Milk gone bad: A1 beta-casein and GI distress

Milk is an important food for young infants and a common source of nutrition among adults. However, many humans stop producing the lactase enzyme responsible for digesting the milk sugar lactose after weaning, a condition called lactose intolerance. When individuals with lactose intolerance consume lactose through milk or other forms of dairy, they may experience varying forms of gastrointestinal (GI) distress, including abdominal pain, bloating, gas, nausea, and diarrhea. These symptoms are caused by the fermentation of lactose in the colon, since it was not absorbed in the small intestine.

Roughly 65% of the human population is considered to have a reduced ability to digest lactose after infancy. However, the prevalence of true lactose intolerance is difficult to discern because studies have varied in their interpretation of what constitutes this condition. Many surveys rely on self-reported lactose intolerance, but many individuals who self-report lactose intolerance show no evidence of lactose malabsorption.

An alternative explanation^[1] for the high levels of self-reported lactose intolerance may be the type of protein in milk. The two major protein groups in milk are whey and casein, with the latter accounting for about 80% of total protein. The most common genetic variants^[2] of casein protein in milk are A1 beta-casein and A2 beta-casein.

A2 beta-casein is recognized as the original form of beta-casein and is the only beta-casein found in the milk of purebred Asian and African cattle. The A1 beta-casein variant is found among cattle of European origin and is believed to have arisen more than 5,000 years ago. Accordingly, most milk sold commercially is a combination of A1 and A2 beta-caseins, as it is sourced from European cattle or other cattle that have been crossbred with European cattle. Examples include Guernsey cows, Holsteins, and Ayrshires. Human milk and milk from goats and sheep contains only A2 beta-casein. The beta-casein proteins are degraded into beta-casomorphins (BCMs) during the digestive process. The main difference between A1 and A2 beta-casein is that A1 beta-casein produces BCM-7 upon digestion while A2 beta-casein does not. There is a growing body of evidence^[2] suggesting that BCM-7 is bioactive and is associated with inflammation and several disease states, such as diabetes and coronary heart disease. However, these associations are not without criticism.

Up until now, nearly all the evidence investigating health effects of BCM-7 and the beta-casein variants has been observational or conducted in test tubes and animals. The current study was designed to compare the human health effects of consuming milk containing only A2 beta-casein with milk containing A1 beta-casein type in terms of GI function, symptoms, and inflammation.

The two common forms of casein present in milk are A1 beta-casein and A2 beta-casein, which differ as a result of a genetic mutation in cattle over 5,000 years ago. There is observational, test tube, and animal evidence to suggest that A1 beta-casein may promote inflammation and be linked to inflammatory disease states. The study under review put this to the test in humans.