Sorry if I was unclear. What I meant to ask is, what is it in dairy products that causes the thickening of mucus in some people?
The reason I am asking is that I am a student of voice - and voice students/singers refrain from dairy due to its effect on mucus. The thick mucus acts on the vocal cords, etc.
So, I'm wondering, what is it about dairy that causes this thickening? Is it the lactose? Whey? Casein? Some protein that is throughout milk, and therefore unavoidable?