updated 1/4/2007 6:54:17 PM ET 2007-01-04T23:54:17

Blood levels of folate in young women are dropping, a disturbing development that could lead to increased birth defects and may be due to low-carb diets or the popularity of unfortified whole-grain breads.

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Government health officials could only speculate on the reasons but called the backslide in this important B vitamin disturbing.

It's not clear how the decline in folate levels has affected newborns, but preliminary data suggest the dramatic declines in neural tube defects seen in the late 1990s may have leveled off by 2004, said officials with the U.S. Centers for Disease Control and Prevention.

"This is a cause of substantial concern," said Dr. Nancy Green, medical director for the March of Dimes, which campaigns for birth defects prevention.

Folate is a naturally occurring B vitamin. An artificial version, which is more easily metabolized by the body, is folic acid.

Years ago, scientists concluded that folate deficiencies contributed to the occurrence of serious birth defects of the spine and brain, known as neural tube defects.

So the government has long urged women to eat cereals and breads fortified with folic acid to help prevent birth defects. By the late 1990s, the fortification campaigns were succeeding: Folate levels increased, and neural tube defects dropped by as many as 1,000 a year.

But a CDC study released Thursday found an 8 percent to 16 percent decline in folate levels in U.S. women of childbearing age, according to large blood-drawing surveys done between 1999 and 2004.

First decline since health campaigns began
It was the first time such a decline has been seen since the start of government health campaigns urging women to make sure they get enough folic acid.

The decline was most pronounced in white women, although black women continue to be the racial group with the least folate in their blood, health officials said.

The study was based on a regular national survey that involves not only interviews but physical examinations and blood tests. It measured the blood of about 4,500 women, ages 15 to 44, between 1999 and 2004.

It's being published this week in a CDC publication, Morbidity and Mortality Weekly Report.

It's not clear why blood folate levels dropped in this decade, but there are several possible explanations, experts said.

Increasing obesity rates among young women may be a factor. Research has found obese people metabolize folate differently than thinner folk, and some doctors believe heavier women need more folic acid to prevent neural tube defects, Green said.

The Atkins affect
Diet trends may have been be another factor, said Dr. Joseph Mulinare, a CDC epidemiologist who was the study's lead author.

He noted that in 1998, the Food and Drug Administration began requiring that folic acid be added to breads, cereals and other products that use enriched flour. Whole-grain breads were not under that mandate because they already contain some folate.

Low-carb diets increased in popularity during the early 2000s. Women who avoided flour and bread products because of their carbohydrates may have also taken in less folic acid, Mulinare said.

Vitamins and supplements are the best way to get the recommended daily dose of 400 micrograms of folic acid. But only a third of women of child-bearing age take a folic acid-containing supplement every day, he said.

Eating certain foods also helps, especially breads, cereals and other products containing enriched flour.

While whole-grain breads contain natural folate, it's a smaller amount than the folic acid in enriched breads. So the popularity of whole-grain breads "may be a factor" in the drop in folate levels, Mulinare said.

Copyright 2007 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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