Does uppercase (all-caps) text in headings and navigation reduce the overall user comprehension of the website?

I'm interested to know how (and why) the user experience is affected by the way the text is displayed on the screen.

I'm not thinking about how the experience is affected by the way text and the rest of the site interacts and affects the user but if the text by itself can have a measurable impact on user's comprehension of the website's message and purpose.

So, this is my question: does uppercase (all-caps) text in headings and navigation reduce the overall user comprehension of the website? Supposing there are no acronyms in the reading and nav.