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The belief that milk might increase mucus production has been around for centuries, but it seems to be more myth than fact.

“The evidence is very scarce to support any relationship between dairy consumption and either symptoms of mucus or worse asthma control,” said Dr. Sonali Bose, assistant professor of pulmonary and critical care medicine at Icahn School of Medicine at Mount Sinai in New York.

In one study, researchers infected 60 Australians with the common cold and then had them live communally for 10 days. Participants tracked their dairy intake and collected their used tissues in sealed bags for later weighing by a research assistant. They found no relationship between intake of dairy products and the weight of nasal secretions, though participants could have adjusted their milk intake depending on whether they were feeling phlegmy or not, and their beliefs about milk and mucus.

Indeed, one thing that makes a milk-mucus link so hard to study is that it is such a common belief that our expectations can skew results. A randomized trial tried to tease out this so-called “nocebo” effect by giving 125 people either cow’s milk or soy milk, made indistinguishable with chocolate mint flavoring. Participants reported that both drinks made their saliva feel thicker and created what felt like a coating over their mouth, throat or tongue. But neither beverage seemed to affect symptoms like coughing, postnasal drip, sinus congestion or difficulty breathing.

Several small studies have also found little or no link between milk consumption and lung function in people with asthma, but “effects in certain subpopulations may be missed by these small studies,” Dr. Bose said. “If my patients feel like eliminating dairy from their diet helps, then I support that.”

However, particularly for children with asthma, cutting out milk poses a risk of nutritional deficiencies, with no evidence of benefit, said Dr. Ran Goldman, a professor of pediatrics at the University of British Columbia. “Avoiding milk may expose children to problems caused by reduced calcium levels and restricted growth,” he said.