Key points
- A1 beta-casein has been linked to a range of illnesses like heart disease, diabetes, schizophrenia, and autism spectrum disorder in population and animal studies 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 suggested an association between A1 casein and diabetes and heart disease
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.
β-casomorphin-7 is theorised to be a trigger for diabetes, cardiovascular disease, autism spectrum disorder, schizophrenia, sudden infant death syndrome, apnoea, and constipation
One of the chemicals linked to health effects (especially diabetes but also to autism spectrum disorder and schizophrenia) is a relatively powerful opioid compound β-casomorphin-7.3 This opioid results from the digestion of A1 casein (bot not A2) and is thought to be one of the reasons for the theorised negative health effects of A1 casein from dairy. This compound is also theorised to be a trigger for auto-immune (type 1) diabetes,1 along with cardiovascular diseases, autism spectrum disorder, schizophrenia, sudden infant death syndrome, apnoea, and constipation,4 along with affecting opioid receptors throughout the body including in the nervous system, endocrine, and immune systems.5 While it has been suggested that casomorphins are not demonstrably absorbed in humans,1 it is likely that children with immature digestive systems and those with increased gut-permeability (i.e. a ‘leaky gut’) can absorb these endogenous opiates.5
It should also be considered that (as shown in the research presented in this issue of CARR) IF A1 casein was a strong and independent contributor to heart disease or diabetes, we would see increased incidence and prevalence of these with increasing intakes of dairy consumption, something that is not seen.
Emerging human trials
In the last few years, human trials have begun to appear comparing functional outcomes from the consumption of mixed A1/A2 dairy and A2-only dairy.
In a pilot study comparing A1 to A2 milk to placebo for recovery from exercise (21 participants), both A1 and A2 milk both improved 20 m sprint performance and trended towards improved maximal isometric contraction when compared with placebo, but there were no meaningful differences between A1 and A2 milk.6 A recent honours thesis has suggested a possible, small improvement in recovery from high-intensity netball performance from A2 vs A1 milk.7
In a recent study of Chinese children with dairy intolerance, A2 milk was compared to standard milk (containing both A1 and A2 casein). The children who consumed A2-only dairy experienced significantly less severe gastrointestinal symptoms, reduced stool frequency (urgency to defecate is common in food intolerances and dairy allergy like this), and improvements in stool consistency, compared to those drinking A1-A2 milk. Immune and inflammatory markers (interleukin-4, immunoglobulins G, E, and G1) along with the opiate-like protein beta-casomorphin-7 were only elevated in those consuming conventional milk and this was also associated with lower glutathione levels (a key endogenous antioxidant). Interestingly, cognitive testing showed a significant improvement in test accuracy only after consumption of A2 milk.8 A recent thesis containing both in vitro and human in vivo studies showed that A2 milk had a tendency to lower bloating and abdominal cramps and reduce c-reactive protein (a marker for systemic inflammation).9
Children who consumed A2-only dairy experienced significantly less severe gastrointestinal symptoms
While the evidence is mixed, emerging research does suggest that those with milk intolerances (especially related to gut symptoms and effects on cognition following ingestion) might be able to tolerate and benefit from A2 milk versus conventional dairy.
References
1. Truswell AS. The A2 milk case: a critical review. European Journal of Clinical Nutrition. 2005;59(5):623-31.
2. Allison AJ, Clarke AJ. Further research for consideration in ‘the A2 milk case’. European Journal of Clinical Nutrition. 2006;60(7):921-4.
3. Woodford KB. A critique of Truswell’s A2 milk review. European Journal of Clinical Nutrition. 2006;60(3):437-9.
4. Şahin Ö, Boztepe S, Aytekin İ. A1 and A2 Bovine Milk, the Risk of Beta-casomorphin-7 and Its Possible Effects on Human Health:(II) Possible Effects of Beta-casomorphin-7 on Human Health. Selcuk Journal of Agriculture and Food Sciences. 2018;32(3):640-5.
5. Bell SJ, Grochoski GT, Clarke AJ. Health Implications of Milk Containing β-Casein with the A2 Genetic Variant. Critical Reviews in Food Science and Nutrition. 2006;46(1):93-100.
6. Kirk B, Mitchell J, Jackson M, Amirabdollahian F, Alizadehkhaiyat O, Clifford T. A2 Milk Enhances Dynamic Muscle Function Following Repeated Sprint Exercise, a Possible Ergogenic Aid for A1-Protein Intolerant Athletes? Nutrients. 2017;9(2):94.
7. Wallingford S. Comparing A2 chocolate milk to standard chocolate milk as a post exercise recovery aid: University of Chichester; 2018.
8. Xiaoyang S, Zailing L, Ni J, Yelland G. Effects of Conventional Milk Versus Milk Containing Only A2 beta-Casein on Digestion in Chinese Children: A Randomized Study. J Pediatr Gastroenterol Nutr. 2019.
9. Almuraee AA. The comparative effects of milk containing A1/A2 β-casein vs milk containing A2 β-casein on gut and cardiometabolic health in humans: University of Reading; 2019.
