Rates of two rare childhood cancers declined after the U.S. began requiring grain products to be fortified with the B vitamin folic acid, a new study finds.

Reported in the journal Pediatrics, the study does not prove that folic acid deserves the credit.

But researchers say the findings at least offer reassurance that folic-acid fortification has not led to an increase in children's cancers -- which has been a theoretical concern.

"The good news is that there doesn't seem to have been an increased risk of childhood cancer," said lead researcher Amy M. Linabery, of the University of Minnesota in Minneapolis.

In 1996, the U.S. mandated that enriched flours, breads, pastas and other grain products be fortified with folic acid, the synthetic version of the B vitamin folate.

The goal was to help reduce rates of neural tube defects, severe birth defects of the brain and spine. They include spina bifida, when the spine fails to close during early fetal development, and anencephaly -- usually fatal -- when much of the brain never forms.

Experts advise women of childbearing age to get 400 micrograms of folic acid per day, and urge them to establish that level of intake before conception, since neural tube defects take shape very early, before many women know they are pregnant.

Some other countries, though, are still debating whether to add folic acid to the grain supply. One concern has been the vitamin's potential to contribute to cancer; some research has linked folic-acid fortification to an increased rate of colon cancer in adults.

But no one knows if folic acid is to blame. In fact, other research has tied higher intakes of folate from food to a lower risk of colon cancer.

Since women should already be getting folate and folic acid to curb the risk of birth defects, these latest findings should offer them some reassurance that it's safe as far as their future children's cancer risk, according to Linabery.

For their study, Linabery and her colleagues used government data on cancer rates among children younger than five between 1986 and 2008.

Over those years, 8,829 children were diagnosed with cancer. Overall, cancer rates were similar before and after mandatory folic-acid fortification.

But for two cancers, the rates dipped in the folic-acid era, compared with the decade before.

Wilms tumor, a type of kidney cancer, dipped from about 18 cases for every one million children each year, to just under 15 cases per million.

The other cancer, known as a primary neuroectodermal tumor, declined from roughly four cases per one million each year, to just over two cases per million.

Both cancers are quite rare, and no one is suggesting that women take folic acid to prevent the diseases in their children.

"But we already believe that women should be getting folic acid for the proven benefit of preventing neural tube defects," Linabery said.

The study has limitations. It looked at overall national patterns -- and not the relationship between a woman's actual folic acid intake and her child's individual risk of cancer.

Still, Linabery said, "taking folic acid during the childbearing years is important." And these findings, she added, offer reassurance that fortification has not led to any increases in childhood cancer in the U.S.