A2 Milk: A Gentler Choice for Digestion? Understanding A2 β-Casein and Its Role in Digestive Comfort

Did You Know? Human Milk Is Naturally A2-Type

The β-casein found in human breast milk is naturally A2-type,1 containing proline at the corresponding amino-acid position that distinguishes A2 from A1 β-casein.

This is interesting because A1 and A2 β-casein behave differently during digestion. A1 β-casein can release the bioactive peptide β-casomorphin-7 (BCM-7) more readily, whereas A2 β-casein has a much lower tendency to release bovine BCM-7 under normal digestive conditions.

Why Does BCM-7 Matter?

BCM-7 is a bioactive peptide with opioid-like activity that can interact with opioid receptors in the gastrointestinal tract. Research suggests that this interaction may slow gut motility and prolong intestinal transit time. Experimental studies have also linked BCM-7 and A1 β-casein with increased markers of intestinal inflammation.2

These effects may help explain why A1-containing milk has been associated in some human studies with digestive symptoms such as bloating, abdominal discomfort and changes in stool patterns.3

Why A2 May Be Gentler on the Tummy

Human studies comparing conventional milk containing A1 and A2 β-casein with milk containing only A2 β-casein have reported potential digestive advantages associated with A2-only milk, including:

● Less abdominal discomfort

● Less bloating

● Improvement in some gastrointestinal symptoms

● Differences in stool consistency and bowel habits

● Potentially better tolerance in people who experience discomfort after drinking conventional milk4

This suggests that sometimes, discomfort associated with drinking milk may not be related to lactose alone; the type of milk protein may also play a role.

References
1. Kay SIS, Delgado S, Mittal J, Eshraghi RS, Mittal R, Eshraghi AA. Beneficial effects of milk having A2 Β-Casein protein: myth or reality? Journal of Nutrition. 2020;151(5):1061-1072. doi:10.1093/jn/nxaa454
2. Brooke-Taylor S, Dwyer K, Woodford K, Kost N. Systematic Review of the Gastrointestinal Effects of A1 Compared with A2 β-Casein. Advances in Nutrition. 2017;8(5):739-748. doi:10.3945/an.116.013953
3. Cieślińska A, Fiedorowicz E, Rozmus D, Sienkiewicz-Szłapka E, Jarmołowska B, Kamiński S. Does a little difference make a big difference? Bovine Β-Casein A1 and A2 variants and Human Health—An update. International Journal of Molecular Sciences. 2022;23(24):15637. doi:10.3390/ijms232415637

4. Sheng X, Li Z, Ni J, Yelland G. Effects of conventional milk versus milk containing only A2 Β‐Casein on digestion in Chinese children. Journal of Pediatric Gastroenterology and Nutrition. 2019;69(3):375-382. doi:10.1097/mpg.0000000000002437