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Is there any data that shows dairy is bad for kids (or really, anyone) when they have a cold? That it makes the cold worse? If you have a milk allergy aside... just curious.
There are times when you read an academic article and it is just pure joy. (To be fair, this is not common.) So I was delighted when your question prompted one of those experiences. The article in question is here, entitled “Milk, mucus and myths,” which gives you a good idea of the conclusions.
The article contains lots of fascinating nuggets (e.g. that this idea may have originated with Maimonides, who suggested that cheese might be problematic for asthmatics). The researchers also note that the belief that “milk causes phlegm” is widespread, and many people in surveys, including parents asked about children, report that they believe in this link and avoid giving their children dairy when sick.
However (you knew it was coming), there is no evidence that it is actually true. The authors go through a review of studies that look at dairy consumption and actual phlegm volume: no link. They report on a study of mucus perception. Volunteers were given either dairy milk or soy milk (blinded to which one) and then asked about their perception of mucus. Both milks generate a perception of more mucus, suggesting, as they put it, that there is something about the mouthfeel of these products that generates this sense.
I will quote their conclusion in full: “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. Milk is an important source of calories, calcium and vitamins for children. The milk-mucus myth needs to be rebutted firmly by healthcare workers.”
Research! It’s the best. Enjoy your milk.