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Answer: Your workout probably will feel easier, but that's not a reason to switch to decaf.
Caffeine acts as a central nervous system stimulant, producing effects -- such as increased heart rate and blood pressure -- that can make you feel more alert and energetic. Caffeine can also function as an ergogenic aid, meaning that its physiological effects enhance athletic performance. Most experts agree that caffeine's impact on the nervous system alters one's perceived level of exertion, making your workout seem easier and allowing you to exercise longer before feeling tired.
"It is strongly suggested that the effects are due to centrally stimulated mechanisms and subsequent reduced perception of effort," explains Victor Maridakis of the University of Georgia's department of kinesiology. "Studies have reported an increase in time to exhaustion in aerobic exercise with a moderate dose of caffeine."
Some studies suggest that caffeine also stimulates the release of stored fats, which muscles can then burn for energy. This delays the depletion of muscle glycogen stores and as a result, delays the onset of fatigue.
Whatever the exact mechanism responsible, in allowing you to prolong your workout, caffeine use can lead to added caloric expenditure. "Any time you increase the duration of exercise you increase the amount of calories burned," Maridakis points out.
But relying solely on caffeine to lengthen your workouts can be dangerous, warns Maridakis: "Caffeine's psychomotor effects may lead individuals to believe they can exert themselves past their tolerance levels and be hazardous." He also cautions that individuals react to caffeine differently based on factors such as body fat percentage, cigarette smoking, and use of oral contraceptives. "Each of these must be taken into account before any inferences into caffeine's ergogenic effects during exercise can be made."
How do you know if you've overdone it? "If you feel nervous or jittery, or feel like your heart rate is up before you've even gotten past 2 miles an hour, then you've probably had too much caffeine," says Lona Sandon, a registered dietitian and spokesperson for the American Dietetic Association. "It depends on the person, but for someone who doesn't typically have caffeine throughout the day, one cup of coffee or even a diet Coke is going to be more than enough to give them that extra edge."
If you do decide to go for that "extra edge," remember that caffeine often stimulates the digestive tract. "Timing becomes an issue," says Sandon. "The more caffeine you have, the more likely you're going to have to stop in the middle of your workout to find the restroom."
Originally published on FitnessMagazine.com, August 2006.