Berkeley - Fruits and vegetables keep you healthy, but some of their chemical components, concentrated and sold in high doses as flavonoid supplements in health food stores, are likely to make you sick, warn scientists at the University of California, Berkeley's School of Public Health.

The warning applies to such popular products as ginkgo pills, quercetin tablets, grape seed extract and flaxseed, which contain high concentrations of flavonoids.

Unlike vitamins C and E, flavonoids become dangerous at the high doses available in some supplements, which are not regulated by any governmental agency, according to Martyn Smith, UC Berkeley professor of toxicology.

Although they may protect against some forms of cancer when consumed in the diet, plant flavonoids actually have the capacity to become carcinogenic at higher levels, said Smith. High doses of these chemicals also carry other health risks including a small but documented risk for a rare form of leukemia in young children.

"I think some Americans could be poisoning themselves with these supplements." said Smith. "The potency of concentrated plant flavonoids in some of these products has been radically underestimated."

"Please do eat fruits and vegetables," he said. "But stop taking these supplements."

Smith and UC Berkeley graduate student Christine F. Skibola are publishing their findings on the health effects of high flavonoid intake this week in the English scientific journal, "Free Radical Biology and Medicine."

Their paper describes the many biological activities of flavonoids, showing that high levels of plant flavonoids can bind with and damage chromosomes and DNA in cell cultures. The effects follow a gradient, with protective effects at low levels and mutagenic effects at high levels.

Flavonoids also alter the activity of various enzymes and interfere with hormone metabolism, particularly estrogen and thyroid hormones, according to Smith and Skibola.

None of these adverse effects is likely to occur at dietary levels of consumption. In fact, Asians and vegetarians are known to have less cancer than other people, in part because of their high consumption of flavonoids in soy, green tea and vegetables.

The authors point out, however, that no one could swallow in food anywhere near the amounts of flavonoids provided in some supplements.

Studies in the United States, Europe and Asia, for instance, show that people get 5-68 milligrams of quercetin in their diet per day. But a popular health food supplement recommends taking 1,000 milligrams in one swallow - 10 to 20 times more than even a high dietary intake of quercetin.

"That's when we get worried," said Skibola. "There is no rhyme or reason for the dosages recommended on these bottles. These compounds need to be regulated."

"The idea that natural is safe is completely wrong," Smith added. "The ability to take these compounds in pill form has transformed positive, protective vegetable elements into potentially dangerous compounds."

More research is needed into the adverse effects of excessive flavonoid intake. But Smith and Skibola say that for now, "the public should be wary of the beneficial claims made for these supplements and limit their intake until their safety is established. Otherwise, they may be doing more harm than good."