Caffeine can boost exercise performance, but watch the dosage

Caffeine can boost exercise performance, but watch the dosage

By Howard LeWine, M.D., Harvard Medical School

Q: A friend drinks a cup of coffee one hour before exercise. She claims it improves her performance. Is this true?

A: Yes, assuming she drinks caffeinated coffee. A cup of decaf might make her think she’s exercising harder, but it’s the caffeine that boosts performance.

Caffeine’s positive effect on athletic performance is most significant for endurance events, such as long-distance running, cycling and swimming. Normally, after 30 minutes of exertion, glycogen (the body’s most immediate source of glucose for energy) begins to deplete. Caffeine can slow the depletion of glycogen.

When you exercise at an intense pace, glycogen will not run out for one or two hours, but once it does, you can “hit the wall.” By preserving glycogen stores longer, you can potentially increase your exercise time by 20 percent, or perhaps finish a race a little faster. Caffeine may also stimulate earlier and greater use of fat-burning for energy.

The other potentially positive effects of caffeine for exercise include decreased awareness of feeling tired, mental alertness and improvement in mood to help you maintain a workout program.

Caffeine definitely has its downside, though. There are important side effects that can be quite detrimental to performance. For example, some people are very sensitive to caffeine and get too hyped up and jittery. Caffeine even in small amounts can cause insomnia.

The usual recommended caffeine dose is 200 to 400 milligrams taken one hour before an event. I suggest starting at the lower dose. Depending on your weight, that might be a cup to 1.5 cups of strong regular coffee. Drink it one to two hours before exercise. A person who never drinks coffee may need less, while someone who drinks two or more cups daily might need more.

Caffeine doses above 500 milligrams don’t provide any extra boost. In fact, they can decrease performance.

Athletes required to do urine tests must be very careful about how much caffeine they consume. Caffeine is present in many teas, sodas, chocolate, energy supplements and over-the-counter medicines. The amount of caffeine can quickly add up and result in disqualification from competition.

(Howard LeWine, M.D., is an assistant professor of medicine at Harvard Medical School.)

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