Medical myths are stubborn. Ancient Chinese texts associated the consumption of milk with a "thickening of phlegm," and 12th-century Jewish physician Moses Maimonides wrote in his Treatise on Asthma that milk could cause "a stuffing in the head" that exacerbated symptoms of asthma. Eight hundred years later, Dr. Spock, a 20th-century American pediatrician who wrote a popular book about baby care, directed parents to limit dairy while babies were sick, especially with upper respiratory infections.
For nearly a thousand years, medical professionals have been telling us to lay off the dairy or face the mucus consequences. But is it true? A review of the literature led by pediatric respiratory specialist Dr. Ian Balfour-Lynn, of London's Royal Brompton Hospital, published Sept. 6, 2018, in Archives of Disease in Childhood, concludes that there is virtually no connection between dairy products and upper respiratory mucus — and that the myth might be preventing children with conditions like asthma, cystic fibrosis or even the common cold from getting enough nutrients like calcium.
According to Balfour-Lynn's literature review, studies dating back to 1948 have indicated that there's no link between milk and phlegm, but a 2003 study in the journal Appetite asked 345 random Australian shoppers whether they thought milk causes mucus. Of the 111 shoppers who drank whole milk, almost half of them were of the opinion that it did. Only 25 percent of the 121 reduced fat milk drinkers said they believed milk caused mucus. The difference in perception might have something to do with how milk feels in the mouth — it is, after all, just droplets of fat suspended in water.
"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 in the review. "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."
Balfour-Lynn suggests that, since milk is a good source of calories and minerals like calcium, the long-standing cultural belief that milk is bad for children with respiratory problems could mean kids aren't getting the nutrients they need. This could lead to children growing up with weaker bones and shorter stature.