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Many of the Olympians competing in London are juiced — though not in the colloquial sense that someone is doping. Instead, the juice these athletes are imbibing is literal, with beetroot juice and tart cherry juice two of the most popular choices. Growing numbers of elite athletes are turning to these natural beverages to provide what they hope will be a legal performance benefit.

Phys Ed Gretchen Reynolds on the science of fitness.

Recent studies, however, raise questions about whether the athletes are necessarily receiving the benefits that they think they are and what that means for the rest of us who’d love to find fitness in a glass.

Beetroot juice, as the name implies, is created from the knotty parts of a beet. Who first imagined that liquefying beetroots might improve physical performance is unknown. But he or she appears to have been on to something. In a series of studies in the past two years, beetroot juice has been found to enhance certain types of athletic performance. In a representative study published last year, for instance, cyclists who ingested half a liter of beetroot juice before a 2.5-mile or a 10-mile time trial were almost 3 percent faster than when they rode unjuiced. They also produced more power with each pedal stroke.

Since in the world of elite sports a 3 percent improvement in performance is enormous, athletes quickly embraced the juice as news of the studies spread. Today, beetroot juice is reportedly a staple among British track and field athletes at the Olympics, including Mo Farah, who won the gold medal this week in the men’s 10-kilometer race, and among several of the United States Olympic marathon runners, many other nations’ runners, swimmers, rowers and cyclists, and quite a few Olympic soccer players.

Although it isn’t clear just how beetroot juice improves performance, it seems to improve blood and oxygen flow to muscles, says Andrew Jones, a professor of applied physiology at the University of Exeter in England, who’s led many of the studies of beetroot juice and athletic performance. It also prompts muscles to use that augmented oxygen more efficiently. “There is a lower oxygen cost” to exercise when someone is drinking beetroot juice, he says. That may be one reason it allowed volunteers who drank it for a week beforehand to walk or run for significantly longer on a treadmill than those who had drunk a placebo juice.

But that advantage may not exist in all types of exercise, other new research suggests. A cautionary study published last month found that a single dose of beetroot juice ingested several hours before a one-hour cycling time trial did not noticeably improve the riders’ performance.

What that finding suggests, says Naomi Cermak, a researcher at the University of Maastricht in the Netherlands who led the study, is that beetroot juice, while effective at improving performance in short, extremely strenuous bouts of exercise, may have less effect during longer, relatively less intense types of exertion. In other words, the juice might help an 800-meter runner but perhaps not a marathoner.

Based on the currently available science, Dr. Cermak adds, it’s also likely that benefits will be most evident in someone who drinks the juice regularly, not someone who tries it for the first time on the day of a race.

So if you wish to experiment with beetroot as a performance booster, begin at least a week before a race or strenuous event. In many experiments, volunteers drank a half-liter of the juice per day. (Some studies have used smaller, concentrated beetroot “shots.”) And be prepared for a period of acclimation. Beetroot juice is “an acquired taste,” says Dr. Jones.

Somewhat more palatable is tart cherry juice, which also has a wide following among Olympians. Created using sharp, almost sour-tasting Montmorency cherries, it is not, strictly speaking, a performance-enhancing beverage. Instead, it affects the body’s ability to recover from hard exertion, says Malachy McHugh, the director of research at the Nicholas Institute of Sports Medicine and Athletic Trauma at Lenox Hill Hospital in Manhattan.

In studies by Dr. McHugh and colleagues, tart cherry juice reduced muscle pain and weakness after bouts of intense strength training as well as after a marathon. In a similar experiment by other researchers, racers in the annual Hood to Coast 196-mile relay race in Oregon reported significantly less pain after the race if they drank tart cherry juice in the week beforehand.

The juice has notable anti-inflammatory and antioxidant capabilities, Dr. McHugh says, although the particular components of the juice that are most active in that context are still being teased out. When he asked food scientist colleagues to analyze tart cherry juice, he said, “I was given a list of 30-plus compounds” that were likely to contribute to the drink’s benefits.

As for dose, his and other experiments have usually provided volunteers with two 8- or 12-ounce bottles of tart cherry juice per day, the equivalent of close to 100 Montmorency cherries a day. (Sweet cherries, by the way, have shown little efficacy in exercise-related experiments.)

Dr. McHugh and virtually all other exercise scientists looking into the potential benefits of fruit and vegetable juices caution that much science remains to be done to understand who will benefit and how, as well as whether there is such a thing as too much of a good thing. (Nitrate, for example, a key component of beetroot juice, has been found in extremely high doses to be carcinogenic and to contribute to other diseases.)

Still, Dr. McHugh, for one, is a tart cherry juice convert. A dedicated player of the brutal sport of Gaelic football, he downs “a bottle a day and two bottles on days of heavy training sessions or games,” he says. “My teammates are all at least 20 years younger than me. I would attribute the ability to maintain the fitness required to play in part to the tart cherry juice.”