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

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

The two most common forms are:

• A1 beta-casein. Milk from breeds of cows that originated in northern Europe is generally high in A1 beta-casein. These breeds include 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. These include Guernsey, Jersey, Charolais, and Limousin cows (1, 2).

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

Some studies suggest that A1 beta-casein may be harmful and that A2 beta-casein is a safer choice.

Thus, there is some public and scientific debate over these two types of milk.

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

SUMMARYA1 and A2 milk contain different types of beta-casein protein. Some studies indicate that A2 milk may be the healthier of the two.

Beta-casomorphin-7 (BCM-7) is an opioid peptide released during the digestion of A1 beta-casein (3, 4).

It’s the reason why some people believe regular milk to be less healthy than A2 milk.

A few research groups suggest that BCM-7 may be linked to type 1 diabetes, heart disease, infant death, autism, and digestive problems (5, 6, 7, 8).

While BCM-7 may affect your digestive system, it’s still unclear to what extent BCM-7 is absorbed intact into your blood.

Studies have not found BCM-7 in the blood of healthy adults who drink cow’s milk, but a few tests indicate that BCM-7 may be present in infants (7, 8, 9).

While BCM-7 has been extensively researched, its overall health effects remain unclear.

Type 1 diabetes

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

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

However, these studies are observational. They cannot prove that A1 beta-casein causes type 1 diabetes — only that those who are getting more of it are at a higher risk.

While some animal studies have found no difference between A1 and A2 beta-casein, others show 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.

Heart disease

Two observational studies link A1 milk consumption to an increased risk of heart disease (6, 11).

One test in rabbits showed that 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. Still, the human relevance of the results has been debated (2).

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

In one study in 15 adults at high risk of heart disease, no significant adverse effects were observed. A1 and A2 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).

Sudden infant death syndrome

Sudden infant death syndrome (SIDS) is the most common cause of death in infants under 12 months old.

SIDS is 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. Yet, further studies are needed before any firm conclusions can be reached.

Autism

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 in infants found higher levels of BCM-7 in those fed cow’s milk compared to those who were breastfed. Notably, levels of BCM-7 dropped quickly in some of the infants while remaining 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 indicates that drinking cow’s milk may worsen behavioral symptoms in children with autism. But other studies found no effects on behavior (23, 24, 25).

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

SUMMARYA few studies suggest that A1 beta-casein and the peptide BCM-7 may be linked to diabetes, heart disease, autism, and SIDS. Still, results are mixed and more research is needed.

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

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

In fact, 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 study in 41 people showed that A1 milk causes softer stools than A2 milk in some individuals, while another study in Chinese adults found that A2 milk led to significantly less digestive discomfort after meals (28, 29).

Additionally, animal and human studies suggest that A1 beta-casein may increase inflammation in the digestive system (29, 30, 31).

SUMMARYGrowing evidence suggests that A1 beta-casein triggers adverse digestive symptoms in some people.

Debate about the potential health effects of A1 and A2 milk is ongoing.

Research suggests that A1 beta-casein causes adverse digestive symptoms in certain individuals.

But the evidence is still too weak for any solid conclusions to be made about the supposed links between A1 beta-casein and other conditions, such as type 1 diabetes and autism.

That said, A2 milk could be worth a try if you struggle to digest regular milk.