The scientists say folic acid given over a period of more than three years may feed the growth of cancers that were too small to be detected otherwise. They say their study raises questions about the benefits of fortifying food with folic acid.
Many countries, including the United States add it to flour and other grains. Getting enough of it is crucial for women before pregnancy to prevent serious birth defects like spina bifida.
In Norway, food is not fortified with folic acid.
The study, conducted by researchers at Haukeland University Hospital in Bergen, Norway was published in the Journal of the American Medical Association.