A picture of different types of milk products in glasses on a bench.

A1 and A2 Proteins in Milk

There are many different types of milk proteins. One particular protein, beta-casein, comes in two forms: A1 and A2. How much A1 and A2 protein is in milk varies by the different breeds of dairy cows, and the genotype of the individual cows in a herd.

Cows will produce either A1, A2 or a mix of A1 and A2 protein in their milk, depending on their genes.[1] Farmers can change the beta-casein protein produced by their herds by selecting bulls which carry only the A2 protein marker. The offspring of these bulls will have either the A2 genotype or A1/A2 genotype, and therefore produce more, or exclusively, A2 protein in their milk. To exclusively produce A2 protein only milk, all cows in the herd must be of the A2/A2 genotype. Some farmers selectively breed to develop herds of this kind.

Some studies have been published in Australia and internationally which evaluate the benefits of milk which contains only the A2 protein. Broadly, study participants with milk intolerance, lactose maldigestion or lactose intolerance report improvements in digestive symptoms, such as stomach discomfort on consuming A2 protein only milk.[2] While further research is needed in larger populations, long term, these findings are promising for those who experience digestive discomfort from regular dairy milk, allowing for the consumption of dairy milk, without the symptoms. This is important as dairy milk provides a unique package of more than 10 essential nutrients that are readily absorbable and important for bone health, muscle and nerve function, growth and repair.[3]

For people who experience digestive discomfort, it is recommended to try spreading milk consumption over the course of the day, consuming milk with a meal[4] or trying milk containing only the A2 beta-casein protein.

[1] Givens I, Aikman P, Gibson T et al. Proportions of A1, A2, B and C β-casein protein variants in retail milk in the UK. Food Chem. 2012; 139:549-552

[2] Ramakrishnan M et al. Milk Containing A2 β-Casein ONLY, as a Single Meal, Causes Fewer Symptoms of Lactose Intolerance than Milk Containing A1 and A2 β-Caseins in Subjects with Lactose Maldigestion and Intolerance: A Randomized, Double-Blind, Crossover Trial. Nutrients. 2020 Dec 17;12(12):3855.

[3] Haug A, Høstmark AT, Harstad O. Bovine milk in human nutrition--a review. Lipids Health Dis. 2007 Sep 25;6:25.

[4] O’Connor L, Eaton T, Savaiano D. Improving Milk Intake in Milk-Averse Lactose Digesters and Maldigesters. Journal of Nutrition Education and Behaviour. 2015;47(4):325-330.e1.