A number of studies have shown significant performance increases in various endurance disciplines, including running, following caffeine ingestion. In one study, elite runners improved their time in a treadmill run to exhaustion by 1.9 percent with caffeine. Caffeine boosted time to exhaustion in a cycling test by 15 minutes in another study. And in a study involving swimmers, caffeine was found to enhance performance in maximal-effort swims of up to 25 minutes' duration.

How does the world's most widely used drug achieve these effects? It appears caffeine enhances performance in shorter events by stimulating the nervous system in ways that enable the muscles to contract faster and more efficiently. In longer events, caffeine delays fatigue by reducing the athlete's perception of effort. Specifically, it increases the concentration of hormone-like substances in the brain called ?-endorphins during exercise. The endorphins affect mood state, reduce perception of pain, and create a sense of well-being.

Caffeine has also been found to delay fatigue during exercise by increasing the level of free fatty acids in the bloodstream and thereby boosting fat burning and conserving muscle glycogen (which is the limiting fuel source for muscle work). This latter effect of caffeine used to be considered the major mechanism by which it enhanced endurance performance, but it is now known to be a minor factor. In fact, for those who normally maintain a high-carbohydrate diet, it is virtually a non-factor.

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Caffeine is a diuretic, meaning that it increases urine production, which could theoretically exacerbate dehydration during exercise. However, exercise negates this effect. In a recent scientific review, researchers from the University of Connecticut wrote, "Dietitians, exercise physiologists, athletic trainers and other sports medicine personnel commonly recommend that exercising adults and athletes refrain from caffeine use because it is a diuretic, and it may exacerbate dehydration and hyperthermia." However, "contrary to popular beliefs... caffeine consumption does not result in... water-electrolyte imbalances or hyperthermia and... reduced exercise-heat tolerance."

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When and How

Runners commonly take caffeine in pill form (proven to be more effective than equal amounts of caffeine consumed in coffee) 30 to 60 minutes before races to enhance competitive performance. What's the optimal amount? The ergogenic effect of caffeine is dose-dependent. The maximum effect is seen with doses of 5 to 6 milligrams per kilogram of body weight. For a 150-lb runner this translates to roughly 340 to 400 mg, or the amount of caffeine you'd get in 14 to 17 oz. of drip brewed coffee. The minimum amount of caffeine the average runner must consume for a measurable ergogenic effect is about two mg per kilogram of body weight.

More: Caffeine, the Ergogenic Aid