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)?

  • 1
    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. – iamnotmaynard Jun 15 '16 at 16:48

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.

  • 3
    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

Milk does not cause mucus to thicken. However, thin clear liquids, especially warmed liquids thins mucous more effectively, such as tea or broths. Milk is still recommended as a healthy drink for people with colds and upper respiratory infections, since the body needs the nutrition while you are sick. (Milk shouldn't be used to take some antibiotics and if you have a milk allergy you should avoid it.)

To demonstrate that this is really true and not bad advice, people with cystic fibrosis have a common genetic disorder in which their bodies produce too much mucus and it is life-threatening, leading to difficulty breathing and lung infections. It builds up in the lungs and digestive tract. They usually need lung transplants sometime in their lives or they will die. It's much worse than a cold, in other words.

Breast milk is considered very important for babies with cystic fibrosis and full fat milk products are highly recommended for children and adults with the disease as a nutritious, high-calorie and high-protein drink (they have trouble keeping on weight). The linked article is arguing that all high fat and animal based, high protein foods cause mucus build up. Several dairy, animal and animal fats are on the following list.

Here are recommended foods, according to Medline Plus for the disease

Eating patterns:

  • Eat whenever you are hungry. This may mean eating several small meals throughout the day.

  • Keep a variety of nutritious snack foods around. Try to snack on something every hour. Try cheese and crackers, muffins, or trail mix.

  • Make an effort to eat regularly, even if it's only a few bites; or include a nutritional supplement or milkshake.

  • Be flexible. If you aren't hungry at dinner time, make breakfast, mid-morning snacks, and lunch your main meals.

Getting more calories and protein:

  • Add grated cheese to soups, sauces, casseroles, vegetables, mashed potatoes, rice, noodles or meat loaf.

  • Use whole milk, half and half, cream, or enriched milk in cooking or beverages. Enriched milk has non-fat dry milk powder added to it.

  • Spread peanut butter on bread products or use it as a dip for raw vegetables and fruit. Add peanut butter to sauces or use on waffles.

  • Skim milk powder adds protein -- try adding 2 tablespoons of dry skim milk powder in addition to the amount of regular milk in recipes.

  • Add marshmallows to fruit or hot chocolate. Add raisins, dates, or chopped nuts and brown sugar to hot or cold cereals or for snacks.

  • A teaspoon of butter or margarine adds 45 calories to foods. Mix it into hot foods such as soups, vegetables, mashed potatoes, cooked cereal, and rice. Serve it on hot foods -- hot breads, pancakes, or waffles absorb more butter than cool ones.

  • Use sour cream or yogurt on vegetables such as potatoes, beans, carrots, or squash. They can also be used as a dressing for fruit.

  • Breaded meat, chicken, and fish have more calories than broiled or plain roasted.

  • Add extra cheese on top of frozen prepared pizza.

  • Add coarsely chopped hard cooked egg and cheese cubes to a tossed salad.

  • Serve cottage cheese with canned or fresh fruit.

  • Add grated cheeses, tuna, shrimp, crabmeat, ground beef, diced ham or sliced boiled eggs to sauces, rice, casseroles, and noodles.

Some of the information contained in this post requires additional references. Please edit to add citations to reliable sources that support the assertions made here. Unsourced material may be disputed or deleted.

  • 1
    You've provided references to discuss CF and hot liquids, but nothing that supports your first nor third sentences, which are the only ones addressing the question. – Oddthinking May 27 '16 at 5:59
  • @Razie Mah, why can you not mix milk and antibiotics? – fledermaus May 28 '16 at 8:44
  • This answer assumes, but shows no evidence, that the fact that dairy is recommended (by Medline Plus) for those with CF (in order to increase calorie and protein intake) means that dairy does not increase mucus. None of the sources support this answer's assertion. – iamnotmaynard Jun 15 '16 at 16:54

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