For hundreds of years, at least as far back as the 12th century, it has been thought that milk produces more mucus in our bodies – and as a result should be avoided when you have a cold or other respiratory illness. Now one children's health expert is saying it's a myth.
Not only that, but he argues avoiding giving children milk in a mistaken belief that it's going to cause a build up of phlegm is denying kids an important source of nutrition.
According to Ian Balfour-Lynn from Royal Brompton Hospital in the UK, there's no evidence linking milk with mucus – and in newly published research he's been through numerous studies as far back as 1948 to check.
"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," writes Balfour-Lynn.
"The milk-mucus myth needs to be rebutted firmly by healthcare workers."
The idea that milk produces phlegm – and, by the way, that chicken soup gets rid of it – seems to have been promoted early on by spiritual leader and court physician Moses Maimonides, who died in 1204.
This hypothesis was given a boost by children's health guru Benjamin Spock: his 1946 book on baby and child health – The Common Sense Book of Baby and Child Care – was hugely popular, selling more than 50 million copies before he died in 1998.
And a large portion of the public – nearly half of whole milk drinkers in Australia, for example – still think that milk causes mucus build-up. A smaller percentage even think milk can cause asthma.
Yet the evidence that milk produces mucus is thin on the ground, with some studies saying there's no evidence at all.
One current hypothesis suggesting there is a link centres on the protein beta-casomorphin-7 – it's known that this protein, involved in the breakdown of certain types of milk, is responsible for boosting mucus production.
However, Balfour-Lynn points out that this all happens in the bowel. While those with bowels weakened by infection – people with cystic fibrosis perhaps – could find more mucus ending up in their respiratory system, it shouldn't be able to travel that far around the body in the majority of cases.
Instead, Balfour-Lynn suggests, the sticky compounds of saliva could be interacting with the emulsive properties of milk to produce more of the liquid and give the impression of excess mucus. People think there is a link when it fact there isn't.
"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," writes Balfour-Lynn.
"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."
This all matters because of the energy, calcium and vitamins present in milk. Children shouldn't be denied milk as a source of all this goodness as a result of a proposed link that has yet to be proven, Balfour-Lynn argues.
There's evidence that children who don't drink enough milk have been shown to have reduced bone density and to be more vulnerable to bone fractures.
The research has been published in Archives of Disease in Childhood.