Nutritional supplement stores and exercise magazines often advertise ergogenic supplements that supposedly improve workout performance, although many doctors and personal trainers advise against the potentially dangerous side effects of these supplements (they can cause heart failure and dizziness). Are there any safe stimulants that may improve your exercise endurance and performance?

According to the American College of Sports Medicine, consuming caffeine (which is found in coffee, espresso, tea, cola drinks, and chocolate) before exercising may increase your endurance and performance during both prolonged and short-term exercise sessions. They're not exactly sure how caffeine works with the body to improve performance, but a study showed that consuming the caffeine equivalent of between 2 to 6 cups of coffee one hour prior to exercise increased both the endurance and performance of elite athletes.

If you drink caffeinated beverages, drinking one before your workout may help you run or walk a little farther or cycle a little longer. Studies have shown that moderate caffeine intake - about 300 milligrams, the equivalent of three cups of drip coffee - is safe in most adults. There's conflicting evidence about whether caffeine has a diuretic effect (which means it helps eliminate water from the body). Whether you drink caffeine prior to your workout or not, you should drink plenty of water before, during and after your workout to remain adequately hydrated.

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Sources:  International Food Information Council, www.ific.org; American College of Sports Medicine, www.acsm.org.