-Quercetin has been shown to boost endurance performance and support a healthy immune system in active individuals
-However, the quercetin content in foods and its oral bioavailability has led to inconsistent research findings
Quercetin is a potent antioxidant that has been linked to various health-promoting benefits. We can’t synthesize it in the body but fortunately, it’s found in common food sources including cherries, onions, tomatoes, apples, broccoli, tea, red wine, and capers. Quercetin is a flavonoid, a phytonutrient sub-category found in plants that play important roles related to pigmentation, UV filtration, and chemical messaging. Phytonutrients have been an area of interest for scientists because of the link between their consumption and health.
Quercetin consumption supports general health
Epidemiological studies have shown that high dietary intake of flavonoids is associated with a reduced risk for chronic diseases and all-cause mortality. [1,2]
In large population studies, higher dietary intake of quercetin has been associated with decreased risk for cardiovascular diseases.  A typical western diet will provide anywhere from 0-40 mg of quercetin per day, with intakes of 33 mg or more being associated with better health outcomes.
Because quercetin is such an abundant flavonoid, scientists have examined its isolated effects on health. A study examining the effects of quercetin supplementation on healthy male smokers saw improved cardiometabolic biomarkers across the board following quercetin supplementation. 
A clinical trial measured the effects of quercetin supplementation on upper respiratory tract infections (URTI) in a large community. The researchers noted that URTI total sick days and severity was reduced in physically fit middle-aged and elderly subjects that supplemented with quercetin. 
Quercetin helps support the immune systems of active individuals
While exercise is beneficial for general health and longevity, hard training individuals can be susceptible to illness shortly after a workout when their immune systems are temporarily suppressed. Researchers wanted to see if quercetin supplementation could support the immune response following exercise.
In a study on well-trained cyclists, quercetin supplementation greatly reduced the incidence of URTIs the 2-weeks following an intense exercise period. Only 1 out of 20 cyclists in the quercetin group had a URTI incident compared to 9 out of 20 in the placebo group. 
Quercetin can help boost endurance performance
Exercise induces free radical production that can contribute to muscle fatigue. Because quercetin has shown the ability to scavenge free radicals, researchers wanted to see if this could extend to increased exercise performance. A study examined the effects of quercetin supplementation over the course of 7 days and found it was beneficial for improving ride time to fatigue and VO2 max.  VO2 Max is a test to measure the maximal amount of oxygen an individual can use during intense exercise. It’s an indicator of aerobic capacity, the more oxygen you can use the more energy you can produce.
In a systematic review of 11 studies examining 254 subjects, for the effects of quercetin on endurance exercise capacity. The researchers found that quercetin exhibited a statistically significant benefit on VO2 Max and endurance performance of about 2%. 
The Problems with Quercetin
While there’s promising research on quercetin, there’s also inconsistent findings.
A study on short-term quercetin supplementation on soldier performance found no positive effect on any aerobic performance measurements. 
A 12-week study was done on females supplementing with quercetin, observing its effects on immune function and inflammation. At the end of the 12-weeks, there were no effects on immune function or inflammation. 
Some researchers think the sources of quercetin and its oral bioavailability may contribute to these inconsistencies.
The quercetin content in food can vary greatly depending on where it’s grown, sunnier climates will have a higher content. The storage methods used will also affect content, onions lose some quercetin when stored whereas in strawberries their content increases if they’re frozen. Cooking methods used will also have an effect, with boiling being shown to greatly reduce content. Quercetin also has notoriously poor oral bioavailability and relatively low absorption. Individual differences like genetics and gut microbiota will also impact bioavailability. Further complicating matters is that different quercetin forms have been used in studies including aglycone, glucuronide, isoquercetin, and rutin.
 Kim, Y., & Je, Y. (2017). Flavonoid intake and mortality from cardiovascular disease and all causes: A meta-analysis of prospective cohort studies. Clinical Nutrition ESPEN, 20, 68-77. doi:10.1016/j.clnesp.2017.03.004
 Knekt, P., Kumpulainen, J., Järvinen, R., Rissanen, H., Heliövaara, M., Reunanen, A., . . . Aromaa, A. (2002). Flavonoid intake and risk of chronic diseases. The American Journal of Clinical Nutrition, 76(3), 560-568. doi:10.1093/ajcn/76.3.560
 Larson, A., Symons, J. D., & Jalili, T. (2010). Quercetin: A Treatment for Hypertension?—A Review of Efficacy and Mechanisms. Pharmaceuticals, 3(1), 237-250. doi:10.3390/ph3010237
 Lee, K., Park, E., Lee, H., Kim, M., Cha, Y., Kim, J., . . . Shin, M. (2011). Effects of daily quercetin-rich supplementation on cardiometabolic risks in male smokers. Nutrition Research and Practice, 5(1), 28. doi:10.4162/nrp.2011.5.1.28
 Heinz, S. A., Henson, D. A., Austin, M. D., Jin, F., & Nieman, D. C. (2010). Quercetin supplementation and upper respiratory tract infection: A randomized community clinical trial. Pharmacological Research, 62(3), 237-242. doi:10.1016/j.phrs.2010.05.001
 Nieman, D. (2008). Quercetin Reduces Illness but Not Immune Perturbations after Intensive Exercise. Yearbook of Sports Medicine, 2008, 115-116. doi:10.1016/s0162-0908(08)79224-9
 Davis, J. M., Carlstedt, C. J., Chen, S., Carmichael, M. D., & Murphy, E. A. (2010). The Dietary Flavonoid Quercetin Increases VO2maxand Endurance Capacity. International Journal of Sport Nutrition and Exercise Metabolism, 20(1), 56-62. doi:10.1123/ijsnem.20.1.56
 Kressler, J., Millard-Stafford, M., & Warren, G. L. (2011). Quercetin and Endurance Exercise Capacity. Medicine & Science in Sports & Exercise, 43(12), 2396-2404. doi:10.1249/mss.0b013e31822495a7
 Sharp, M. A., Hendrickson, N. R., Staab, J. S., Mcclung, H. L., Nindl, B. C., & Michniak-Kohn, B. B. (2012). Effects of Short-Term Quercetin Supplementation on Soldier Performance. Journal of Strength and Conditioning Research, 26. doi:10.1519/jsc.0b013e31825cf22d
 Heinz, S. A., Henson, D. A., Nieman, D. C., Austin, M. D., & Jin, F. (2010). A 12-week supplementation with quercetin does not affect natural killer cell activity, granulocyte oxidative burst activity or granulocyte phagocytosis in female human subjects. British Journal of Nutrition, 104(06), 849-857. doi:10.1017/s000711451000156x