Health myths are remarkably persistent. A particularly enduring one is the belief that drinking milk increases the production of phlegm and thus worsens the symptoms of the common cold, asthma and other respiratory illnesses.

That belief is simply not true. In fact, scientists debunked the milk-mucus myth decades ago. Yet, as Dr. Ian Balfour-Lynn, a pediatric pulmonologist at Royal Brompton Hospital in London, explains in a recent review article for the journal Archives of Diseases in Childhood, he continues to hear from many people — the parents of his young patients — who believe it.

“They stop their children from having milk,” he writes. That is unwise as well as unnecessary, he adds, for milk is an important energy source for many children, particularly those with cystic fibrosis.

How the myth got started

Balfour-Lynn traces the belief that milk produces phlegm back to Moses Maimonides, a 12-century Jewish philosopher and physician. In a treatise on asthma, Maimonides claimed that several foods generated excess mucus and worsened the symptoms of the illness. He included cheese among those foods. Elsewhere Maimonides warns that all types of milk should be avoided because they can cause “a stuffing in the head.”

(Maimonides also famously recommended chicken soup to people to assist “in the stirring up and ejection of pulmonary phlegm.” That advice might have some scientific validity.)

Traditional Chinese medical texts were more positive about milk, says Balfour-Lynn, although they did link dairy foods to thicker phlegm.

The myth that milk triggers the production of phlegm continued into modern times, making its way into many popular health books, including Dr. Benjamin’s Spock’s enormously influential “The Commonsense Book of Baby and Child Care,” which has sold more than 50 million copies since first published in 1946. As Balfour-Lynn points out, the latest 2011 edition of Spock’s book still states that “[d]airy products may cause more mucus complications and more discomfort with upper respiratory infections.”

No wonder that a 2004 survey of parents of children treated at a U.S. pediatric clinic for illnesses such as asthma and cystic fibrosis found that most of them avoided giving their children milk.

Past research

Balfour-Lynn describes the studies that have been done on the topic, including the first one, which was published 70 years ago, in 1948:

[The study] divided 647 patients according to their daily milk consumption (0–5, 6–9, 10+ glasses per week) and found no difference between the groups in terms of whether they complained of mucus in the throat. Nose and throat examinations were carried out on 157 of the subjects and there was no excess mucus noted in those who drank milk versus those who did not.

Fifty years later, an Australian study involving people infected with a common cold virus had similar results, as has additional research involving people with asthma.

There is a proposed scientific explanation for a milk-mucus connection in respiratory illnesses. It’s based on research that has shown a protein in milk breaks down into chemicals that can trigger mucus production. But, as Balfour-Lynn points out, that happens in the bowels, not the airways.

A sensory perception

Why, then, do people believe that drinking milk worsens their respiratory symptoms? It’s likely because of how milk feels in the mouth, says Balfour-Lynn.

Compounds in saliva mix with those in the milk, increasing the milk’s viscosity, or thickness. “This could well affect the sensory perception of milk mixed with saliva, both in terms of its thickness coating the mouth and the after feel — when small amounts of emulsion remain in the mouth after swallowing,” he writes.

But feeling thicker is not the same as being thicker.

“While certainly the texture of milk can make some people feel their mucus and saliva is thicker and harder to swallow, there is no evidence (and indeed evidence to the contrary) that milk leads to excessive mucus secretion,” Balfour-Lynn concludes.

“Milk is an important source of calories, calcium and vitamins for children. The milk–mucus myth needs to be rebutted firmly by healthcare workers,” he adds.

FMI:  You’ll find the review article on the Archives of Diseases in Childhood website, although the full article is behind a paywall.