From Paige Waehner,
Your Guide to Exercise.
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A. Caffeine has long been used by endurance athletes and sleepy office workers alike to help increase energy and endurance, but is it safe and does it work?

Studies have found that, when it comes to exercise, caffeine:

• Delays fatigue
• Slows the breakdown of muscle glycogen, which means your body has more fuel to keep going
• Enhances endurance
• Keeps you more alert
• Can reduce muscle pain during exercise
• May lower perceived exertion, making exercise feel more comfortable

While caffeine can have a positive effects on endurance exercise, you don't need it to have a good workout. If you're a competitive athlete thinking of using caffeine, keep in mind that it is a diuretic, so you may need more bathroom breaks. That could affect your workouts as well as your hydration levels.

How Much Caffeine Is Enough?

Studies have shown that 3 mg/kg to 6 mg/kg is enough to enhance endurance. The average cup of coffee has about 60 mg to 120 mg, so it doesn't take much caffeine to do the job.

Cautions

Caffeine is a stimulant, so it does have side effects that could cause problems for certain people:

• Increased urination
• Stomach upset
• Trembling or shaking
• Trouble sleeping
• Increased anxiety
• Headaches
• Nausea

If you're pregnant, you should avoid caffeine and you should also be cautious if you're on medication or taking other performance-enhancing supplements that contain other stimulants. Too much caffeine can increase the side effects and could be dangerous to your health if you have other illnesses or conditions.

_Sources:

Graham TE, Rush JW, van Soeren MH. Caffeine and exercise: metabolism and performance. Can J Appl Physiol. 1994 Jun;19(2):111-38.

Motl RW, O'connor PJ, Tubandt L, Puetz T, Ely MR. Effect of caffeine on leg muscle pain during cycling exercise among females. Med Sci Sports Exerc. 2006 Mar;38(3):598-604.

Demura S, Yamada T, Terasawa N. Effect of coffee ingestion on physiological responses and ratings of perceived exertion during submaximal endurance exercise. Percept Mot Skills. 2007 Dec;105(3 Pt 2):1109-16.