Flavonoids have been found to be very powerful antioxidants when tested in the laboratory.

But newer studies suggest they may not be as useful within the body. Some, such as quercetin, can become oxidized and turn into oxidants themselves.

Taking the other antioxidant vitamins, minerals, and phytonutrients along with quercetin allows it to remain in its most powerful antioxidant form.

Normally, when ascorbate (vitamin C) encounters a free radical, the ascorbate is converted into an oxidized form called dehydroascorbate (DHA). This oxidized form of ascorbate is not a strong radical, and is rapidly taken up by white blood cells and converted back to the reduced antioxidant form.

Vitamin E plays a vital role in this conversion. But there are conditions under which ascorbate can become a dangerous free radical-producing substance.

High levels of free iron and copper in the presence of ascorbate can trigger the generation of a very powerful free radical called the hydroxyl radical.

That’s why everyone should exercise caution in taking iron or copper supplements, as well as eating foods high in iron and/or copper.

It is also why you shouldn’t cook with iron or copper-plated cookware. Also avoid aluminum and nonstick pots and pans. Cooking with stainless steel is always best.

Like vitamin E, another antioxidant called glutathione plays a critical role in preventing vitamin C from oxidizing. People with low glutathione levels need higher doses of ascorbate for protection.

Most antioxidants — including N-acetyl-L-cysteine, R-lipoic acid and many flavonoids — will raise glutathione levels in cells.

Toxic metals, especially mercury, cannot only lower glutathione levels, they also prevent oxidized glutathione from being converted back to its antioxidant form.