The Dairy Council denies there’s an association between milk and increased mucus production, and they blame it on the jews. The original myth, they claim, stems from a 12th century Jewish physician. Not just any 12th century Jewish physician though, but none other than Maimonides himself.
What have we learned in the last 800 years? The latest review on the subject was published last year. Does Milk increase mucus production? Fact… or fiction?
It appears to be, fact.
The milk protein casein breaks down in the stomach to produce a substance called casomorphin, which as it’s name implies, has opioid effects, which makes sense from an evolutionary standpoint, as species survival may depend on a close maternal bond between infant and mother.
The guess is that opioid receptors on the mucus glands in the respiratory tract may respond to the casomorphin from milk, which could potentially "Stimulate the production and secretion of mucus from these respiratory glands." This may explain why "a subgroup of the population who have increased respiratory tract mucus production, find that many of their symptoms, including asthma, improve on a dairy elimination diet.”
Maybe Maimonides was right.