According to a new analysis, people taking high doses of the B vitamin folic acid are not at an increased risk of cancer, easing some concern about possible side effects of national fortification programs.
The U.S. and Canada have required white flour to be fortified with folic acid since 1998, after deficiencies in pregnant women were tied to brain and spinal cord birth defects in their babies.
However, researchers noted, fortification is not mandatory in Western Europe, for example, in part because of concern that extra folic acid might slightly increase people's risk of cancer due to its role in cell growth. Cells, including cancer cells, need folate - the natural form of folic acid - to grow and divide.
While this new finding is good news, researchers say there are still enough questions about the long-term effects of folic acid that people should still "be a little cautious" about loading up on the vitamin.
For the new analysis, a team of international researchers combined data from 13 separate trials that randomly assigned participants to daily folic acid or a vitamin-free placebo and recorded who went on to develop cancer.
The studies included a total of close to 50,000 volunteers who were followed for just over five years, on average.
During that time, 7.7 percent of people in the folic acid groups and 7.3 percent in the placebo groups were diagnosed with any kind of cancer - a difference that could have been due to chance.
Likewise, there was no increased risk of individual cancers - including colon, prostate, lung or breast cancer - attributed to folic acid.
Most trials used daily doses of folic acid between 0.5 and 5 milligrams. In the one study that used a much larger dose - 40 mg daily - there was still no difference in cancer diagnoses between people who were and weren't taking the vitamin.
The total daily amount of folic acid delivered through flour fortification is less than 0.5 mg per day for most North Americans.
Folate, the natural form of the vitamin, is found in spinach, asparagus, lettuce and other greens. The recommended upper daily limit of folic acid from a supplement is 1.0 mg.
"The conclusion you can make from this is that over a relatively short period of time, there was no significant benefit or harm," said a researcher who worked on the review. Most cancers take 10 to 20 years to develop, so it's hard to tell from shorter studies if there really is no folic acid-cancer link or if the researchers didn't follow people for long enough to see an association, whether positive or negative.
Folic acid has a dual nature: extra folic acid could actually prevent cancers from developing in the first place, but once cancer is formed, then they're like any other growing cells which need folate to proliferate.
The researchers agreed this study shouldn't be the last word on the potential side effects of folic acid.
For now, the researcher added, people might want to avoid piling supplements on top of multivitamins and fortified food.
Source: The Lancet, online January 25, 2013.
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