Make of this what you will:
The human evidence is from people who did not know they had a lesion. In the Aspirin/Folate Polyp Prevention Study, adults who had already had a colorectal adenoma removed were randomised to 1 mg folic acid a day or placebo. Folic acid did not reduce new adenomas. Over the later follow-up it was associated with more advanced adenomas, and with a higher rate of cancers outside the colon. In the men in that trial, the estimated 10-year chance of a prostate cancer diagnosis was 9.7% on folic acid and 3.3% on placebo, an age-adjusted hazard ratio of 2.63. Dietary folate at baseline pointed the other way. The trialists themselves noted that the prostate signal might be a chance finding among many endpoints, and other B-vitamin trials have been weaker or null. It is still the clearest randomised signal that a supplement dose can increase diagnoses in people who were not being treated for cancer.
The trial ran from 6 July 1994 to 31 December 2006. The main adenoma results were published in 2007 in the Journal of the American Medical Association. The prostate-cancer analysis of the men in that same trial was published in 2009 in the Journal of the National Cancer Institute.
The dose was 1 mg of folic acid a day, against placebo, in people who had already had a colorectal adenoma removed. Median follow-up for the prostate report was 7 years, with some participants followed for up to 10.8 years.
