Measurements of long-term supplementation with quercetin yield a wide variance in absorption rates. A 12-week study of 1,000 subjects who received either placebo or 500 mg or 1,000 mg of quercetin per day yielded a range of unpredictable responses in plasma levels. A study of ileostomy patients given single 100 mg doses of quercetin found a mean absorption rate of 24%. Another study found a 52% absorption when the quer¬cetin was administered as a serving of fried onions.17 Polymor¬phisms in intestinal enzymes, transporters, and liver enzymes, and effects of coadministered foods (dietary compounds), may explain some of these variances.
Many of these recent studies on quercetin, including the one highlighted above, have been conducted by researchers at Appalachian State University in Boone, North Carolina. In one of their earlier quercetin studies published in 2008, the researchers gave trained cyclists either 1,000 mg/day daily doses of quercetin or placebo for 5 weeks. After 3 weeks of normal training during the winter season, these athletes rode intensely (3 hours per day at 57% wattsmax) for 3 days in a row and were tested for multiple measures of inflammation, oxidative stress, and immune function and monitored for incidence of upper respiratory tract infection (URTI). Quercetin supplementa¬tion increased plasma quercetin levels and reduced incidence of URTI during the 2-week period following the 3-day burst of extreme exertion but had no effect on laboratory measurements of immune dysfunction, inflammation, or oxidative stress. Although, “F2-isoprostanes, nitrite, ferric-reducing ability of plasma, trolox equivalent antioxidant capacity, and C-reactive protein were significantly elevated as a result of exercise,” no difference was seen between those taking quercetin and those not taking it.
The Boone researchers conducted a second parallel study of the same duration but, rather than cyclists, used ultra-marathon runners who competed in the 160-kilometer Western States Endurance Run (WSER). Subjects took 1,000 mg of quercetin or placebo each day for 3 weeks prior to the race. Quercetin supplementation again had no effect on inflammation, immune dysfunction, or oxidative stress in serum samples from the runners after completing the race. URTI rates did appear lower in the following weeks in those who took quercetin but not to the point of statistical significance. Combining data from this WSER study with data from their earlier cyclist study did yield a statistically significant reduction in post exertion illness; URTI rates were two-thirds lower in the quercetin group than in the placebo group. Yet the lack of measurable effects in serum testing suggests the parameters being measured may not accurately predict the benefit of supplementation.
The data were not what the researchers had expected and prompted re-evaluation about half-life, absorption, and bioavailability of quercetin. In these two studies, the last dose of quercetin had been taken 10–24 hours before the exercise bout, a length of time that might have exceeded quercetin half-life, which may be as short as 3.5 hours.
In rat studies, omega-3 fats, vitamin C, vitamin E, and green tea extracts have a synergistic effect on quercetin’s action. The Boone researchers used this information to design their next trials.
In July 2009, the Boone group described a trial utilizing a combination of quercetin, green tea, and fish oil. Quercetin (1,000 mg/day), with or without the addition of epicallocat¬echin 3–gallate (EGCG; 120 mg), isoquercetin (400 mg), and EPA-DHA (400 mg), was administered to a group of cyclists who went through the same 3-hour, 3-day period of extreme exertion. This time the treatment period lasted only 2 weeks. In this trial, the last dose was taken only 1 hour before the start of the heavy exertion. This trial yielded far better results: the mixture reduced post-exercise measures of inflammation, oxidative stress, and immune disruption. Plasma quercetin levels of those who took the supplement combination were almost twice as high as in those who took quercetin alone. This suggests that improved absorption was at least partly responsible for this improved effect.