Food and Behaviour Research

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Does milk increase mucus production?

Bartley J, McGlashan SR. (2010) Med Hypotheses.  74(4) 732-4. doi: 10.1016/j.mehy.2009.10.044. Epub 2009 Nov 25. 

Web URL: View this and related abstracts via PubMed here.

Abstract:

Excessive milk consumption has a long association with increased respiratory tract mucus production and asthma. Such an association cannot be explained using a conventional allergic paradigm and there is limited medical evidence showing causality.

In the 
human colon, beta-casomorphin-7 (beta-CM-7), an exorphin derived from the breakdown of A1 milk, stimulates mucus production from gut MUC5AC glands. In the presence of inflammation similar mucus overproduction from respiratory tract MUC5AC glands characterises many respiratory tract diseases. beta-CM-7 from the blood stream could stimulate the production and secretion of mucus production from these respiratory glands.

Such a hypothesis could be tested in vitro using quantitative RT-PCR to show that the addition of beta-CM-7 into an incubation medium of respiratory goblet cells elicits an increase in MUC5AC mRNA and by identifying beta-CM-7 in the blood of asthmatic patients.

This association may not necessarily be simply cause and effect as the person has to be consuming A1 
milk, beta-CM-7 must pass into the systemic circulation and the tissues have to be actively inflamed.

These prerequisites could explain why only a subgroup of the population, who have increased respiratory tract mucus production, find that many of their symptoms, including asthma, improve on a dairy elimination diet.

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 people. 

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: 
  1. 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'),
  2. 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: