What is A2 Protein milk?

Protein is an essential nutrient which contributes to many different physiological functions. Milk contains proteins in different forms, predominantly casein and whey. Cow’s milk is casein dominant, and A2 protein milk refers to a particular type of casein called ‘beta-casein’. In cow’s milk there are 12 types of beta-casein1. Of these 12 types of beta-casein, the most common are A1 and A2 beta-casein1. It is the genetics of the cows that determine which beta-caseins they produce in their milk. Cows can produce A1, A2 beta-casein or more commonly a mixture of both. Milk from the Guernsey breed of cows is mainly A2 beta-casein, but most European cattle produce a mixture of A1 and A2 beta-casein2. A2 protein milk is simply milk that comes from A2 protein milk-producing cows.

What is the difference between A1 and A2 beta-casein?

The A1 and A2 beta-casein proteins are long chains of amino acids that are identical apart from the position of one amino acid: A1 has histadine at position 67 of the amino acid chain, and A2 beta-casein has proline at position 67 of the amino acid chain3.

The difference this makes is very subtle.  These large protein chains are broken down into shorter peptides which then differ in each milk due to the histadine and proline4. These shorter peptides are digested slightly differently from each other.

Human milk beta-casein

Feeding babies with breastmilk is the best nutrition that they can get and is recommended as a sole source of nutrition for the first six months of life, and then continued on until a child’s second birthday. The protein content and structure of human milk differs from cow’s milk. Human milk does contain beta-casein, and like A2 protein milk also has a proline at the same position of the beta-casein protein chain5.

The evidence base for A2 protein milk

At the moment, the research in this area is in its infancy. There are some clinical trials so far that have shown some adults who experience digestive symptoms when they drink milk containing A1 protein, find they can tolerate A2 protein milk6-8. More research is needed to confirm any benefits of drinking A2 protein milk.

Why would parents use a formula based on A2 protein milk?

The protein composition of formula milk has been a mixture of A1 and A2 beta-casein for many decades and has been safely used. Formula milks based on A2 protein milk are shown to be safe and support normal growth in healthy term infants9. The composition and manufacturing process for A2 protein milk are similar to standard formula milks. The difference is in the digestion of the protein chains. Some families already choose to buy milk from farmers who have cows that produce A2 protein milk, and formula based on A2 protein milk is simply for those who would prefer to offer that to their babies as well.

IMPORTANT NOTICE: Breastfeeding is best. Infant milk is suitable from birth when babies are not breastfed. Follow-on milk is only for babies over 6 months, as part of a mixed diet and should not be used as a breastmilk substitute before 6 months. We advise that the use of formula milks and the decision to start weaning should be made only on the advice of a doctor, dietitian, pharmacist or other professional responsible for maternal and child care based on baby’s individual needs. Use Growing Up milk as part of a varied, balanced diet from 1 year.

1. Farrell et al. Nomenclature of the proteins of cow’s milk – Sixth Revision. J Dairy Sci 2004;1641-1674

2. De Noni I, FitzGerald R, Hannu J, et al. Review of the potential health impact of β-casomorphins and related peptides. EFSA J. 2009;7(2):231r.

3. Lonnerdal B. Nutritional and physiologic significance of human milk proteins. Am J Clin Nutr 2003;77(6):1537S–1543S

4. Pal et al. Milk Intolerance, Beta-Casein and Lactose. Nutrients 2015;7(9):7285-97

5. Sadler M & Smith N. Beta-casein proteins and infant growth and development. Infant 2013;9(5):173-176

6. He M. et al. Effects of cow’s milk beta-casein variants on symptoms of milk intolerance in Chinese adults: a multicentre, randomised controlled study. Nutr J 2017;16:72

7. Ho 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):994-1000

8. 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;15:35

9. Oropreza-Ceja LG. Lower Protein Intake Supports Normal Growth of Full-Term Infants Fed Formula: A Randomized Controlled Trial. Nutrients 2018;10(7):886


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