The Differences Between A1 and A2 Beta-Casein

June 15th, 2021

The dairy conundrum is never ending. Is it good? Is it bad? Full-fat or fat-free? As functional nutrition practitioners, we often refer to the latest research to guide our recommendations, but the answer is hardly black and white.

We know lactose intolerance and CMPA (cow milk protein allergy) are well-established diagnoses, all of which, if left unmanaged, can cause painful, unpleasant, and sometimes rather embarrassing symptoms. 

Over the years, research has focused on the health implications of two specific dairy proteins: A1 and A2 beta-casein. 

Let’s take a closer look at the differences and potential health effects of the two. 


A1 vs. A2

Casein proteins make up roughly 80% of proteins in cow milk. There are four casein subtypes: alpha S1, alpha S2, beta, and kappa-casein. 

Beta-caseins, specifically A1 and A2 beta-casein, are the most abundant. (1,2,3)

Cow milk contains a combination of A1/A2-beta casein, despite being conventional, organic, grass-fed, full-fat, or skim. 

Goats, sheep, camels, and buffalo produce predominantly A2 milk. Human milk is A2-exclusive. (4) 

Furthermore, companies like The a2 Milk Company and Alexandre Family Farm produce and sell milk products containing only A2 beta-casein. They’re able to do this by milking cows without the A1 beta-casein gene. (5,6)

A1 and A2 are similar, with only one difference at position 67 in the amino acid sequence: histidine for A1 and proline for A2. Research suggests that the structural difference, albeit small, has a significant impact on digestion. (2,3)

The theory is that digestion of A1 beta-casein, but not A2, releases beta-casomorphin 7 (BCM-7). BCM-7 is an opioid peptide that’s believed to be an antecedent to various health problems like gut issues, heart disease, type 1 diabetes, autism, and cognitive decline. (2,3,4)


Gut Health

A 2014 randomized cross-over study evaluated the gastrointestinal effects of A1 milk vs. A2 milk on 41 adults between the ages of 19-68. 

The study participants underwent a 2-week dairy-free washout period. After the washout, each individual consumed 3 cups of cow milk per day (either containing both A1/A2 or A2-only) for 2-weeks. They repeated this process twice and asked participants to record GI symptoms and stool consistency using the Bristol stool scale. 

The results showed that drinking milk containing A1 beta-casein, but not A2, led to more abdominal pain and higher stool consistency values (i.e., more diarrhea) on the Bristol stool scale. (7)

Another study conducted in 2016 evaluated the effects of regular milk vs. A2-only milk in 45 Chinese subjects with self-reported lactose intolerance. Researchers assessed inflammation-related biomarkers, BCM-7, symptoms of post-dairy digestive discomfort (PD3), and cognitive processing.

They found that participants who drank regular milk (A1/A2 combination) had higher inflammation-related biomarkers and BCM-7, significantly more digestive complaints, longer transit time, and fewer short-chain fatty acids. (8)

While both studies are small and more extensive studies need to be conducted, the results are notable. 



The 2016 study discussed above found that in some participants, stomach inflammation and small bowel inflammation improved after switching from regular A1/A2 milk to A2-only milk. However, a small subset of participants showed inflammatory improvement on the flip side after switching from A2-only milk to A1/A2 milk. (8)


Cognitive Function 

A 2019 study evaluated the cognitive effects of regular A1/A2 milk vs. A2-only milk in healthy preschoolers with mild-to-moderate milk intolerance. Participants consumed one cup of A1/A2 milk daily for five days and repeated this process with A2-only milk. The researchers evaluated average stool consistency and frequency, serum inflammatory and immune markers, and cognitive behavior based on Subtle Cognitive Impairment Test scores.

The results showed that A2-only milk reduced GI symptoms and improved cognitive performance and test scores. It’s important to note, though, that this study received funding from The a2 Milk Company. (9)


Type 1 Diabetes

Some studies purport that drinking milk containing A1 beta-casein at a young age increases the risk of type 1 diabetes. Conversely, other studies show no association. 

A 2017 article published in Nutrition & Diabetes suggests that the ingestion of A1 beta-casein combined with other genetic and environmental triggers are connected to the development of type 1 diabetes. (10)

However, a separate study published in the European Journal of Clinical Nutrition refutes the A1 beta-casein hypothesis and claims there’s insufficient scientific evidence to link A1/A2 milk to medical conditions like type 1 diabetes. (11)



While there’s research to support the potential adverse health effects of A1 beta-casein, the jury is ultimately still out, and more extensive studies are needed. It’s up to us as functional nutrition practitioners to use a critical lens to create an N-of-1 personalized plan for every client we counsel. 

If you’re interested in learning more about dairy and its health implications, sign up today for IFN Academy Track 1 to access Dr. John Bagnulo’s “Food as Medicine” presentation! He reviews ancestral diets vs. modern-day diets in-depth and the health effects of A1 and A2 beta-casein.

by Tori Eaton, RDN, IFNCP


1) Amidor T. A1 vs A2 milk proteins. Today’s Dietitian. 2021; 23(3):10. Accessed May 15, 2021. 

2) Sodhi M, Mukesh M, Kataria RS, Mishra BP, Joshii BK. Milk proteins and human health: A1/A2 milk hypothesis. Indian J Endocrinol Metab. 2012;16(5):856. doi: 10.4103/2230-8210.100685

3) Kaskous S. A1 and A2 milk and their effect on human health. J Food Eng. 2020;9(1):15-21.

4) Pasin, G. A2 milk facts. California Dairy Research Foundation website. February 9, 2017. Accessed May 15, 2021.

5) Why Drink A2/A2 Organic Milk. Alexandre Family Farm website. Accessed May 15, 2021.

6) Heins B. Breeding for A2 milk. University of Minnesota website. May 2019. Accessed May 15, 2021.

7) Ho S, Woodford K, Kukuljan S, Pal S. Comparative effects of A1 versus A2 beta-casein on gastrointestinal measures: a blinded randomized cross-over pilot study. Eur J Clin Nutr. 2014;68(9):994-1000.

8) Jianqin et al. Effects of milk containing only A2 beta casein versus milk containing both A1 and A2 beta casein proteins on gastrointestinal physiology, symptoms of discomfort, and cognitive behavior of people with self-reported intolerance to traditional cows’ milk. Nutri J. 2016;15:35. doi: 10.1186/s12937-016-0147-z

9) Sheng X, Li Z, Ni J, Yelland S. Effects of conventional milk versus milk containing only A2 b-casein on digestion in chinese children: a randomized study. J Pediatr Gastroenterol Nutr. 2019 Sep; 69(3): 375–382. Published online July 9, 2019. Accessed May 15, 2021. doi: 10.1097/MPG.0000000000002437

10) Chia, J., McRae, J., Kukuljan, S. et al. A1 beta-casein milk protein and other environmental pre-disposing factors for type 1 diabetes. Nutr. Diabetes 7, e274 (2017).

11) Truswell, A. The A2 milk case: a critical review. Eur J Clin Nutr 59, 623–631 (2005).