It's hardly surprising that a stimulant like caffeine can affect athletic performance. Runners, soldiers and watch-keepers have been using it for centuries.
But researchers keep stirring things up with new findings about caffeine precisely because it seems to influence nerve and muscle and blood cells, among other things, in a variety of ways.
One of the more notable findings is that taking in the equivalent of a cup of coffee before exercise helps limit muscle pain while working out.
Scientists at the University of Georgia reported as far back as 2003 that caffeine reduces muscle pain during exercise. Their study, involving muscle pain during cycling, found that riders reported substantially less pain after taking a caffeine pill rather than a placebo tablet.
Other researchers have detected a surge of endorphins -- those feel-good hormones released during exercise -- in people who take caffeine before heavy exercise, like a race.
Robert Moti, a former competitive cyclist and now physical performance and health professor at the University of Illinois-Champaign, reports that caffeine indeed works on a nerve signal processing system in the brain and spinal cord linked to pain.
Moti started out studying possible links between caffeine intake, spinal reflexes and physical activity, but his most recent work looks at the effect of caffeine on muscle pain during high-intensity exercise. The study appears in the April issue of the International Journal of Sport Nutrition and Exercise Metabolism.
And "what we saw is something we didn't expect: caffeine-naive individuals and habitual users have the same amount of reduction in pain during exercise after caffeine consumption (a pill equal to two to three cups of coffee)."
It had been expected that participants (all fit, college-aged males) who seldom or never drank caffeine would get a bigger impact over those who regularly drank three or four cups a day. Many earlier studies had showed this, but not all have.
"Sometimes you see the tolerance effects, sometimes you don't," Moti said. "Clearly, if you regularly consume caffeine, you have to have more to have that bigger, mental-energy effect," but the tolerance effect doesn't necessarily hold true for pain or other stimuli.
What about using caffeine during exercise?
British researchers have found that athletes who drank a glucose solution mixed with caffeine during two-hour cycling tests burned carbohydrates at a rate 26 percent higher than those who got the same solution without caffeine. They think the caffeine may boost the rate of glucose absorption through the intestine, which would take fuel to working muscles faster and boost endurance.
Other researchers have found that using caffeine combined with carbs puts more glycogen -- muscle's primary source of fuel during exercise -- back into muscles following an exhaustive workout.
Australian researchers, again studying cyclists, found that those who took in caffeine equal to five or six cups of coffee along with a pasta meal had 66 percent more glycogen in their muscles four hours later than did those who consumed the carbs alone.
"If you have 66 percent more fuel for the next day's training or competition, there is absolutely no question you will go farther or faster," said John Hawley, senior author of the study and a researcher at the Royal Melbourne Institute of Technology University.
However, all the apparent advantages of caffeine for exercise don't mean that indiscriminately chugging those high voltage sport-energy drinks is necessarily a good idea. Many of them that are sold as supplements vary widely in caffeine content -- to upward of 500 milligrams (equal to at least four cups of coffee) in one serving. Soft drinks with caffeine can't have more than 71 mg in a 12-ounce can under Food and Drug Administration limits.
Lack of restrictions on the supplement drinks raises the risk of caffeine intoxication -- which can cause jitters, insomnia, stomach upset, tremors and rapid heartbeat, among other symptoms, yet "few include warnings about the potential health risks" said Roland Griffiths, a researcher at Johns Hopkins who has been studying adverse events from the drinks in children and teens.
