This thread inspired me to begin eating low vitamin A about 6 weeks ago. I've supplemented A heavily in the past, I've eaten a lot of liver in the past, and I've eaten a lot of colorful vegetables for over a decade because "it's healthy."
@Blossom inspired me to find out that I'm a poor converter of beta carotene, too.
I've never been a consistent coffee drinker, but my threshold for becoming jittery has always been pretty low. Anecdotally, I feel like my tolerance for coffee/caffeine has increased dramatically since eating less A. I haven't truly tested this by intentionally over-consuming coffee during this time, but I really feel like I handle it much better than in the past.