Regular bouts of endurance exercise can increase mitochon¬drial density in muscle tissue by 50% in a period of weeks. In animals, similar increases in mitochondrial density can be triggered by caloric restriction, as well as by certain plant polyphenols, including isoflavones from soy, resveratrol, and EGCG. Quercetin has a similar effect, at least in mice.
In April 2009, Davis et al from the University of South Carolina reported that feeding mice 25 mg/kg of quercetin for a week almost doubled their muscle mitochondrial DNA. Feeding the mice even half that dose of quercetin increased their SIRT1 mRNA by 200% and increased their treadmill running time by 37%; however, repeating these experiments on humans yielded mixed results.
In December 2009 researchers had a study published looking for a similar effect in humans. Dumke and colleagues at the University of Montana recruited 40 cyclists, gave them either 1,000 mg/day of quercetin or placebo for 3 weeks, and then tested muscle biopsies from the riders. The researchers could not find any difference between those taking quercetin and those taking placebo in muscle efficiency, muscle mRNA expression, or other measurements.
Yet a study in 2006 had shown benefit. In this earlier paper researchers from Pepperdine University recruited 11 elite cyclists and fed them a mixture of antioxidant vitamins and green tea extract either with or without quercetin for 3 weeks. Those who received the mixture plus the quercetin improved their times on a 30-kilometer time trial by 1.7% more than those who didn’t receive the quercetin. Of note, the Boone group’s 2009 study using athletes showed that quercetin, EGCG, and fish oil taken together decreased inflammatory markers, but did not produce changes indicative of increased mitochondria growth.
In the most recent study, published in February 2010, the Boone group used quercetin alone, but not in athletes. Instead they conducted their trial with untrained individuals. Even without the addition of green tea and fish oil, they reported a distinct benefit. In this study, 26 adult males took quercetin (1,000 mg/day) or a placebo. They were given 12-minute time trials on treadmills. Those taking quercetin went almost 3% farther during their 2nd time trial, while those taking placebo actually went about 1% less than their initial distance on their 2nd attempt. Increases from 16% to 25% in the measured levels of mitochondrial RNA and DNA suggest that muscle cells were building new mitochondria, but these numbers did not reach statistical significance.
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Not all studies of untrained subjects have shown benefit in exercise performance. An October 2009 paper reported the work of Cureton et al from the University of Georgia who randomly gave 30 untrained individuals either 1,000 mg/day quercetin or placebo mixed with PowerAde for time periods that ranged between 9 to 16 days. This study was funded by the Coca Cola Company, the makers of PowerAde.34 Perfor¬mance was tested via cycling tests and indirect muscle oxidative capacity testing and showed no improvement in either of these parameters. There appear to be valid reasons to critique this study, so its inability to show benefit may have an explanation.