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Milk, mucus and myths
  1. Ian M Balfour-Lynn
  1. Department of Paediatric Respiratory Medicine, Royal Brompton Hospital, London, UK
  1. Correspondence to Dr Ian M Balfour-Lynn, Department of Paediatric Respiratory Medicine, Royal Brompton Hospital, London SW3 6NP, UK; i.balfourlynn{at}

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‘Science must begin with myths, and with the criticism of myths’. Karl Popper, 20th century philosopher


A myth is a widely held but false belief. If it is repeated often by enough people, those who know the truth may start to doubt themselves. Indeed, George Orwell said that myths which are believed in tend to become true. Our department has repeatedly been told by parents that drinking milk increases mucus production from the lungs, and so they stop their child having milk. This is particularly so in patients with conditions associated with excess mucus, for example, cystic fibrosis and primary ciliary dyskinesia, but also includes children with infant wheeze or asthma. Indeed, many people believe milk should be avoided with any respiratory illness, even a common cold.

Origin and propagation of the myth

It is often written that the belief started with Moses Maimonides, the Jewish spiritual leader and Court Physician in Egypt who died in 1204. In his Treatise on Asthma, written for an asthmatic relative of Saladin the Great, he warns against eating several foods that generate phlegm.1 These include fatty food; scalding hot gas-generating foods (eg, black beans, peas); food made from coarse wheat flour and heavy meats. However, while he warns against cheese (especially if very old), his only mention of milk is that all kinds cause ‘a stuffing in the head’ and it is best to keep away from them.2 He does of course recommend chicken soup that ‘assists in the stirring up and ejection of pulmonary phlegm’. Traditional Chinese medical texts have also linked dairy consumption with a humidifying effect and thicker phlegm,3 although in reality most of their texts are positive about drinking milk.4 The belief is repeated in the influential Dr Spock Baby and Child Care book5; first published in 1946, it had …

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  • Funding None declared.

  • Competing interests None declared.

  • Provenance and peer review Commissioned; externally peer reviewed.

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