A1 beta-casein has been linked to a range of illnesses like heart disease, diabetes, schizophrenia, and autism spectrum disorder in population and animal stuides but this evidence is not strong
A1 casein and its metabolic byproduct, the opioid beta-casomorphin-7 have been linked to many illnesses in humans and animals
The emerging evidence in controlled trials of human subjects suggests that A2 dairy may improve gut symptoms, cognition, and perhaps even performance in those sensitive to dairy
While the evidence overall has demonstrated a likely beneficial effect on inflammation from dairy consumption, those with allergies to dairy proteins are likely to experience increased inflammation from dairy proteins. There is also a large body of evidence that suggests improved outcomes resulting from the reduction or elimination of dairy from the diet for people with autoimmune and inflammatory conditions.
An emerging area of research has begun to look at the different effects of the two major dairy proteins; A1 or A2 β-casein. Both A1 and A2 proteins are found in most dairy proteins, while A2 type casein is found in dairy derived from Jersey or Guernsey cows, as well as Charolais, Limousin, and the African Zebu. It is available as A2 milk, cheese, and other dairy for consumers and these products contain only the A2 form of β casein.
The original research on A1 vs A2 casein relied on population data which suggested an association between A1 casein and diabetes and heart disease in people who consumed higher amounts of A1 protein (in countries like New Zealand) compared to higher intakes of A2 within populations from Western Samoa, the Masai in Eastern Africa,1 along with regional analysis of Nordic and northern European countries.2 Animal research at this time also suggested the link between A1 casein and both diabetes and heart disease.1
However, population data is always confounded and other factors are likely to have a stronger effect on the incidence of cardiometabolic disorders than the consumption of mixed A1-A2 milk or dairy vs A2-only. Animal data similarly is unreliable because the physiology and dietary predilection of both rats and rabbits (from which the experimental data was derived) are very different from humans and the results from these subjects is notoriously unreliable when translated to the human experience.