However, a numbers of studies across the world (many done by independent researchers not associated with dairy councils) have shown there is no truth behind that statement.
The studies, including a major one in 1990 published in the American Review of Respiratory Disorders Journal, concluded that it wasn’t dairy itself that increased the production of phlegm, but rather that the fat in whole dairy products thickens already existing mucus.
That same year, the South Australian journal Appetite published the results of a double-blind study in which subjects drank either milk or a soy-based drink, which shed more light on this common belief.
Although subjects reported feeling thicker mucus after drinking both liquids, none of them had actually produced more phlegm. Rather, what they were describing was the creamy texture in both drinks that can linger in the palate for a few moments.
I can attest from personal experience that dairy alternatives always have that effect on me.
Some people go as far as saying that going dairy-free is a way to avoid asthma. Again, clinical studies have shown respiratory airways are not negatively affected by dairy consumption.