Posted on by Joy Erdile, MSc, RD

Milk Intolerance May be Caused by A1 Beta-Casein

NOTE: This article was written and posted on July 21, 2019, and was completely updated and expanded on December 29, 2025.

Introduction

Digestive issues resulting from milk consumption are frequently attributed to lactose intolerance. However, emerging clinical research indicates that for many, the culprit is actually an intolerance to a specific protein variant found in most commercial cow’s milk: A1 beta-casein. Understanding the difference between A1 and A2 milk proteins is essential for those who suffer from persistent gastrointestinal distress, as well as those managing complex chronic conditions involving mast cell activation and autonomic dysfunction.

The History of A1 and A2 Beta-Casein

Casein makes up approximately 80% of the protein in milk, and beta-casein accounts for about 30% of that total. Originally, all domesticated cows produced milk containing only the A2 variant of beta-casein. This is the same variant found in human breast milk, as well as milk from goats, sheep, and buffalo. Approximately 8,000 years ago, a genetic mutation occurred in Holsteins, leading to the production of the A1 protein. Today, Holsteins are the primary dairy breed in North America and Northern Europe, meaning most commercially available milk contains a mix of both A1 and A2 proteins.

Why A1 Milk Causes Inflammation

The difference between A1 and A2 beta-casein comes down to a single amino acid at the 67th position of the protein chain. In A2 milk, this position is held by proline. In A1 milk, it is held by histidine. When A1 milk is digested, this histidine bond breaks, creating a peptide called beta-casomorphin-7 (BCM-7). BCM-7 is an opioid peptide that can slow down gastrointestinal transit time and increase inflammatory markers, leading to symptoms that mimic lactose intolerance, such as bloating and abdominal pain.

The “Trifecta” Connection: MCAS, POTS, and EDS

For patients managing the “Trifecta” of Mast Cell Activation Syndrome (MCAS), Postural Orthostatic Tachycardia Syndrome (POTS), and Ehlers-Danlos Syndrome (EDS), the choice of dairy is a critical clinical intervention rather than a minor dietary preference.

MCAS: The Histamine Liberator

BCM-7, the peptide produced during A1 digestion, is a potent “histamine liberator.” In MCAS, mast cells are hyper-reactive; BCM-7 can act as a direct chemical trigger, causing these cells to degranulate and release histamine and other inflammatory mediators systemically [4, 9, 11].

POTS: Autonomic and Motility Interference

Because BCM-7 binds to opioid receptors in the gut, it significantly slows motility [5, 6]. Furthermore, the histamine release triggered by A1 milk causes vasodilation (widening of blood vessels). For POTS patients already struggling with blood pooling, this can exacerbate heart rate spikes and lightheadedness [12].

EDS: Connective Tissue and Permeability

Fragile connective tissue in EDS often results in increased intestinal permeability (“leaky gut”). This may allow larger fragments of the inflammatory BCM-7 peptide to enter systemic circulation more readily, driving the chronic inflammatory state that often accompanies hypermobility disorders [13].

Practical Steps for Relief

If you suspect a sensitivity to A1 proteins, consider switching to A2-only sources. This includes goat’s milk, sheep’s milk, or milk from specific “heritage” cow breeds like Jerseys or Guernseys. It is also worth noting that butter and full-fat whipping cream are almost entirely fat and do not contain significant amounts of casein, meaning they are generally well-tolerated regardless of the source. For many, switching to A2 dairy allows them to enjoy milk products again without inflammation or GI upset.

More Info?

I used to offer a Food Allergy/Food Sensitivity Package; however, most people would be seeking services based on IgG Food Sensitivity tests purchased through a naturpath. rather than IgE-mediated test results obtained through an allergist.  I continue to support those diagnosed with IgE-mediated food allergies and, depending on the severity, will usually do so within the context of add-on services to the Comprehensive Dietary Package. You can learn about me here.

To your good health!

Joy

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References

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  13. The EDS Clinic. (2025). POTS and MCAS: What is the link? [https://www.eds.clinic/articles/mast-cell-activation-syndrome-postural-orthostatic-tachycardia-syndrome]
 
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