Dr Ian Balfour-Lynn, a children’s respiratory consultant at London’s Royal Brompton Hospital, argues in the latest Archives of Disease in Childhood that there is no clinical evidence for avoiding giving milk to children with asthma, cystic fibrosis, or respiratory infections like the common cold.

He believes the notion that milk generates excess phlegm dates back to a 13th century treatise and was perpetuated by children’s health guru Dr Spock’s 1946 book Baby and Child Care that sold 50 million copies last century.

Balfour-Lynn says a link that had been mooted was protein produced by the breakdown of certain types of milk, which is known to boost the activity of a gene that stimulates mucus production.

But he said this breakdown process happened in the bowel, and could only affect the respiratory tract if the integrity of the bowel was weakened by infection, so allowing the milk protein to travel elsewhere in the body. He said it was “highly unlikely” that the common cold would do this, although it may be possible in people with cystic fibrosis, which is associated with gut inflammation.

The more likely explanation lies in how milk feels in the mouth, said Balfour-Lynn. Milk is an emulsion, while saliva contains compounds that make it stickier and which quickly interact with the emulsion, boosting its volume.

“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.

“This may explain why so many people think there is more mucus produced, when, in fact, it is the aggregates of milk emulsion that they are aware of lingering in the mouth after swallowing.”

He said it mattered if children weren’t given milk because of this persistent myth because milk was an important source of energy, calcium, and vitamins. Calcium was also critical for good bone health and warding off osteoporosis in later life: with evidence showing that fractures were more common in children who didn’t drink milk. He added this was particularly important in conditions like cystic fibrosis or asthma when sometimes repeated large doses of steroids, which sap bone strength, are part of the treatment.

“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,” he concludes. “The milk-mucus myth needs to be rebutted firmly by healthcare workers.”

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