Caffeine (1,3,7-trimethylxanthine) is a purine alkaloid that
occurs naturally in coffee beans (Figure 1).1 At intake levels associated
with coffee consumption, caffeine appears to exert most
of its biological effects through the antagonism of the A1 and
A2A subtypes of the adenosine receptor.6 Adenosine (Figure 1)
is an endogenous neuromodulator with mostly inhibitory
effects, and adenosine antagonism by caffeine results in effects
that are generally stimulatory. Some physiological effects
associated with caffeine administration include central nervous
system stimulation, acute elevation of blood pressure, increased
metabolic rate, and diuresis.7 Caffeine is rapidly and almost
completely absorbed in the stomach and small intestine and distributed
to all tissues, including the brain. Caffeine metabolism
occurs primarily in the liver, where the activity of the cytochrome