Explain the Toxicity of Folate?

There is no evidence to suggest that it is possible to consume sufficient natural folate to pose a risk of toxicity. However, this clearly does not apply to folic acid given in supplements or fortified foods. The main concern with fortification of high levels of  folic acid is the masking of the diagnosis of pernicious anaemia, because high levels  of folic acid correct the anaemia, allowing the neuropathy to progress undiagnosed  to a point where it may become irreversible, even upon treatment with vitamin B. Consumption of large amounts of folic acid might also pose other less well-defined risks. The United States National Academy of Sciences (NAS), after reviewing the literature, has suggested an upper level of 1000 mg. There is probably no great risk of toxicity at a range of intakes between 400 and 1000 mg of folic acid per day, with the exception of some increased difficulty in diagnosing pernicious anaemia.