There is so much we don't know about nutrition science. I was just reading about how vitamin C excess in supplements can release iron stores in the body, which can be harmful, since iron is an oxidant...especially for men in relation to heart disease.
The article below talks mainly about 'red super foods', but I thought it was interesting about quercetin and other flavonoids.

Junkfood Science: Super reds and super foods — Will more antioxidant flavonoid foods make us healthier?
Excerpt:
To illustrate how illogical our fears about foods and chemicals can be, many people fear any detectable amount in food of any chemical that has ever been shown to cause cancer in rodent studies. No one would argue, though, that fruits, vegetables, nuts and teas can’t be part of a healthful diet. Yet, quercetin is one of the flavonoids that’s been tested and found to be carcinogenic in rodent studies (which use high exposures), according to Dr. Bruce N. Ames, Ph.D., renowned researcher on mutagenic and carcinogenic risks and professor in Biochemistry & Molecular Biology at the University of California, Berkeley and Dr. Lois Swirsky Gold, Ph.D., director of the Carcinogenic Potency Project at University of California, Berkeley. But like all chemicals, natural or synthetic, they caution, the low levels of chemicals our bodies are naturally exposed to every day are highly unlikely to be a risk for cancer. “Humans have many natural defenses that buffer against ordinary exposure to toxins and these are usually general, rather than tailored for each specific chemical.”

Researchers now understand that the antioxidant capacity of flavonoids is decreased when broken down by digestion; the body perceives them as foreign compounds and tries to excrete them rapidly. It also appears that it only takes small amounts to see healthful benefits, but more is not necessarily better. Just because certain foods are bursting with some nutrient that appears healthful does not mean that even more is very, very healthful.

The logic used by those promoting antioxidants for anti-aging also reflects a misunderstanding of how cells detect and repair the damage caused by free radicals and the important role that free radicals play in normal physiological processes (such as the immune response and cell communication), according to anti-aging experts. While free radicals promote beneficial oxidation, in excess they produce harmful oxidation that can cause cell damage, doctors Stephen Barrett, M.D., and Victor Herbert, M.D., J.D., M.A.C.P., F.R.S.M. have explained. Vitamins C and E and beta-carotene, for example, are mischaracterized as ‘antioxidants,’ when they are really redox agents (antioxidants in some instances and pro-oxidants in others, producing harmful free radicals). That may explain why antioxidant supplements or eating more fruits and vegetables high in antioxidants have not proven out to be beneficial in clinical trials on real people, and in some cases suggest harmful effects.

Dr. Martyn T. Smith, Ph.D., professor of toxicology at the University of California, Berkeley, has particularly cautioned about consuming high concentrations of flavonoids being sold as health food supplements. Writing in the journal, Free Radical Biology and Medicine, he and co-author Christine F. Skibolaa said that at low concentrations, flavonoids appear to acts as antioxidants and potentially have anti-cancer actions to block and inhibit cell division. But the amounts in some popular supplements they tested found levels 10 to 20 times what is recommended for the human body. “At high concentrations, certain flavonoids can act as pro-oxidants and become mutagenic, meaning that they could cause oxidative damage and cause DNA and chromosome damage,” they said. At these levels, flavonoids can also alter normal body functions, hormones, interfere with metabolism of drugs and interfere with the absorption of needed minerals.

Women of childbearing age who might become pregnant should be especially careful, they wrote, as flavonoids cross the placenta. The fact is, high levels of flavonoid consumption has never been tested and shown to be safe in humans, and the potential adverse effects may outweigh benefits. “Just because something comes from a natural source doesn't mean it can't hurt you,” Skibola said. “The dose makes the poison.... unfortunately people tend to forget that.”