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  • Writer's picturefranceswalker@thefoodinto

Why you may tolerate A2 milk

Updated: Apr 8

Do you react to normal milk but seem to be able to tolerate A2 milk? Read on to see what the issue might be for you.

A2 milk is almost exactly the same as normal milk, even with being able to get lactose free versions- so what is it about the A2 milk that is marketed as being better for us, and is it, in fact a better choice than normal cow's milk?

It comes down to A1 and A2 proteins.


The history of the A1 and A2 is completely fascinating.

A really, really long time ago, cow's milk only contained A2 protein. THEN A MUTATION OCCURRED: around 5000 - 10,000 years ago in the cows of Northern Europe and suddenly A1 appeared in cow’s milk for the first time.

Generations on, we have cows that produce different proportions of A1 and A2 milk proteins, with the A2 milk coming from cows that produce only A1 and no A1 protein in their milk.


A1 and A2 proteins in milk refer to a difference in these milk proteins.

Cow's milk is made up of a lot of proteins: there are the CAESIN proteins (the part of milk that is used to make cheese) and the WHEY proteins (the proteins used to make nutritional supplements, and fun fact, is the by product of making cheese and used to be thrown away.)

It is the casein proteins here that we are focussing on: casein is made up of 3 different types of protein:

  1. alpha (α) casein

  2. beta (β) casein

  3. kappa casein.

Now we are focusing on the beta (β) casein: this is the part of the milk protein where that mutation happened all those thousands of years ago.

Proteins are made up of amino acids lined up- like a string of beads. 209 amino acids strung together makes up the beta (β) casein protein.

Thousands of years ago, a mutation happened at the 67th amino acid which was HISTIDINE and was replaced with the amino acid PROLINE.

This means that all the A1 beta (β) casein proteins has histidine as the 67th amino acid while the A2 beta (β) casein has proline as the 67th amino acid.

  • A1 milk 67th amino acid = histidine

  • A2 milk 67th amino acid = proline

How A1 milk produces BCM7

One difference with potentially BIG impacts.

When both these different proteins are broken down in our gut during digestion very different protein fragments are formed.

The A1 β-casein breaks at the 67th amino acid forming the peptide betacasomorphin-7 (BCM-7). Break down of the A2 β-casein results results in very little BCM-7 (6).

Animal and human studies suggest that BCM-7 affects movement of the gut and increases gut inflammatory markers (4, 7).

Limited data suggests that milk containing more A2 protein may make the milk move more quickly through the gut (reduce intestinal transit time) and result in fewer gut issues than milk containing only A1, although more evidence is needed to confirm this (2-5).

It may be of particular interest with those with constipation: based on the small number of human studies which have shown that A2 β-casein shortens gut transit time. However relatively large intakes may be needed to achieve these effects (2-3 cups of milk per day).

Should you try A2 milk if you seem to be intolerant to normal milk?

A2 milk is safe as long as you do not have a cow's milk allergy (2). Use the lactose free A2 milk if you think you are also sensitive to the lactose in cow's milk.

If you get very serious side effects from any milk protein, only trial A2 milk with medical adcice/supervision.

In people with IBS, or those avoiding dairy due to a suspected intolerance trialling A2 dairy is worthwhile, and if successful will go a long way in making sure you don't over eliminate foods you don't ned to in your diet.

Updated 24/7/23


  1. De Noni, R.J.; FitzGerald, H.J.T.; Korhonen, Y.; Le Roux, C.T.; Livesey, I.; Thorsdottir, D.; Tomé, R.W. Scientific Report of EFSA prepared by a DATEX Working Group on the potential health impact of  casomorphins and related peptides. EFSA Sci. Rep. 2009, 231, 1–107.

  2. Jianqin, S., 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. Nutr J, 2016. 15: p. 35.

  3. Crowley, E.T., et al., Does milk cause constipation? A crossover dietary trial. Nutrients, 2013. 5(1): p. 253-66.

  4. Barnett, M.P., et al., Dietary A1 beta-casein affects gastrointestinal transit time, dipeptidyl peptidase-4 activity, and inflammatory status relative to A2 beta-casein in Wistar rats. Int J Food Sci Nutr, 2014. 65(6): p. 720-7.

  5. Ho, S., et al., Comparative effects of A1 versus A2 beta-casein on gastrointestinal measures: a blinded randomised cross-over pilot study. Eur J Clin Nutr, 2014. 68(9): p. 994-1000.

  6. Scientific Report of EFSA prepared by a DATEX Working Group on the potential health impact of β-casomorphins and related peptides. EFSA Scientific Report (2009) 231, 1-107.

  7. Ul Haq, M.R., et al., Comparative evaluation of cow beta-casein variants (A1/A2) consumption on Th2-mediated inflammatory response in mouse gut. Eur J Nutr, 2014. 53(4): p. 1039-49.


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