I've been told to avoid eating dairy during a heavy cold or sinus infection because dairy makes mucus very thick. I've also read elsewhere that dairy doesn't impact thickness of mucus... But anecdotal evidence makes me think that consumption of dairy does impact the thickness of mucus.
So is it correct to avoid dairy if one is having nasal congestion issues?
If it indeed does cause thick mucus... why?
According to this study, no.
We conclude that no statistically significant overall association can be detected between milk and dairy product intake and symptoms of mucus production in healthy adults, either asymptomatic or symptomatic, with rhinovirus infection.
It seems that you're already aware of this given your comment:
I've also read elsewhere that dairy doesn't impact thickness of mucus
There's a saying, "the plural of anecdote isn't data", which applies here. Unless you find strong evidence on the opposing side, or you see glaring problems with the study, it's best then to assume that dairy products don't increase mucus production.
That's not to say that your experiences are unimportant. If you feel otherwise (and that's not due to a nocebo effect), then do as you please and decrease your dairy consumption. However, I'll leave you with this article, which states:
Some doctors say that milk thickens saliva, which may coat the throat and give the perception of more mucus, but it does not cause the body to produce more mucus or phlegm.
