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It is a common advice, perpetuated by many parents, and even some doctors, that if you have a cold, you should avoid dairy products, because they cause phlegm to build up or thicken. See e.g. Foods that cause mucus buildup.

Do they (and if they do, how/why)?

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    Really? I never heard that one before. Instead I heard so many times that when you have a cold you should take a warm cup of milk with honey... – nico Feb 5 '12 at 9:50
  • They can if you are allergic to dairy products – Wayne In Yak Mar 6 '13 at 17:56
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    1) What's the difference between mucus and phlegm? As far as I know, there isn't one, medically. It's what people call the same substance that varies. 2) Lactose intolerance is not the same thing as a milk allergy. 3) Regarding the study cited, although people report an increased feeling of mucus, when nasal secretions were measured there was no increase that correlated to the amount of milk consumed. Much of what we believe to be an effect of drinking milk is related to our own perception and the consistency of milk itself. Also, are we all talking skim milk here or are we comparing skim to 2 – user19045 Apr 29 '14 at 17:11
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    The link used to show notability for this claim says itself, "Some people think that milk increases mucus production or makes it thicker or harder to swallow, but this is likely not the case, according to the Australasian Society of Clinical Immunology and Allergy." It does not support the claim, but in fact claims the opposite. – Reinstate Monica -- notmaynard Jun 15 '16 at 16:48
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The Wikipedia article List of common misconceptions states:

Drinking milk or consuming other dairy products does not increase mucus production. As a result, they do not need to be avoided by those suffering from flu or cold congestion.

They back it up with this article:

which claims:

Milk and dairy product intake was not associated with an increase in upper or lower respiratory tract symptoms of congestion or nasal secretion weight. [...] We conclude that no statistically significant overall association can be detected between milk and dairy product intake and symptoms of mucus production in healthy adults, either asymptomatic or symptomatic, with rhinovirus infection.

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    So, two things about that study: one, they are studying people with increased mucus production already (ie, they have a cold)-- what about people who have no such additional component? Secondly, what about people who are lactose intolerant? Even scientists who get paid by dairy researchers (who also back up this claim, btw: ncbi.nlm.nih.gov/pubmed/2154152) must allow that those with milk allergies can still develop respiratory problems. – mmr Feb 5 '12 at 21:58
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    I respond (despite a preponderance of articles that tell me that I'm wrong) because we've definitely noticed the effect in our 2 year old son, to the point that he's not allowed to drink milk. His breathing becomes so labored that he can't sleep. His uncle is similarly affected, and has tested positive to lactose intolerance. I know that anecdote != data, but at least in our house, there shall be no milk. Cheese, though, does not seem to be a problem, for whatever reason, so lactose might not be the ultimate cause. – mmr Feb 5 '12 at 22:01
  • I hate to argue with a well-researched answer, but the article cited explicitly cites "mucus" and not "phlegm". Indeed, there is no increase in mucus production (outside of expected results) that same Doctor was also quoted saying that the people might be describing a thick milk sensation on the back of their throat that they attribute to mucus. – TheCompWiz Feb 10 '12 at 17:05
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    Regarding cheese: It doesn't harm people with lactose allergies because cheese is made by feeding milk to bacterial cultures. These cultures break down the lactose into something more digestible, so cheese has much less lactose than milk. – Noah Jul 25 '12 at 14:42
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    @mmr, I don't think lactose intolerance/dairy allergy is relevant to this claim, which appears to refer to the population at large. – Brian S Apr 30 '14 at 14:22

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