• franceswalker@thefoodinto

Why you may tolerate A2 milk

Updated: Jun 4, 2021

Do you react to normal milk but seem to be able to tolerate A2 milk?

What about lactose- how does that fit into the picture?

Lactose = milk sugar. Lactase is the enzyme produced by specialised gut cells to digest the lactose into its simple sugars (glucose and galactose) which then can be absorbed across the gut wall.

If lactose is not broken down then it cannot move across the gut wall. It stays in the gut, attracts water and travels down to the large intestine where it is rapidly fermented by the gut bacteria resulting in the production of gas. This extra water and gas expands the gut, triggering pain receptors and can result in IBS symptoms of diarrhoea, bloating and gut pain. If you react to lactose in dairy food in this way you are said to be lactose intolerant.

Even if you are lactose intolerance you may still make some lactase enzyme so will find you cope with smaller amounts of lactose in a snack or meal. Taking lactase enzymes at the same time as eating lactose containing foods is a useful strategy to avoiding the effects of a high lactose load.

A2 milk has the same content of lactose of regular milk, so why do some people with lactose intolerance actually do better on A2 milk?

The short answer is: if you react to regular milk but can tolerate A2 milk then it is not lactose (= milk carbohydrate) that is the problem here but the difference in 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 appears in cow’s milk.

These days Northern European cows harking back from these mutators tend to contain A1 and A2 in equal amounts. As do Australian cows and those also in America and New Zealand.

Pure-bred cows from Southern Europe, Asia and Africa tend to have more A2 than the A1, although both are present (1).

Limited data suggests that milk containing more A2 protein may reduce the time it takes for gut contents to move through the gut (reduce intestinal transit time) and result in fewer gastrointestinal symptoms than milk containing only A1 (even in lactose intolerant patients) but at this stage there is not enough evidence to confirm this (2-5).


Well first what are the proteins in milk?

Well surprisingly, the proteins in milk only make up a small 3-4% of the milk. The majority of this is casein (80%) and the remaining 20% is whey.

It is the casein here that we are focussing on:

Casein is made up of 3 different types of protein:

  1. alpha (α)

  2. beta (β): a lot of this type further classified into A1 and A2 β-casein

  3. kappa casein.

As you can see, the A1 and A2 come from the beta fraction. Got that? Well it gets more interesting the more you drill down.

A1 and A2 β-casein are very close cousins. Both are made up of exactly 209 amino acids with identical structures EXCEPT for ONE 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).

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

A2 milk is safe as long as you do not have a cow's milk allergy (2).

In people with IBS, or those avoiding dairy due to a suspected intolerance including lactose intolerance, trialling A2 dairy is worthwhile, and if successful will go a long way in expanding the diet and providing adequate calcium.

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).


  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.