The report, from Tufts University, is the latest to raise a cautionary note about a public-health policy that has been largely viewed as a success.

"Have we done more harm than benefit?" said Dr. John Potter, a colon cancer expert at the Fred Hutchinson Cancer Center in Seattle, who was not connected to the latest research.

Writing last month in the journal Cancer Epidemiology Biomarkers and Prevention, scientists reported that colon cancer cases in the U.S. spiked after manufacturers began fortifying cereal grains with folic acid in the late 1990s. They saw a similar trend in Canada, which began fortification around the same time.

The pattern was surprising, researchers said, because colon cancer rates had been steadily dropping since the mid-1980s. Greater consumption of folic acid looked like the explanation.

Joel Mason, lead author and professor of nutrition science and policy at Tufts, said the report does not prove that extra dietary folic acid causes colon cancer but does suggest fortification may have unforeseen trade-offs. Nutritionists have long known that younger women need 400 micrograms of folic acid daily to reduce their chances of giving birth to infants with neural tube defects, caused by the failure of the fetal spinal column to fully close. Although rare, such defects are devastating.

Since 1998, U.S. food manufacturers have been required to add 140 micrograms of folic acid to each 100 grams of cereal grains that are labeled "enriched." Breads, cereals and other grain-based foods shipped across state lines are all fortified with folic acid, a B vitamin naturally found in green leafy vegetables, fruits, dried beans and nuts.

In only a few years, the rate of neural tube defects in the U.S. fell, from 10.6 per 10,000 births in 1996, before fortification, to 7.6 per 10,000 births in 2000. Canada also saw a sharp decline. Those results deepened the desires of some scientists and health advocates for even greater improvements.

The nonprofit March of Dimes, which has long endorsed increased fortification, is preparing a petition asking the Food and Drug Administration to further boost folic acid levels in cereal grains. The March of Dimes said government surveys show that many women ages 18 to 45 do not receive adequate amounts of folic acid in their diets. But some researchers have cautioned against increased fortification because of possible downsides. Folic acid can mask symptoms of vitamin B-12 deficiency, common in the elderly. Unaddressed, a B-12 deficiency can lead to neurological problems.

Not long ago, it was thought that almost everyone stood to benefit from taking large amounts of folic acid, which had been linked to a decreased, not increased, risk of colon cancer. That belief was challenged in June when researchers reported in the Journal of the American Medical Association that large daily doses of folic acid did not reduce development of precancerous growths called adenomas. The 1,000-patient study of people with a history of adenomas also found folic acid seemed to increase their colon cancer risk.

About 130,000 Americans are diagnosed with colon cancer each year, and 56,000 of them die from it. Still, Mason and others said it was premature to stop folic acid fortification altogether.

"Can we make a case that more people are at risk of developing colon cancer than are at risk of developing neural tube defects? We really don't have the answers," Potter said.

From the August 13, 2007, Prepared Foods e-Flash