I can't handle caffeine myself anymore but in the past I always found it to be an appetite suppressant. I know some very thin people who drink coffee all day for that reason (not that I recommend that.) Aren't there a bunch of diet pills that are basically caffeine based? (not that I recommend those either.)
Coffee also has diuretic effects. Red Bull I would stay away from. And coffee and Red Bull? Danger Will Robinson. IMHO.