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→How much?
When you take caffeine as coffee, it may not improve performance as it does when taken in other forms. One study showed that caffeine in coffee does not give a performance improvement where isolated caffeine does<ref name="CaffCoffee"/> and another showed no benefit from caffeinated coffee over decaffeinated coffee<ref name="CaffCoffeeButts25"/>. Some studies show a performance benefit from caffeinated coffee<ref name="CaffCoffeeWiles"/><ref name="CaffCoffeeCostill"/><ref name="CaffCoffeeCasal"/>, but did not compare with caffeine alone. This may be because coffee contains hundreds if not thousands of compounds besides caffeine<ref name="CaffMetaAndPerf"/><ref name="CafCoffeeCancer"/>. Some of these extra compounds of been shown to affect glucose metabolism<ref name="CafT2DMSysRevDam05"/>. It is been observed that decaffeinated coffee can reduce the absorption of glucose<ref name="CaffDecaffCoffeeGlucose"/>.
=How much?=
Most studies use 3-13mg/Kg, average 6mg, but within that dose range there was no obvious dose response<ref name="CaffMeta"/>. A smaller intake of 3 to 5 mg/kg dose before exercise and then 1 to 2 mg/kg intakes during prolonged exercise has been recommended<ref name="CaffMetaAndPerf"/>. Too much caffeine (9 mg/kg), especially for those that do not regularly take caffeine, can cause impairment, such as becoming talkative, giddy, and unable to perform simple tasks such as telling the time<ref name="CaffMetaAndPerf"/>. Low levels of exercise (30% [[VO2max|V?O<sub>2</sub>max]]) seem to increase the metabolism of caffeine<ref name="CafModEx"/>, but higher intensities have no impact<ref name="CafExThermal"/>.Below is a listing of caffeine in common beverages. {| class="wikitable"!Source!!Caffeine (mg)|-|Brewed coffee||100-150+ per 8oz|-|Brewed decaf coffee||5 (range: 3-12)|-|Starbucks Espresso||75 per shot|-|Black Tea||28-46 per 8Oz|-|Oolong Tea||12-55 per 8Oz|-|Green Tea||8-36 per 8Oz|-|Soda - Cola||35 per 12oz Can (see 'soda' for details)|-|Soda – Mountain Dew||54 per 12oz Can |-|Red Bull||80 per 8.3oz Can|-|}
=Caffeine and Dehydration=
Caffeine does not impact performance in hot/humid conditions, nor act as a diuretic when running<ref name="CaffDiuretic"/>. Caffeine at 360mg is a diuretic at rest, but not at 180mg or less<ref name="CaffDiureticDose"/>. Caffeine does not cause long term dehydration<ref name="Caff11day"/>, and black tea has been shown to hydrate as well as water<ref name="CaffTea"/>. Caffeine does result in increased sodium excretion in the urine<ref name="CaffSodium"/>, but the significance of this unclear.