Here's a recent study that moves beyond epidemiology:
Altered Folate Availability Modifies The Molecular Environment Of The Human Colorectum: Implications For Colorectal Carcinogenesis, Cancer Prevention Research, April, 2011
Ten volunteers received supplemental folic acid (1 mg/d) for 8 weeks. A second group of 10 consumed a low-folate diet for 8 weeks (they were admitted to hospital as inpatients for this purpose).
Biopisies of the sigmoid colon and rectum revealed an increase in markers of inflammation in the supplemented group. The authors hypothesized:
"Excessive folate supplementation might promote colorectal carcinogenesis by enhancing proinflammatory and immune response pathways."Participants were taking a milligram (mg) of folic acid. It's not hard to get a mg (equal to 1000 micrograms (mcg)) of folic acid in a day, not since the FDA in 1996 required all flour and uncooked cereal grains be fortified with folic acid (at 140 mcg/100 grams). The recommended allowance (or DRI: Dietary Reference Intake) for folic acid in this country is 400 mcg. A basic multivitamin usually contains that, but so does a bowl of fortified breakfast cereal. Add some pasta and a piece or two of bread, which are now fortified, and you're well over 800 mcg, the amount found to increase cancer risk in this study.2 (And the amount in those Twinlab Folic Acid Caps pictured.)
Taking vitamins is not as innocent as supplement manufacturers lead us to believe.
1 Role of Folate In Colon Cancer Development And Progression, The Journal of Nutrition, 2003
2 Cancer Incidence And Mortality After Treatment With Folic Acid And Vitamin B12, JAMA, 2009