Quercetin's respiratory benefits limited to over 40s

In middle aged and older subjects, however, daily supplements of the polyphenol was associated with fewer sick days, and a reduction in the severity of infections, according to findings published in Pharmacological Research​.

“In the present study, however, a significant reduction in URTI duration and severity associated with quercetin supplementation was experienced only in subjects over 40 who rated themselves as physically fit,”​ wrote the researchers, led by Professor David C. Nieman at Appalachian State University.

“This was a surprising finding, because these older and fitter subjects were already at low risk for URTI,”​ they added.

The study involved 12 weeks of supplementation with quercetin at doses of 500 and 1000 mg/day.

Common illness​

Upper respiratory tract infections (URTI) are the most common of all human illnesses. Such infections occur in the nose, sinuses, pharynx, larynx, trachea, and bronchi, and are epitomized by the common cold.

According to the National Institute of Allergy and Infectious Diseases, the average adult suffers from between two and four colds every year, whilst children can catch between six and 10.

Quercetin is an antioxidant flavonoid found in fruits and vegetables. It has been shown to possess strong anti-viral properties when cultured with target cells and causal agents of URTI.

However, almost all reported quercetin-related effects are from in vitro​ studies, with only a few small-scale human supplementation studies – that have produced conflicting results.

The new study aimed to assess the effects of quercetin on URTI outcomes. The randomized, double-blinded, placebo controlled trial measured URTI rates and severity in a large community group (1002 participants), over a 12 week period.

No effect​

While no significant group differences were reported for URTI outcomes between all subjects combined, or for specific gender, BMI, or age groups, the authors do report that physically fit over 40 year olds experienced a 36 percent reduction in URTI severity and a 31 percent reduction in total URTI sick days when receiving the high dose.

However in the under 40’s age category, a 29 percent increase in URTI sick days was observed, along with a 27 percent increase in severity and a 19.5 percent increase in symptoms for the high dosage supplementation compared to placebo.

Despite the lack of effects on URTI outcomes, quercetin supplementation in doses of 500 and 1000 mg/day were seen to significantly increase plasma quercetin levels with no reported side effects.

Future work​

“These findings are consistent with published studies linking URTI risk to age, gender, and exposure to stressful life events,”​ wrote the authors.

“Future work is needed to elucidate the potential connection between quercetin affectivity and exercise status,” ​they added.

Source: Pharmacological Research​
Published online ahead of print, doi: 10.1016/j.phrs.2010.05.001
“Quercetin supplementation and upper respiratory tract infection: A randomized community clinical trial”​
Authors: S.A. Heinz, D.A. Henson, M.D. Austin, F. Jin, D.C. Nieman