A number of new reports suggest folic acid fortification in foods may have detrimental health effects, including increasing the risk of cancer. In many of these reports it is noted that consuming liberal amounts of folate rich foods appears to pose no risk while too much folic acid, an isolate not found in nature, may be detrimental. From a physiological standpoint the liver may have a limited capacity to metabolize folic acid into folate.
In a recent study published in the European Journal of Gastroenterology and Hepatology. Hirsh et al observed a 162% increase in the rate of hospital discharges owing to colon cancer in Chile immediately following the country's implementation of a folic acid fortification program. Hirsh et al go on to to report that their findings are "in conflict" with epidemiological results showing inverse associations for colon adenomas and colorectal cancer and plasma folate levels. In other words, folic acid in fortified foods appear to increase cancer risk, yet low plasma folate appears to be corelated with increased risk as well.
The stark differences in data may owe to the form of folate taken. Hirsh et al note, for instance, that the epidemiological studies suggesting that low plasma folate levels correlate with higher cancer risk were among individuals not taking multivitamins or in deficient populations. In other words, such individuals were deficient in folate rich foods (which pose no risk) and did not take multivitamins with folic acid, the chemical isolate used to fortify foods and also found in most supplements.
Study: Hirsh, S., European Journal of Gastroenterology and Hepatology 2009, 21:436-439