In this combined analysis of the three largest randomized clinical trials of folic acid supplementation for the prevention of colorectal adenomas, we found that up to 42 months of treatment and follow-up (mean duration=30.6 months), daily use of folic acid supplements of 0.5–1.0 mg does not prevent the occurrence of new colorectal adenomas in the large bowel among men and women with a previous history of adenomas. There was a non-significant trend towards a beneficial effect of folic acid supplementation among those with lowest plasma levels of folate, and adverse effects among those with the highest. Mortality was significantly lower in the folic acid arms.
Daily use of alcohol, a known folate “antagonist,” 30
did not significantly modify the effect of folic acid supplementation on adenoma risk, but we did observe a non-significant trend of decreasing risk of all adenomas associated with folic acid with increasing number of drinks. Many observational studies of colorectal and other cancers31
have indicated an interaction such that among individuals who drink alcohol, folate intake and blood levels are inversely associated with risk, in contrast to lesser or no benefit among those who abstain.
We also evaluated whether there may be other subgroups who are more susceptible to the effects of folic acid in the large bowel. Unlike observational findings for colorectal cancer from the NHS/HPFS study,32
we did not observe any difference in the effect of folic acid supplementation by family history. We observed no indication that other personal demographic characteristics, lifestyle and dietary factors significantly modified the risk of adenomas or advanced lesions aside from the trend in alcohol intake.
We found no differences in the rates of adverse events comparing folic acid and placebo treatment group in terms of myocardial infarction, stroke, colorectal cancer and all cancer types combined. Interestingly, we observed a potential beneficial effect of folic acid supplements on overall mortality. This statistically significant result is a marked contrast with the lack of mortality benefit seen in the much larger combined analysis of trials conducted to investigate the effect of folic acid supplementation (typically with other B-vitamins) for the possible prevention of cardiovascular outcomes among high-risk individuals.33
The reasons for this difference are not clear, although the cardiovascular trials all studied individuals at high risk of cardiovascular endpoints, in contrast to the generally healthy population in the adenoma trials.
We obtained data from all larger folic acid randomized clinical trials to conduct this combined analysis. Two small clinical trials were not included. A trial in Michigan randomized 137 individuals to placebo or 5 mg of folic acid and reported a significantly lower number of adenomas per patient in the folic acid compared to placebo group after 3 years of follow-up.11
The second small trial, which investigated 60 patients randomized to 1 mg of “folate” (presumably folic acid) or placebo and followed them for 2 years, reported a non-significant reduction in adenoma recurrence associated with folic acid use.12
Results from these trials differ from those reported here, for reasons that are not clear. The small Michigan trial used a 5 mg folic acid dose, and it is possible that this would have a different effect on adenoma occurrence than the lower doses used in the larger studies. In addition to the limited sample size and the high drop out rate, another limitation of this trial was that associations were only assessed in terms of numbers of adenomas per participant, i.e. a few participants with a large number of adenomas could have artificially inflated those observed associations. There was a substantial but non-significant increase in mortality and adenoma numbers in the folic acid group among subjects over 70 years old.
Results from this combined analysis of clinical trials also are not consistent with earlier observational studies that suggested that high intake and blood levels of folate are inversely associated with colorectal adenomas or cancer risk.1, 2
There are several potential explanations. For one thing, the trials have studied folic acid rather than the reduced folates found in natural food sources, and it is possible that the synthetic and natural folates differ in their effects.34
However, the fact that studies of folic acid supplements (largely from multivitamins) have also found apparent protective associations with risk of colorectal cancer argues against this possibility. Even if folic acid and natural folates have similar effects, the folate doses used in the trials represent folate intakes considerably higher than those obtained from diet, and it is a possibility that at these high doses, folate from any source may be detrimental. Of course, it is also possible that the observational studies are affected by confounding by the effects of other nutrients in folate-rich foods and other lifestyle characteristics of those who have high folate intake or use multivitamins. There were also few individuals with deficient levels of folate (<7nmol/L) in these trials, and there is a growing appreciation that while adequate levels of folate are important at some yet undefined threshold of folate, higher levels may not be beneficial.35
Therefore, our results suggest with greatest certainty that most people in fortified or similar populations are unlikely to benefit substantially from folic acid supplements within 3.5 years of follow-up and treatmen. However, it is unclear whether an effect may be observed in alcoholics with poor diets or populations with low folate levels or with a longer follow-up.
A further possible reason for the difference between the observational and clinical trial findings relates to the timing of the folate administration. Evidence from animal models suggests that folate may play a dual role in the colorectum depending on the timing of supplementation. In a Apc+/− Msh2−/− mouse model, modest doses of folate supplementation given after the formation of neoplastic foci appeared to increase the development of new tumors.8
Folic acid supplementation was also shown to incrementally increase the total number of aberrant crypt foci, the earliest precursor for colorectal cancer, and the mean number of crypts per focus.7
Since all the subjects in the trials included in this analysis had previous adenomas, presumably they already had transformed crypts that could be promoted by folic acid supplementation.
Studies of colorectal cancer in populations where the food supply is fortified with folic acid have suggested that folate may increase risk or benefit only those who do not take multivitamins.4, 36–38
Even in a population in which folic acid fortification of foods does not occur, a U-shaped association between folate levels and colorectal cancer risk has been observed in one study.5
Our findings focus on pre-cursor lesions to colorectal cancer, and the effects of folic acid may differ for adenoma development compared to those for invasive colorectal cancer.
Folate deficiency may increase risk of colorectal cancer via alteration in normal methylation patterns, which can affect maintenance of DNA integrity and stability and expression of oncogenes, tumor suppressor genes, and genes associated with changes in the expression of apoptosis and cell cycle genes.10, 39, 40
Folate supplementation might then inhibit carcinogenesis in deficient individuals, but have no particular effect in folate-replete populations.
On the other hand, high levels of folate may promote colorectal carcinogenesis, for example via their role in nucleotide synthesis.41, 42
Neoplastic cells have a relatively high rate of proliferation43
and an up-regulation of folate receptors44
compared to normal tissue. Folate antagonists, such as methotrexate, have been used as chemotherapeutic agents for several cancers including those in the colorectum.45
Furthermore, folic acid is a pharmaceutical fully oxidized, monoglutamyl form of folate. Unmetabolized folic acid may be detected in blood after folic acid supplementation 46
and may be associated with reduced natural killer cell cytotoxicity.34
Our meta-analysis has several limitations. First, only one clinical trial, the ukCAP trial, examined the effect of 0.5 mg of folic acid. This was also the only study conducted in a country that did not fortify food products with folic acid. However, in none of our analyses did we find substantial indications of between-trial heterogeneity. Second, results from this meta-analysis address only the short-term effects of folic acid supplementation, during follow-up of up to 42 months. Longer studies are needed to examine potential differences by duration of use and duration of follow-up. Follow-up in the AFPPS and NHS/HPFS is continuing, though not in the ukCAP trial.
We were also unable to examine potential effects on other cancers and cardiovascular events due to small numbers. All participants in this clinical trial were volunteers who had a previous history of at least one colorectal adenoma, and our findings may therefore not pertain to individuals without a history of such neoplasia. In general, colorectal lesion size was estimated by the endoscopist during the endoscopy at the time of resection, and so is subject to measurement error. Of the three trials, only the AFPPS had a central pathology review.
This meta-analysis included the three large randomized clinical trials that have tested folic acid as a chemopreventive agent against colorectal adenomas. The trials were well conducted, with high compliance. Follow-up rates were high, although in two of the trials only about 2/3 of subjects had an endoscopy during the initially intended window. The sample size in the pooled studies was substantial, providing good statistical power to detect moderate effects. Additional strengths include the systematic collection of risk factor and dietary information. Because of the randomized analysis, confounding is unlikely and the fixed doses of folic acid reduce or eliminate the measurement error associated with dietary assessment. In addition, inclusion only of individuals with a “clean” (“no polyps”) endoscopy in the AFPPS and ukCAP trials allowed them to assess the effect of folic acid on incident rather than prevalent adenomas. However, the NHS/HPFS did not restrict entry to subjects with a clean colon (although 80% had had a endoscopy within 2 years prior to the start of the trial and the rate of recurrence was 24%) and it is still possible polyps were missed during endoscopy from participants in the AFPPS and ukCAP trials. Uniform, blinded follow-up prevented differential ascertainment of endpoints according to folate intake.
In this large-scale collaborative combined analysis, we found no evidence overall that folic acid is beneficial for the prevention of colorectal adenomas in the large bowel after a follow-up of up to 42 months. These data, together with other findings dampen enthusiasm for the potential chemopreventive effects of folic acid. More follow-up is needed to assess the longer-term effects of folic acid use.