Caffeine

The doctor gave me a routine that included "Drink x glasses of water a day. Caffeinated drinks don't count toward your requirement." The instructions did not say "avoid caffeine," only caffeine-containing drinks don't count.

Why Is It?

By this criterion, drinking a litre of iced tea (150 mg caffeine after the ice melts) is evil but drinking a mug of coffee (133 mg caffeine) chased by 3/4 litre of is water acceptable.

I have always assumed that whatever nutrients I ate over a short period became effectively integrated into a single dosage in my body.

References:
Caffeine table from the Center for Science in the Public Interest.
(c) 2007 Bill Barnes