Can You Overdose on Folic Acid?

folic acidVitamins have a great public image. Even the term itself derives from the Latin “vita,” meaning life. And indeed vitamins are essential to life. Our bodies cannot make these nutrients, so we must get them from our diet. But just because a little is good, more is not necessarily better. Folic acid, one of the B vitamins is a case in point. It plays a role in a number of biochemical reactions, including the production of DNA. Each time our cells multiply, they need to produce more DNA. And life of course depends on the multiplication of cells. Unfortunately, so does cancer. And since cancer cells multiply very quickly, they have a greater requirement for folic acid. This suggests that as far as a link between cancer and folic acid goes, dosage may be very important. We want to make sure that our body is well supplied with the amount of folic acid needed for all the essential biochemical reactions, but we also want to guard against furnishing cancer cells with the dose they need to proliferate. Indeed, methotrexate, one of the medications used in cancer treatment works by blocking the conversion of folic acid into tetrahydrofolate, the active form of the vitamin. Folic acid is especially critical in the earliest stages of development. Birth defects, such as spina bifida and anencephaly, a tragic condition in which the brain has not properly formed, can be caused by a deficiency in folic acid. That’s why folic acid is added to “enriched” flour. This fortification program has worked remarkably well, with birth defects attributed to folic acid deficiency having being significantly reduced.

When it became clear that folic acid supplementation had such a protective effect against birth defects, researchers began to examine its potential to prevent other health problems. Animal experiments and theoretical considerations had suggested protection from heart disease and cancer. While no evidence was found for heart disease in humans, the Nurses Health Study which had followed the health status of thousands of nurses over decades revealed that women with the highest folic acid intake were least likely to get colon cancer. This stimulated a study at Dartmouth College of more than a thousand men and women who previously had polyps removed from their colon. They were given a thousand micrograms of folic acid a day, with hopes of preventing cancer. Not only did that not happen, there was actually a slight increase in cancerous polyps. But this study involved people who already had polyps and therefore says nothing about folic acid preventing polyps in the first place, as was evidenced in the Nurses Health Study. Also disturbing was the finding of an increased incidence of prostate cancers. But we have to keep in mind that the supplemental dose of 1000 micrograms is considerably larger than the 400 micrograms found in most supplements. What are we to conclude from all this? That the effects of folic acid supplementation on babies may be different from that on adults, and that in adults folic acid may prevent cancer but make existing cancers worse. And of course, dose matters, more is certainly not better. At this point, there is no reason to take any supplement that has more than 400 micrograms of folic acid.

Joe Schwarcz

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