Caffeine acts as an antagonist on adenosine, thereby diminishing the
usual decrease in concentrations of norepinephrine. Phosphodiesterase
is inhibited by caffeine, and protein kinase increases. Catechins also inhibit
pancreatic and gastric lipases and attenuate fat emulsification,
whereby fat absorption is decreased [14]. Taken together, the methylation
of catechins by catechol-O-methyltransferase (COMT), and the inhibition
of phosphodiesterase by caffeine appear to be the principal
mechanisms behind the stimulating properties of a green tea catechins
caffeine mixture [14]. The inhibition of both enzymes activates a signal
cascade that stimulates the SNS (sympathetic nervous system); together
with HSL (hormone-sensitive lipase) and upregulation of UCPs
(uncoupling protein) this leads to increased energy expenditure and
fat oxidation [