The twelfth century Egyptian court physician Moses Maimonides has a lot to answer for. Before his death in 1204 he wrote a document titled Treatise on Asthma, full of advice for a wheezy relative of Saladin the Great.
Alongside recommendations to avoid fatty foods, black beans and very old cheese, he noted that milk causes “a stuffing in the head”, and should be avoided.
From this ancient reference, according to paediatrician Ian Balfour-Lynn from Royal Brompton Hospital in the UK, arose the stubborn conviction that drinking milk increases phlegm production. The notion – a cherished and usually unquestioned piece of household wisdom – received a further boost in 1946 when it was included Benjamin Spock’s runaway bestseller, The Common Sense Book of Baby and Childcare.
This was unfortunate, contends Balfour-Lynn, writing in the journal Archives of Disease in Childhood, because there is no clinical evidence of milk’s mucous-producing effect. The belief, however, leads many a caring parent to withhold it from children with asthma, infant wheeze, or chronic conditions such as cystic fibrosis – thus depriving them of an important source of energy, calcium and vitamins.
The researcher cites experiments conducted as far back as 1947 that compared people’s perceptions of the relationship between milk and phlegm with actual phlegm production. None of them recorded a link between the white stuff and snotty stuff.
In 2010, researchers did find partial evidence to support a milk-mucous link. New Zealand scientists Jim Bartley and Susan Read McGlashan established that a protein called beta-casomorphin-7, derived from the breakdown of some types of milk, could affect gene expression associated with mucous production.
“However, this is in the colon, so can only lead to more mucous in the respiratory tract if intestinal permeability is increased during inflammatory states, thus allowing milk proteins to enter the systemic circulation,” writes Balfour-Lynn.
“There is no reason though that a viral cold should lead to increased intestinal permeability, so this theory seems unlikely.”
He concedes, however, that the process might be of concern for people with cystic fibrosis, which is associated with gut inflammation.
The most simple explanation for the long-held link between drinking milk and making mucous, he suggests, aside from the enduring influence of Moses Maimonides and Dr Spock, is simply mouth-feel. Milk feels thick on the palate, and so does phlegm.
“Milk is an important source of calories, calcium and vitamins for children,” he concludes. “The milk–mucus myth needs to be rebutted firmly by healthcare workers.”
Lest the reputation of Moses Maimonides seem permanently sullied by his dairy-related error, it should be pointed out that this is not his only enduring contribution to public health. In the same treatise, he also became the first writer to recommend chicken soup of the alleviation of, well, just about everything, really.