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It’s become a common belief worldwide that gulping down a glass of milk will make you phlegmy. But... there seems to be no real scientific evidence to back up that claim.

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It’s a belief almost as old as the product it’s about. Back in Egypt in the 12th century, a royal physician warned that dairy can make you stuffy in the head.

No one seems to know where he got that idea from, but it’s become a common belief worldwide that gulping down a glass of milk or a grilled cheese will leave you with more mucus than a tissue can hold. But... there seems to be no scientific evidence to back up that claim. The idea that milk worsens congestion is one of the most prevalent and persistent medical beliefs.

And while it might seem like a silly thing to worry about, if dairy does increase mucus secretion in the respiratory tract, it would not only make colds more annoying— it could be harmful to people with respiratory conditions like asthma. So far, only a few rigorous experimental scientific studies have tried to determine if the milk-mucus effect is real, and none have found support for the idea. Several studies of asthmatics found that lung functioning— like how much air you can blow out in one second— weren’t affected by eating dairy.

And, in a study from the early ’90s, people who were infected with a common cold virus didn’t cough, sneeze or make any more nose gunk if they drank milk. That doesn’t stop people from feeling like they’re more phlegmy after a tall glass of 2%, though. In a double-blind study published in the journal.

Appetite in 1993, 169 participants said drinking a flavored milk drink left a coating in their mouths, and that after drinking it, they needed to swallow more and their saliva was thicker. The thing is, they said the same thing about a soy placebo drink which did not contain dairy. And neither beverage significantly increased the severity of mucus-related symptoms like coughing or sneezing.

That could suggest the nocebo effect is at play—the phenomenon where expecting something bad to happen literally makes it happen. Basically, the awful flipside of the placebo effect. Or, it could be the fat in milk-like drinks makes people’s mouths feel a particular way without actually changing mucus production.

You see milk is an emulsion— droplets of fat hanging in water— and saliva makes these fatty drops clump together. Globbier fat drops might feel a bit like mucus or hang around in your mouth for longer, making you think there’s more mucus when there really isn’t. Though more research could really nail the coffin shut on this idea, there’s just no evidence that dairy products actually cause you to produce more mucus.

So go ahead! Drink that milky boba tea, snack on that cheese stick, whatever you wanna do, unless you’ve got lactose intolerance or you're vegan or you just hate cheese... I'm not in charge of your life.

Thanks for watching this episode of SciShow! If you like learning about milk and how it affects your body, we've got more stuff for you like our episode which explains how people can drink milk as adults— something most animals do not do. { ♪OUTRO }.