Milk is a good source of protein, containing 8 grams (g) per 8-ounce (oz) glass. The two major proteins in milk are casein and whey.
Casein accounts for about 80 percent of the protein in milk. There are also different types of casein, one of which is called beta-casein.
Beta-casein makes up about 30 percent of the protein in cow’s milk. A1 and A2 are two variants of beta-casein.
Historically, cows produced milk that contained only the A2 form of beta-casein. Today, most of the milk available from the local grocery store contains mostly A1 proteins.
A1 and A2 proteins affect the body differently
When A1 protein is digested in the small intestine, it produces a peptide called beta-casomorphin-7 (BCM-7). The intestines absorb BCM-7, and it then passes into the blood. Doctors have linked BCM-7 to stomach discomfort and symptoms similar to those experienced by people with lactose intolerance.
Symptoms of stomach discomfort, such as gas, bloating, and diarrhea that occur after consuming dairy products, are typically attributed to lactose intolerance. However, some researchers believe that it is BCM-7, not lactose, that affects digestion and produces symptoms similar to lactose intolerance, in some people.
A study on adults with self-reported milk intolerance compared the effects of drinking regular milk that contained A1 and A2 proteins with A2-only milk on intestinal function, stomach discomfort, and inflammation.
The participants consumed 8 oz of milk twice a day for 2 weeks. They reported worse stomach pain after they consumed the regular milk but no change in symptoms after they drank the A2 milk.
Participants also reported more frequent and looser-consistency stools while they drank the regular milk. These symptoms did not occur after they consumed the A2 milk.