The mild diuretic effect in coffee, tea and soda stems from their active agents, classified as methylxanthines. The caffeine in coffee, tea, and soft drinks is 1,3,7-trimethylxanthine. Tea contains theophylline – 1,3-dimethylxanthine. Theobromine is found in tea, chocolate and cocoa as 3,7-dimethylxanthine. All three compounds are central nervous system and cardiac stimulants, as well as mild diuretics in some situation.
Virtually no evidence exists in the scientific literature that caffeine exaggerates dehydration and electrolyte loss to the point that it impairs exercise performance.