You can't do RCTs on something as common as coffee consumption.
What you can do is mendelian randomization studies. There are genes which influence bitter taste which seem like obvious candidates: If coffee tastes like pure quinine to you, you're probably drinking less of it than you otherwise would.
A quick search shows that there have been M.R. studies on coffee, and they do suggest the relationships between coffee drinking and various good health outcomes aren't causal, just like the ones on alcohol. At this point, we should be suspicious of any "U-shaped" effect curves of extremely common and popular habits.
Yes, but go to the Cochrane library and try to get sensible analysis on anything that doesn't have a few RCTs under its belt, and Cochrane just shrugs.
