This was from the fat burning foods article.
Note: Caffeine and EGCG have both been shown to help you work out harder and longer!
However Caffeine can constrict bloodvessels, potentially reducing the amount of Oxygen to cells, thereby causing more Lactic Acid potential, because the Glucose and Glycoygen you break down, (when eventually in Pyruvate formj, could convert to Lactic Acid more when there is less Oxygen present.
The possibility of having less Oxygen transportation ability in your system, by virtue of Caffeine blocking Iron absorption, thereby affecting Red Blood Cell, (Erythrocyte), levels, would largely depend on daily consumption.
Having Caffeinated substances or Caffeine pills before a workout, would probably have very little to no effect on Erythrocyte levels during a workout, though this effect would vary slightly based on duration.
Personally I don't see any need for caffeinated anything before workouts. A lot of supplement comapnies add it to stuff then brag about it, (forgetting to mention any vasoconstriction potential or possible slight Erythrocyte effects), though promoting possible negatives isn't exactly great for sales.
Even good supplement comapnies that 99.99% of the time don't B.S. consumers about anything, probably skimp on disclosing anomelies or quirks relating to the tiny percentage minority of consumers, that could get such things occuring post--consumption.