Debra and Patrick Doyle of Aspen, Colo., go skiing every weekend in the winter and, in summer, take long mountain bike rides in the hilly terrain where they live.
What helps this 36-year-old couple feel so energetic?
They say it is the cans of Red Bull they consume every morning.
"It jump-starts my day," Ms. Doyle said of the so-called energy drink, made in Austria and sold in supermarkets and delicatessens around the United States.
Once the province of young extreme athletes and the nightclub crowd, which mixes it with vodka, Red Bull has gone mainstream. Sleepy college students drink it because they say they like its amphetamine-like effect; weekend athletes vouch for the buzz it gives them while exercising.
A Red Bull spokeswoman said 1.5 billion cans of the drink were consumed worldwide in 2003, a 10 percent increase from the previous year. In the United States, Red Bull controls roughly 50 percent of the $1 billion energy drink market.
According to the information printed on its slender blue-and-silver can, Red Bull "improves performance, increases concentration, improves reaction speed and stimulates the metabolism."
But if its ingredients are any guide, the boost provided by Red Bull and other energy drinks - SoBe Adrenaline Rush, Snapple Fire and 180 Orange Citrus Blast, for example - may differ little from the lift offered by a strong cup of joe.
Gatorade and other sports drinks are caffeine-free, low in carbohydrates and high in sodium to replace the electrolytes lost when people sweat. Red Bull and other energy formulas, experts say, deliver mainly a jolt of caffeine and a fairly high dose of calories. Each 8-ounce can of Red Bull has 80 milligrams of caffeine, slightly less than a cup of Starbucks coffee and more than twice as much as a 12-ounce can of Coke.
"Caffeine can be ergogenic - a fancy word for performance enhancing - because it appears to perk up the central nervous system, and that becomes increasingly important as the length of exercise increases," said Lawrence Spriet, professor of human biology and nutritional sciences at the University of Guelph in Ontario, who has studied the effects of caffeine on athletes.
Expectation - and the fact that Red Bull is often chugged rather than sipped - may add to the effect.
Although the International Olympic Committee recently removed caffeine from its list of restricted substances, several countries ban the sale of Red Bull to the public because of its high caffeine level.
Carbohydrates are another main ingredient in Red Bull, which tastes like fizzy cherry cough syrup. In addition to glucuronolactone, a carbohydrate found in plant gums and red wine, a can of Red Bull contains about two teaspoons of sugar. Endurance athletes easily burn up Red Bull's 110 calories. But some doctors have concerns about any drink that contains so much sugar.
"The average Joe weekend warrior, he would have to exercise for 15 to 20 minutes just to burn the calories in this drink," said Dr. Lori Mosca, director of preventive cardiology at New York-Presbyterian Hospital in New York. "I would not recommend picking up one of these drinks when you're buying lunch at a deli."
Consuming large quantities of carbohydrates in hot weather can compromise hydration by slowing the rate at which fluid is absorbed into the bloodstream, and it can be tough on the stomach, said Douglas Casa, director of athletic training education at the University of Connecticut in Storrs.
"If someone needs calories in those conditions, there are better alternatives than Red Bull," Dr. Casa said. "I recommend rehydrating with a properly formulated sports drink and getting calories from a properly formulated sports bar."
Like other energy drinks, Red Bull also contains healthful-sounding additives like the amino acid taurine and vitamin B, a total of 30 milligrams in the form of niacin, pantothenic acid, B6 and B12. Taurine, which the body produces on its own, is found in high concentrations in the heart. But as Malcolm Watford, director for the graduate program in nutritional sciences at Rutgers University, puts it, "We haven't got a clue to what it does."
Red Bull's director of corporate communications, Patrice Radden, declined to allow anyone from the company to be interviewed. But in an e-mail message from Austria, the company said taurine loss "can occur in some other physiological situations, such as high stress and physical exertion."
The message went on: "Studies also demonstrated increased excretion of taurine in perspiration and urine during physical exertion. Dietary supplementation of taurine during athletics thus seems reasonable."
A 2001 study from the European journal Amino Acids noted an increase in stroke volume (the amount of blood, oxygen and nutrients that the heart can pump to the working muscles) in a group of athletes who drank Red Bull, and not in a group that drank a caffeinated drink without taurine. According to Red Bull, the company did not sponsor the study but provided the drinks.
Many scientists, however, remain skeptical. Dr. Spriet of the University of Guelph is conducting a study on trained cyclists to determine if taurine can affect performance. So far, he said, "it doesn't look like taurine does anything."
Nor is there any scientific evidence that vitamins affect sports performance.
"Taking B vitamins won't hurt you," Dr. Watford said. "You just get expensive urine." A can of Red Bull sells at a suggested retail price of $1.99, but bars charge as much as $8 for the drink.
At that price, Red Bull is probably not for the frugal.
"At as much as a dollar an ounce, I certainly wouldn't use a fluid like that during a race," said Dr. William O. Roberts, president-elect of the American College of Sports Medicine and medical director for the Twin Cities Marathon in Minnesota. "I'd rather save the money for after the race and have a good glass of wine."
Still, Red Bull's claim for having performance-enhancing powers may be attractive to people who are very active, or very tired.
"There aren't many easy ways of getting energized," said Dr. Paul D. Thompson, director of preventive cardiology at Hartford Hospital in Connecticut. "Wishing doesn't make it so."