In a controlled study, researchers at Appalachian State University in Boone, N.C., proved that the plant compound did reduce a person's susceptibility to viral illnesses.
"These are ground-breaking results, because this is the first clinical, double-blind, randomized, placebo-controlled study that has found a natural plant compound to prevent viral illness," Dr. David Nieman, a professor in Appalachian's Department of Health, Leisure and Exercise Science, said in a statement on Thursday.
An individual eating a healthy diet gets about 25-50 milligrams of quercetin a day.
But the studies participants were given pure, pharmaceutical grade quercetin that was combined with vitamin C and niacin to make it more readily absorbable in the digestive tract.
The test subjects were cyclists who either took 1,000 milligrams of quercetin, or a placebo, daily for five weeks. The cyclists then rode a bicycle three hours a day for three days to the point of exhaustion. Then blood and tissue samples from the participants were examined.
What researchers found was that only 5 percent of the participants who took quercetin reported any sick days while 45 percent of the cyclists who took a placebo reported illness following the extreme exercise.
The "highly significant difference" could not be due to chance in a double-blind, placebo-controlled study, Nieman said in his statement.
But researchers also found quercetin's immune boosting ability didn't kick in until after the three-day intense exercise period.
Nieman will present results from the research study Feb. 9 at the southeastern regional meeting of the American College of Sports Medicine. Then he plans another study. That one will be to see if quercetin has any benefits for people who are undergoing everyday mental stress.
