The health effects of milk may depend on the breed of cow it came from.

Currently, A2 milk is being marketed as a healthier choice than regular milk.

It is claimed to have several health benefits, and to be easier to digest for people who are lactose intolerant.

However, not all scientists agree that A2 milk is better for health.

This article takes an objective look at the science behind A1 and A2 milk.

Casein is the largest group of proteins in milk, making up about 80% of the total protein content.

There are several types of casein in milk, and beta-casein is the second most common. Beta-casein exists in at least 13 different forms (1).

The two most common forms of beta-casein are:

  • A1 beta-casein: Milk from breeds of cows that originated in northern Europe is generally high in A1 beta-casein. A1 milk comes from breeds like the Holstein, Friesian, Ayrshire and British Shorthorn.
  • A2 beta-casein: Milk that is high in A2 beta-casein is mainly found in breeds that originated in the Channel Islands and Southern France. This includes breeds like the Guernsey, Jersey, Charolais and Limousin (1, 2).

Regular milk contains both A1 and A2 beta-casein, but A2 milk contains only A2 beta-casein.

Some studies indicate that A1 beta-casein may be harmful, and that A2 beta-casein is a safer choice. This is the reason for the "A1 vs A2" debate.

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(Image from Food Navigator USA).

A2 milk is produced and marketed by the A2 Milk Company, and contains no A1 beta-casein.

Bottom Line: A1 and A2 milk contain different types of a protein called beta-casein. Some studies indicate that A2 milk may be the healthier of the two.

Beta-casomorphin-7 (BCM-7) is the reason why regular milk is believed to be less healthy than A2 milk.

BCM-7 is an opioid peptide that is released during the digestion of A1 beta-casein (3, 4). A few research groups have suggested that BCM-7 may be harmful (5, 6, 7, 8).

While BCM-7 may affect the digestive system, it is not yet clear to what extent BCM-7 is absorbed intact into the blood.

Studies have not found BCM-7 in the blood of healthy adults after drinking cow's milk, but a few studies indicate that BCM-7 may be present in infants (7, 8, 9).

BCM-7 has been extensively studied, but its health relevance still remains unclear.

Below is a review of the scientific evidence linking A1 milk and BCM-7 with type 1 diabetes, heart disease, infant death, autism and digestive problems.

Bottom Line: Regular milk contains A1 beta-casein, which is partly broken down to beta-casomorphin-7 (BCM-7) in the stomach. BCM-7 has been linked with several adverse health effects.

Type 1 diabetes is typically diagnosed in children, and is characterized by a lack of insulin in the body.

Several studies indicate that drinking A1 milk during childhood may increase the risk of type 1 diabetes (5, 6, 10, 11).

However, these studies are observational in nature.

They cannot prove that A1 beta-casein caused type 1 diabetes, only that those who got more of it were at a higher risk of getting the disease.

Animal studies have provided conflicting results.

Some have found no difference between A1 and A2 beta-casein. Others have shown A1 beta-casein to have either protective or adverse effects on type 1 diabetes (10, 12, 13, 14).

So far, no clinical trials in humans have investigated the effect of A1 beta-casein on type 1 diabetes.

Bottom Line: Several observational studies have found a link between A1 milk consumption during childhood and increased risk of type 1 diabetes. However, the evidence is mixed and more research is needed.

Two observational studies have linked the consumption of A1 milk with an increased risk of heart disease (6, 11).

This is supported by one experiment in rabbits. It showed that consuming A1 beta-casein promoted fat buildup in injured blood vessels. This buildup was much lower when the rabbits consumed A2 beta-casein (15).

Fat accumulation may potentially clog blood vessels and cause heart disease. However, the human relevance of the results has been debated (2).

So far, two human trials have investigated the effects of A1 milk on heart disease risk factors (16, 17).

One of them included 15 men and women who were at a high risk of heart disease. The study had a crossover design, meaning that all participants received A1 and A2 beta-casein at different periods during the study.

The study didn't find any significant adverse effects on risk factors for heart disease. Compared with A2 beta-casein, the A1 type had similar effects on blood vessel function, blood pressure, blood fats and inflammatory markers (16).

Another study found no significant differences in the effects of A1 and A2 casein on blood cholesterol (17).

Bottom Line: There is no strong evidence that A1 milk increases the risk of heart disease. However, the long-term effects have not been studied.

Sudden infant death syndrome (SIDS) is the most common cause of death in infants less than one year of age.

SIDS is defined as the unexpected death of an infant, without an apparent cause (18).

Some researchers have speculated that BCM-7 may be involved in some cases of SIDS (19).

One study found high levels of BCM-7 in the blood of infants who temporarily stopped breathing during sleep. This condition, known as sleep apnea, is linked to an increased risk of SIDS (7).

These results indicate that some children may be sensitive to the A1 beta-casein found in cow's milk. However, further studies are needed before any firm conclusions can be reached.

Bottom Line: There is limited evidence that A1 milk may increase the risk of sudden death in infants. More research is needed.

Autism is a mental condition characterized by poor social interaction and repetitive behavior.

In theory, peptides like BCM-7 might play a role in the development of autism. However, studies do not support all of the proposed mechanisms (20, 21, 22).

One study of infants found higher levels of BCM-7 in those who were fed cow's milk, compared to those who were breastfed. However, levels of BCM-7 dropped quickly in some of the infants, whereas they remained high in others.

For those who retained these high levels, BCM-7 was strongly associated with an impaired ability to plan and perform actions (8).

Another study indicated that drinking cow's milk may worsen behavioral symptoms in autistic children (23).

On the other hand, some studies found no effects on behavior (24, 25).

So far, no human trials have specifically investigated the effects of A1 and A2 milk on symptoms of autism.

Bottom Line: There is no conclusive evidence about the effects of A1 milk on autism. However, the issue is complicated and needs to be studied further.

Lactose intolerance is defined as the inability to fully digest the sugar (lactose) found in milk. This is a common cause of bloating, gas and diarrhea.

The amount of lactose found in A1 and A2 milk is the same. However, some people feel that A2 milk causes less bloating than A1 milk.

Supporting this, studies indicate that milk components other than lactose may cause digestive discomfort (26, 27).

Scientists have suggested that certain milk proteins may be responsible for some people's milk intolerance.

One trial in 41 men and women showed that A1 milk may cause softer stools than A2 milk in some individuals (28).

Additionally, studies in rodents indicate that A1 beta-casein may significantly increase inflammation in the digestive system (29, 30).

Bottom Line: There is growing evidence that A1 beta-casein may affect digestive function. However, further clinical trials are needed to confirm these results.

The A1/A2 debate is still up in the air.

A few studies indicate that A1 beta-casein may have adverse effects in certain individuals.

However, the evidence is still too weak for any strong conclusions to be made.

That being said, if you feel like you tolerate A2 milk better than A1 milk, then you should definitely stick to it.