Although they may protect against some forms of cancer when consumed in the diet, plant fla-vonoids may actually have the capacity to become carcinogenic at higher levels, scientists at the UC Berkeley School of Public Health report in a new study.

In the August Free Radical Biology and Medicine, UC Berkeley toxicologist Martyn Smith and graduate student Christine F. Skibola describe the many biological activities of fla-vonoids, showing that high levels of plant fla-vonoids 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.

The authors point out that no one could swallow in food anywhere near the amounts of flavonoids provided in some dietary supplements. In fact, Asians and vegetarians have less cancer than other people, in part because of their high consumption of flavonoids in soy, green tea and vegetables ( see pages 26 , 33 ).

Studies in the United States, Europe and Asia, for instance, show that people get 5 to 68 milligrams of the flavonoid quercetin in their diets 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,” says Skibola. “There is no rhyme or reason for the dosages recommended on these bottles. These compounds need to be regulated.” Products such as Gingko biloba pills, grape-seed extract and flax-seed may contain high levels of flavonoids.