Folic acid--the scientific debate as a base for public health policy.

Randomized controlled trials have proven that periconceptional folic acid intake reduces the risk of neural tube defects (NTDs). This lead to different public health policies: fortification of foods in many countries and supplementation in some others. We concentrate here on pro's and con's of fortification policies. Meanwhile, new beneficial but also potential adverse effects are being hypothesized. Highest level evidence is available for the protective effect of folic acid on NTDs. Lower level evidence suggests other protective effects, but also some potential adverse effects, such as masking Vitamin B-12 deficiency, increasing twinning rates and an 'acceleration phenomenon' in pre-existing malignant neoplasms. While observational studies show lower cancer rates associated with increased folate intake, some case reports and animal experiments suggest opposite effects. Thus, public health policy makers are facing the question of balancing beneficial and potential adverse effects repeatedly. We propose that the scientific debate no longer focuses on NTDs alone, but that a comprehensive evaluation be undertaken by a public health authority with experience in complex meta-analyses and technology assessment.