The study published this week, in the International Journal of Sports Nutrition and Exercise Metabolism, looked at quercetin’s effects on endurance in healthy nonathletes. For seven days, 12 volunteers received either 500 mg of quercetin dissolved in Tang or a placebo. Their cycling performance was recorded, and then they repeated the experiment with the other substance, serving as their own control group. Quercetin supplementation was associated with a 13.2 percent increase in the amount of time subjects could ride before getting too tired to continue, as well as a nearly 4 percent increase in V02 max, a measure of aerobic fitness.

J. Mark Davis, director of the exercise biochemistry laboratory at the University of South Carolina’s department of exercise science and author of the new study, says quercetin may aid performance through its anti-inflammatory properties or because it increases the number and function of mitochondria, the energy-producing factories found in cells. It may also provide a caffeinelike boost to the central nervous system. Davis suspects quercetin is similar to resveratrol, another plant-derived chemical that’s gotten much attention for its beneficial effects in animal studies. (Results of the study will need to be replicated to be confirmed.)

Another researcher studying quercetin, David Nieman, in the department of health and exercise science at Appalachian State University, is taking a different approach. Rather than studying its use alone, he’s combining it with green tea extract and fish oil, a mixture of flavonoids he believes is better absorbed by the body. “There is more and more evidence mounting that we need to find three, four, or even five unique plant molecules and get the right dose,” he says. The mixture he’s now studying, or some yet-to-be-concocted formulation, he says, might serve as an anti-inflammatory and might boost performance more than quercetin alone, particularly in the un-fit.

Putting potential effects in context, if somebody who isn’t already an athlete just exercises hard, “after a couple of months, you can get a 50 percent to 100 percent improvement in mitochondrial density,” he says. Caloric restriction can produce about a 25 percent improvement. And some kind of flavonoid supplementation, he says, might eventually yield gains of between 10 and 20 percent in the untrained athlete. Gains will be tougher to come by for elite athletes and are likely to be on the margins, though they are possible, he says.

Shoskes warns against “megadoses” of the supplement, saying antioxidants at high doses can actually have pro-oxidant effects and produce symptoms such as pain in small joints. Quercetin in general seems safe, says ConsumerLab.com, though it advises pregnant women to avoid the supplements and says maximum doses for children, nursing women, or people with serious liver or kidney disease haven’t been established.

One problem: Not only is quercetin tough for the body to absorb orally, but that absorption is variable depending on whether the source of the substance is whole foods or purified extracts, says Kathleen Wesa, a physician specializing in integrative medicine at Memorial Sloan-Kettering Cancer Center. So even if there are beneficial properties, the body may simply not have the ability to access them at the level required to achieve results seen in the lab. Researchers are looking at how to make quercetin more bioavailable and whether the substance is both safe and effective, but there are no answers yet, she says.