Folic acid's trade-offs of concern

Willett, the Harvard nutritionist, said it could take another decade or more for the impact of folic acid fortification to be known. But based on what is known, he is optimistic that the many upsides of folic acid will be affirmed.

"Given the overall picture, if there is any negative impact, it is counterbalanced by other positive trends," he said. "I think we've done something that overall is beneficial, and we won't know the full balance of benefits, or possible adverse effects, for many years. But overall the picture looks good."

Still, he added, "Any time you do anything that's beneficial, you take some chance of unintended harm. Almost anything you do in a complex system - and human systems are complex - perturbs something else. Biology is very complicated. There are often unintended consequences, good and bad."

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Folic Acid Facts

What it is

A member of the vitamin B family, folic acid is the synthetic form of the nutrient folate.

Where it's found

Occurs naturally in green leafy vegetables, fruits and nuts. Found in fortified flours, breads and pasta. Highest concentrations are typically found in nutritional supplements.

Known benefits

Prevention of birth defects such as spina bifida; also might prevent some cancers, stroke and cardiovascular disease.

Suspected problems

Might accelerate development of pre-cancerous growths, particularly in the colon, into cancer. Also might increase rate of cognitive impairment in older people.