Can Quercetin Improve Athletic Performance (and Protect Against Cancer)?

The supplement has shown promise in test tubes, but it's too soon to tell if it will work in humans.

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Athletes are always looking for (legal, they hope) ways to boost performance. They take caffeine, which has been shown to enhance endurance, and catch up on sleep, which also seems to improve fitness. And they often take handfuls of supplements in search of an extra edge. A small study published this week sheds light on one popular supplement—quercetin—which is being examined for its potential not only to improve athletic performance but also to prevent or treat a host of other diseases and conditions.

[Read about caffeine's role in improving performance and why sleep helps athletes.]

Quercetin is a kind of plant pigment called a flavonoid and is found naturally in red wine, apples, onions, and other foods. The potential of flavonoids in general to produce health benefits has been studied, and quercetin is no exception; it's sometimes used to treat the symptoms of prostatitis, and it's being looked at for cancer prevention, allergies, glucose absorption in diabetics, childhood asthma, and the lung disease sarcoidosis. And you can buy an energy drink, FRS, containing quercetin. But what's the evidence behind the most common claims?

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.

Quercetin is also used to treat symptoms of prostatitis. Daniel Shoskes, a urologist at the Cleveland Clinic, began studying quercetin's anti-inflammatory properties to protect donor kidneys against damage they incur during a transplant. He turned to it again when searching for an appropriate flavonoid to test against the inflammation of prostatitis, and his research, including a small randomized controlled trial, indicates it does indeed help the condition. (ConsumerLab.com, an independent lab that tests supplements, cites the 1999 study by Shoskes, saying the study suggests 500 mg of quercetin twice daily "may be helpful for chronic non-bacterial prostatitis.")

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.

When it comes to cancer, quercetin has shown promise in test tubes, seeming to slow the growth of, or induce death in, cancer cells, but it's far too soon to stock up on supplements in the hopes of preventing the disease. The American Cancer Society says that "while some early laboratory results appear promising, as of yet there is no reliable clinical evidence that quercetin can prevent or treat cancer in humans."

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.

[See why cancer patients in treatment should skip antioxidant supplements.]

Wesa recommends that rather than take supplements, people eat a balanced, healthful diet and get plenty of exercise. "Those are of proven clinical benefit for disease prevention, lessening symptoms and side effects, and promoting overall survival," she says.