More than 80 percent of the milk protein in ruminants comes from casein, while only half of the protein in human milk does. There are four categories of casein in cows’ milk: beta casein, kappa casein and two types of alpha casein. Whey makes up most of the rest of bovine milk protein.

 “All of the proteins have genetic variants,” Tomasula said of the whey and casein found in cows’ milk. 

Beta casein itself has a dozen genetic variants, including the A1 and A2 mutations, which happened more than 5,000 years ago, but are receiving a lot of attention today. Dairy cows have a combination of the two and can carry the genes: A1A1, A1A2 or A2A2. Health claims involving the superior digestibility of A2 milk (milk harvested exclusively from cows with the A2A2 gene) are making news. 

“I wouldn't expect any differences between the A1 or A2 beta casein proteins as a result of pasteurization alone, but the differences would appear upon digestion of the milk,” Tomasula stated. “A1 would produce casomorphin and A2 would not. However, in one study using human enzymes to digest milk in vitro, both A1 and A2 milk produced casomorphin, with A1 producing more than A2.”

It is the release of casomorphin during digestion which is said to cause gastrointestinal upset and inflammation. A2 milk proponents advocate that the A1 variant of beta casein is primarily responsible for these effects.

The A1 variant of milk produces histidine, an amino acid that has immunomodulating properties, at position 67 in the chain of 209 amino acids that make up the beta casein protein. This mutation causes the protein to break down differently during the digestion process. The histidine, which is the only difference between A2 milk and milk with the A1 variant, allows the formation of the casomorphin, which is related to the opiate family, to break free during the digestive process.

A2 milk has the amino acid proline in place of the histidine. This single difference prevents the undesirable break in the amino acid chain from occurring.  

Peptides, which are compounds of two or more amino acids linked in a chain, can and do have effects on humans, Tomasula said. To date, however, studies only show that inflammation can sometimes occur with the casomorphin peptide.

“It’s the only difference between the two,” she said.

Healthier milk?

The a2 Milk Company, founded in New Zealand, has received various patents for their branded milk, which is free of the A1 variant. The a2 Milk Company introduced its products to the U.S. in 2015, and the company is currently expanding its sales nationwide. Their milk receives a premium price over conventional milk.  

The company has made various health statements warning against drinking milk from cows with the A1A1 or A1A2 gene. They’ve claimed A2 milk is more readily digested, and several of their patent filings claim that milk with A1 contributes to heart disease and diabetes.

“These health claims did not go through the FDA,” Tomasula said. “Types of things that can’t be explained are blamed on the milk. A lot of protocols on these studies have been called into question.” 

Some of those studies claiming health differences between the two have been questioned due to subjectivity, small sample sizes or lack of repeatability. And other research studies have seemed to disprove many of the health claims related to the A2A2 gene, Tomasula said. 

Further testing is needed to prove any health claims being made about A2 milk’s inherent benefits.

Two current studies in the U.S. are being conducted and both are expected to conclude in 2023, which should help to better clarify if there are digestibility differences between A1A1 and A1A2 versus A2A2 milk, she said. 

Genetic difference

According to Chad Dechow, Penn State University associate professor of dairy genetics, any claims that have been made stating A2 milk is the result of human intervention are false. Beta casein protein mutations are found in every lactating species, he noted, and are natural occurrences.

Cows inherit either an A1 or A2 beta casein gene from each of their parents. There is no dominance or recessiveness, and therefore, both genes are expressed. A cow can be typed as A1A1, A1A2 or A2A2. Cows with the A2A2 gene only produce A2 milk. Jersey, Guernsey, Norwegian Reds and Brown Swiss have a higher percentage of A2 genes than Holsteins. 

Using A2 bulls and culling A1A1 cows will increase the amount of A2A2 genetics in a herd, Dechow said. Currently, at major bull studs in the U.S., 47 percent of the Holstein bulls are A2A2, and 10 percent are A1A1. Guernsey bulls are all A2A2. Brown Swiss and Norwegian Red have a small proportion of A1A2 bulls, but no A1A1 bulls. Dechow recommends selecting A2A2 bulls for your sires if all other traits are equal. 

The a2 Milk Company has patented its own genetic tests for cows. But methods available for testing milk for the beta casein protein variant have been available since the 1960s. These tests are now much easier for the laboratories to run, making DNA testing of the cow unnecessary.

 “You can type your milk from your milk, not the cow,” Tomasula said. 

Tomasula urged producers to be cautious when marketing the A2 properties of their milk. It is untested territory as to what producers not associated with the a2 Milk Company can legally say without infringement upon the company’s claims that they alone can market A2 milk, or with their various patents and trademarks.

Milk from A2A2 cows is considered a value-added specialty product, and as such is typically priced higher than conventional milk, although it seems to be priced slightly lower than Organic Valley’s Grassmilk, at least in the regional Northeast marketplace. Cheese and yogurt made with A2 milk are now available, although there has been some controversy about whether or not good cheese can be made from A2 milk, Tomasula said.

What all of this means for dairy farmers remains to be seen.  

Tamara Scully, a freelance writer based in northwestern New Jersey, specializes in agricultural and food systems topics.