The rate of spina bifida and anencephaly birth defects has fallen by more than a third since the addition in 1998 of folic acid to the nation's enriched flours, rice and pastas, according to a study to be released today.
The study, published in the journal Pediatrics, prompted a renewed call from some scientists and health advocates for the Food and Drug Administration to double the required fortification levels to further reduce the rate of the birth defects.
"We're not at maximum prevention," said Dr. Jennifer L. Howse, president of the March of Dimes. "We would like the FDA to reconsider this matter, hold hearings and act as soon as they can."
Other scientists, however, said not enough was known about the consequences of enriching food with folic acid and cautioned that even rare side effects could affect a significant number of people when the entire population was receiving the vitamin through food.
"No one's really looked," said Barry Shane, professor of nutrition at UC Berkeley.
For instance, folic acid can mask the symptoms of a vitamin B-12 deficiency, which is common in the elderly and can lead to neurological problems.
Spina bifida and anencephaly arise when the neural tube of a developing embryo, which forms the brain, skull and spine, does not close properly during the first weeks of pregnancy. The defect can cause paralysis in spina bifida and serious malformation of the brain in anencephaly.
In 1991, scientists demonstrated that the risk of the birth defects could be significantly reduced by giving expectant mothers a synthetic version of folic acid.
The natural form of the vitamin, which is less easily absorbed by the body, is found in green leafy vegetables, whole grains and citrus fruits.
The government recommendation is for women of childbearing age to take 400 micrograms of folic acid each day.
Because the vitamin is needed so early in pregnancy, by the time many women discover they are pregnant it is too late to take a folic acid supplement.
After much debate among health researchers, the FDA mandated that starting in 1998, 140 micrograms of the vitamin be added to each 100 grams of grains that are labeled as "enriched." A higher level was eventually added to flour.
In 2001, the federal Centers for Disease Control and Prevention reported that U.S. rates of spina bifida and anencephaly had fallen by almost 20%.
The new study, also conducted by the CDC, examined birth defect records from 20 states and Puerto Rico from 1995 to 2002. The database, which covered more than 11 million births, turned up 4,468 cases of spina bifida and 2,625 cases of anencephaly.
The scientists found that after the introduction of folic acid, spina bifida decreased 36% among Latino births and 34% among non-Latino white births. Rates of anencephaly also fell in both groups.
The rate of the defects did not fall significantly for African American babies, who have a lower risk to begin with.
Scientists said they did not know why Latinos, African Americans and non-Latino whites have different rates of neural tube defects, but suspected it was related to diet and genetics.
Dr. Godfrey P. Oakley Jr., a research professor at Emory University's Rollins School of Public Health and one of the main advocates of folic acid fortification, said the findings showed that the FDA should have required a higher dose.
Other research has suggested that as much as 75% of all neural tube defects could be prevented through a higher level of folic acid fortification, he said.
Other Americans would also benefit, Oakley said, because folic acid has been shown to reduce blood levels of homocysteine, an amino acid that appears to be a risk factor for heart attacks and strokes.
"It's time to light a fire under this," said Oakley, who coauthored an editorial accompanying the study.
But other scientists said they were uncertain whether higher doses of folic acid would reduce the birth defect rate much more.