Like most people, I prefer that my day include caffeine—in my case, the equivalent of a few cans of Diet Pepsi. That preference extends to my workout routine: I can't remember the last time I took a run or hit the gym without first taking at least a few swigs of soda to clear my head and jump-start my legs. And when I race, I opt for caffeinated chocolate gels over the nonjuiced variety. I'm not breaking any rules—the World Anti-Doping Agency, the group charged with making Tour de France riders and Olympic athletes pee in cups, took caffeine off the prohibited substance list in 2004. But I've wondered what kind of a boost I'm really getting from caffeine and if it has any downside.
I'm not alone in my habit, for sure, either as an American or an athlete. The average U.S. adult consumes 3 mg of caffeine per kilogram of body weight daily (here's a good source for finding out how much caffeine your chosen poison contains), and a recent survey of British athletes showed that 33 percent of track and field athletes and 60 percent of cyclists consumed caffeine for the purpose of boosting performance. Most studies have indeed shown that caffeine enhances performance (by as much as 20 percent), though because the people getting caffeine in a controlled trial can probably feel it, it's possible they may be imagining the athletic boost.
For years, researchers thought caffeine helped endurance athletes by making them burn fat more than glycogen. In a marathon or long bike ride, that meant you'd keep more glycogen in the tank for use later in the event, and avoid or delay hitting the mythical wall. But those studies couldn't be duplicated, says Gordon Warren, a professor of physical therapy at Georgia State University in Atlanta who has researched caffeine. "Now we're throwing our hands up in the air," he says. But even if no one knows exactly why caffeine helps, we do know that it acts on multiple body systems (as opposed to just the muscles, for example).
Some ideas: Caffeine, through the central nervous system, may blunt the perception of pain or of effort, says Kirk Cureton, head of the kinesiology department at the University of Georgia in Athens. It may also mitigate the loss of muscle strength that usually occurs after exercising for a while, he says. And, as anyone who appreciates the morning flood of coffee-related well-being already knows, caffeine can boost alertness and mood, which may also contribute to better performance, says Warren. Not as much research has been done on caffeine and shorter bursts of activity, like sprints or power sports such as weight lifting, says Warren. But what studies have been done hint that caffeine acts via the central nervous system to enhance your ability to activate muscles in your lower body, and that caffeine use boosts performance modestly, by about 4 to 6 percent, he says.
Studies suggest the optimal amount of caffeine for improving performance is about 5 to 6 mg per kg of body weight, says Cureton. But its effects may vary, depending on the individual and his or her normal level of caffeine consumption. So if you plan to hit Starbucks before your next 10K, be sure you've practiced drinking it during training, says Nancy Clark, a dietitian and author of the Sports Nutrition Guidebook. Too much caffeine can make you overly jittery and nervous, and that may hurt performance, says Bülent Sökmen, a researcher in the department of kinesiology at California State University-Fullerton and an author of a recent review on caffeine use in sports, published in the Journal of Strength and Conditioning Research (he was at the University of Connecticut when the review was written). Not to mention that caffeine is renowned for its ability to stimulate the emptying of the digestive system—not an effect you want to feel in the middle of a race.
None of the experts I talked to advocate that you pick up a new coffee habit at the same time you take up jogging. But they said unless you're pregnant, there's no evidence that moderate consumption of caffeine hurts healthy adults. (My colleague Nancy Shute covered the pros and cons of caffeine last year in her cover story on our hyperstimulated nation.) Nor is caffeine going to dehydrate you when you work out, contrary to popular belief.
I still feel a little squeamish about throwing back a Diet Pepsi before a race, even if it's what I'd do on any other day. I know it's not exactly on the level of blood doping, but am I doing something wrong? "Both caffeine and carbohydrate have a [performance-boosting] effect," says Sökmen. "If you think they should ban caffeine, they should also ban the Gatorade."
For more: earlier this year I wrote about another legal performance booster: sleep.