FAB RESEARCH COMMENT:
Milk is a highly nutritious food, but various 'intolerance' symptoms - distinct from 'classic' allergic (IgE-mediated) reactions - have long been reported in association with milk consumption by some people.
Most of these intolerance reactions involve unpleasant gut and digestive symptoms, which can often be explained by intolerance to lactose, the sugar in milk. But others - such as excessive production of mucus or phlegm, which can aggravate symptoms of bronchitis, asthma or allergic rhinitis ('chronic stuffy nose') - remain controversial, as there is little formal scientific evidence to support them, and no obvious explanation for any such link.
This paper offers a novel hypothesis to explain how cows' milk might stimulate mucus production in some
Standard cows' milk contains the A1 form of beta-casein, which breaks down to release an 'opioid peptide' - beta-casomorphin-7 (BCM-7) - and this is known to stimulate mucus production in the gut.
The authors propose that A1 beta-casein could
also stimulate mucus production in the respiratory tract via similar mechanisms, and suggest ways in which this hypothesis could be formally tested.
Most importantly, they point out that milk consumption would increase mucus production by this mechanisms only if two conditions are met:
- BCM-7 would need to reach the general blood circulation (i.e. this would happen only under conditions of high intestinal permeability or 'leaky gut'),
- the tissues would need to be already inflamed (i.e. only people with a pre-existing infection, allergy or other inflammatory condition would be affectd)
This means that only a vulnerable sub-group would be affected, making it unlikely that any such effects would be seen in general population samples.
Milk from other mammals - including humans - does NOT contain the A1 form of beta-casein found in most cows' milk, but instead contains an evolutionarily older form, known as A2 beta-casein. So if this hypothesis is correct, then vulnerable individuals might be able to tolerate e.g. goats' milk - or in the case of infants, breastmilk, rather than standard cows' milk formula.
For further information on A1 vs A2 milk, see also: