risk of heart disease, but robust human clinical trials have not confirmed a causal relationship. Regulatory bodies have not found sufficient evidence to warrant health warnings.

The connection between A1 milk and neurological disorders like autism and schizophrenia is speculative. While BCM-7 can cross the blood-brain barrier in animal models, the evidence for a link in humans is limited and not conclusive.

For some people, yes. Clinical trials have found that individuals who report digestive discomfort from milk often experience fewer symptoms like bloating, gas, and pain when they switch to A2 milk.

There is no widely available blood test to determine sensitivity. The best approach is an elimination trial: remove all conventional dairy for a period, then reintroduce A2 milk exclusively. If digestive symptoms improve on A2 milk compared to regular milk, you may have a sensitivity.

Most modern dairy herds produce milk containing a mixture of A1 and A2 beta-casein. Breeds like Holstein-Friesian, Ayrshire, and British Shorthorn are known to be dominant producers of the A1 variant.