Quercetin protects athletes from colds and flu
Quercetin has an antiviral effect in vitro, we wrote yesterday. Quercetin may also reduce the risk of virus infections in real people, especially if they train intensively. This is evident from a study published in 2007 by David Nieman, a biologist from Appalachian State University, in Medicine and Science in Sports and Exercise.Quercetin
The average westerner consumes 25-35 milligrams per day through his regular diet. In studies in which researchers give lab animals quercetin, the substance appears to accumulate in the lungs. [J Nutr. 2005;135(7):1718-25.]

Quercetin may have antiviral effects, according to some older research. Yesterday we wrote about the inhibition of influenza A viruses by quercetin, and in biomedical literature you will also find studies in which quercetin sabotages herpesviruses, adenoviruses [J Antimicrob Chemother. 2003;52(2):194-8.] and even coronaviruses.

In petri dishes, that is. That is why Nieman, who also conducted research into the immunostimulating effect of oyster mushrooms and an active lifestyle, decided to test quercetin on humans. He conducted this research for the United States Defense Advanced Research Projects Agency.Nieman divided 40 healthy subjects into 2 groups. For 21 days, one group took 1,000 milligrams of quercetin daily. Then, while supplementation continued, the subjects cycled for 3 hours every day for 3 days. After those 3 days, the subjects continued to take quercetin for several weeks, and the researchers monitored whether the subjects became ill.
The other group took a placebo and followed the same course.
Unusually heavy physical exertion temporarily slows down the immune system, so Nieman expected that after the three days of cycling, a relatively large number of subjects would have a cold or the flu. That was indeed the case, but the supplement significantly reduced this risk of disease.

Nieman's research does not clarify how quercetin protects athletes against viruses. The researchers looked at various aspects of the immune system, such as the activity of the Natural Killer Cells or the production of antibodies by immune cells, but found no significant effects.

The researchers think that quercetin turns off the viral protein hemagglutinin. With that protein, influenza A viruses stick to the cells they want to infect. If that protein does not work properly, infection becomes a lot more difficult.Conclusion
"In summary, 3 weeks of 1000 milligrams/day of pure quercetin compared with placebo ingestion in cyclists led to marked differences in rates of illness during the 2-weeks period after intensified exercise", the researchers summarize.

"Upper respiratory tract infections incidence differed between groups despite no measurable influence of quercetin on multiple measures of immune function taken before and after 3 hours of exercise."
"These data support in vitro observations that quercetin supplements may reduce illness rates after periods of heavy training by athletes through direct antipathogenic mechanisms."
Med Sci Sports Exerc. 2007;39(9):1561-9.