In the present systematic review, a meta-analysis of 10 RCTs showed a borderline significant incidence increase of overall cancer in the folic acid group compared to controls. Overall cancer incidence was not reported in the seven observational studies. However, meta-analysis of the seven observational studies including all cancer types reported showed no difference in cancer incidence between the group taking folic acid supplements and abstainers.
In sensitivity analyses, we identified four subgroups where folic acid supplementation increased cancer incidence and/or cancer mortality. These subgroups consisted of studies with a dose of folic acid from 0.4 mg to 1 mg/day, studies with >30% of smokers, studies with >70% of men and studies with a follow-up longer than 5 years. Whether smoking, gender, follow-up time or any other unreported reasons are the factors that contribute to this adverse effect of folic acid has not been studied.
Comparison with other studies
Our finding of no benefit, but a possible adverse effect from folic acid supplementation on cancer incidence is in line with reported results of studies on other nutrient supplements, including vitamin E, β-carotene (vitamin-A precursor) or selenium; all showing no or an adverse effect of supplementation in controlled clinical trials on cancer development.42
A recent meta-analysis of folic acid supplements by Clarke and coworkers, in the ‘B-Vitamin Treatment Trialists' Collaboration’3
with individual patient data, found no increased cancer risk in any subgroups. Our analysis differs from Clarke's by being a systematic review, thus including more studies (five more RCTs and seven observational studies) and by including more diverse populations than the seven B-vitamin trials in populations with cardiovascular disease included in the Clarke's study. In two studies included both in Clarke's meta-analysis and in ours, NORVIT and WENBIT, we included data from the combined analysis with longer follow-up time published in 200912
; whereas Clarke et al
only use cancer data from the original papers13
with a shorter follow-up. When assessing possible harm, a longer follow-up time is beneficial in order to discover more potential cancer cases, as an augmented cancer risk after folic acid intervention may not be confined to a short post-trial time period.
In our sensitivity analyses of the RCTs, the studies with the largest proportions of smokers showed an increased risk of cancer incidence and cancer mortality associated with folic acid supplements. Smoking is a well-known risk factor for many cancers. Furthermore, unlike folate supplements, folate from diet seems to be protective for cancer development.7
Many smokers have poor dietary habits, and folate intake has been shown to be inversely associated with smoking duration.44
Recently, the European Prospective Investigation into Cancer and Nutrition, a large observational study, found a lower risk of developing lung cancer in former and current smokers within the highest quartile of serum folate compared to the lowest (OR 0.68, 95% CI 0.51 to 0.90).45
No effect was apparent in never-smokers (OR 0.84, 95% CI 0.43 to 1.65) or for colorectal cancer risk.46
Interestingly, and in line with a possible adverse effect of folic acid supplementation, another report from the European Prospective Investigation into Cancer and Nutrition study suggests a U-shaped relationship between plasma folate and risk of pancreatic cancer in both men and women, with highest risk for the lowest and highest serum quintile levels.47
A large Swedish prospective study found that pancreatic cancer risk was reduced in people with diets rich in folate from foods but not from supplements.48
This is in line with other observational studies indicating a source-specific effect of folates with dietary folate being protective and folic acid supplements possibly being harmful or having no effect with respect to cancer risk.49–52
In one of the RCTs included in our analysis,41
folic acid supplementation of 1 mg/day increased the risk of prostate cancer, while higher dietary folate and plasma folate levels among non-users of vitamin supplements were associated with decreased risk of prostate cancer.53
It has been speculated that folate may be beneficial in primary prevention of cancer, but potentially harmful in the presence of early cancer.10
Accordingly, the antifolate methotrexate is an effective cytostatic drug used in the standard therapy of several cancer types. We performed sensitivity analyses for the folic acid supplemented population with a history of adenomas and the populations without such a history. This analysis showed no significant increased risk of colorectal cancer in either group. It should be noted, however, that we do not have data to support or weaken the interesting hypothesis of timing of folate supplementation as being important. The relative short latency time from exposure (folate supplementation) to cancer development reported from the recent Norwegian randomised trials12
suggests an effect on late carcinogenesis, as this short time indicates that a precancer situation might have been present during the intervention period.
In our sensitivity analyses of the RCTs, the studies with dose interval 0.4–1 mg/day were associated with a higher risk of cancer development than studies with doses above 1 mg/day, with RRs of 1.21 (95% CI 1.06 to 1.38) and 1.03 (95% CI 0.96 to 1.11), respectively. Our systematic search had no dose limitation and thus allowed us to search for all dose ranges, although our eligibility criteria specified doses ≥0.4 mg/day. We identified no RCTs, but 11 observational studies with doses below 0.4 mg/day reporting cancer incidence, thus allowing us post hoc to evaluate a possible dose–response pattern of observational studies including doses below 0.4 mg/day. Six of these studies were already included in our review, as they also contained groups with daily intakes ≥0.4 mg,32–34
and five were excluded from our main analyses due to daily dose of folic acid below 0.4 mg.49
No increased risk was found in the meta-analyses of doses below 0.4 mg/day, with RR of 1.07 (95% CI 0.92 to 1.26) for the cohort studies33
and 1.17 (95% CI 0.58 to 2.33) for the case–control studies.32
Hence, our analyses did not indicate any dose–response pattern between folic acid dose and cancer incidence.
The reason for the increased risk in the dose interval 0.4–1 mg/day has not been elucidated in our study, and we cannot exclude that other characteristics of these particular studies may be confounders. Although the smokers are randomised and evenly distributed between folic acid and placebo groups, one might speculate that a possible harmful effect of folic acid on smokers will be more evident in studies with more smokers.
A high alcohol intake is associated with higher incidence of several cancer types, including premenopausal and postmenopausal breast cancer.7
Folate intake or blood folate do not seem to influence breast cancer risk, but adequate folate may reduce the increased risk associated with alcohol consumption.57
A similar association between alcohol consumption and folate intake is reported in a prospective cohort study from the Nurses' Health Study on oral cancer risk in women.58
The effect of alcohol consumption on cancer development after folic acid supplementation was not possible to evaluate in the present study, due to restricted information in the original publications on alcohol intake.
When different cancer sites were assessed in our study, prostate cancer was the only cancer type found in the pooled analysis to have a significant risk increase after folic acid supplementation. There was some heterogeneity between the studies (I2
=17%), with one of the trials, AFPPS41
being an outlier compared to the other studies with an RR of 2.68 (95% CI 1.27 to 5.66). However, the increased risk remained even if omitting this study from the pooled effect estimate (RR 1.19, 95% CI 1.01 to 1.39, I2
=0%). In line with our results, a recent meta-analysis on serum folate and prostate cancer59
concluded that high serum folate concentrations were associated with increased prostate cancer risk. We may speculate on a specific effect of folic acid on prostate tissue rather than on cancer development in general due to differences in tissue and cell sensitivity to folic acid effects. Such potential tissue- and cell-specific differences in folic acid effects on cell biology need to be further explored in order to generate a molecular understanding for such a possible diverse tissue effect.
The major group recommended taking folic acid supplements today is women in childbearing age before and in the first trimester of pregnancy in order to prevent neural tube defects in the offspring. Only one of the studies in our review was performed in a population of pregnant women, the 36-year post-trial follow-up after an RCT in Scottish women taking folic acid in pregnancy.29
This study of Charles et al
showed a significant increased risk for total cancer mortality (RR 1.70, 95% CI 1.06 to 2.72) in pregnant women with folic acid supplementation, in the group taking the highest dosage of folic acid, 5 mg/day, but not in the 0.2 mg group (RR 1.20, 95% CI 0.71 to 2.02). However, in our meta-analysis of total cancer mortality, where the study of Charles et al
was analysed together with five other studies, no significant increase in total mortality was showed (RR 1.09, 95% CI 0.92 to 1.30). The 5 mg daily dose far exceeds the folic acid supplementation dosage recommended today for the female fertile population (0.4 mg/day). It is worth noticing also that the proportion of smokers in the study of Charles et al
was high (>40%), which reflects smoking habits in the 1960s when the study was performed. Another study assessing cancer effects after folic acid supplements in pregnancy showed a reduced risk of childhood leukaemia (acute lymphoblastic leukaemia) in the offsprings.60
This study was, however, not included in our present analysis, as folic acid supplement dose was not specified.
The folic acid recommendations in order to reduce risk of fetal neural tube development are poorly followed by fertile women, as few women start folic acid intake prior to pregnancy.61
Several countries, such as Canada and the USA, have started fortification of foods in order to increase periconceptional folic acid intake, aiming at reducing the incidence of neural tube defects. This food fortification strategy has been shown to efficiently reduce the incidence of open neural tube defects in Canada.63
Other countries, such as Norway64
and the UK, have not chosen this general food fortification strategy, mainly due to the unknown effects of augmented folic acid intake on the general population outside the target group of fertile women, such as masking of vitamin B12 deficiency in the older population and a possible risk of cancer development. Longitudinal studies on cancer development in countries that have introduced fortification of foods with folic acid could indicate whether fortification has similar risk of cancer as what seems to be the case for folic acid supplements. At present, there are no studies indicating that the current recommendation of folic acid supplementation (with the present recommended moderate dosages) to fertile women has any negative effects on the woman's or the offspring's future health.
Folic acid may alternatively be administered as 5-methyltetrahydrofolate, as in the SU-FOL.OM3-trial,65
where patients in France with a history of ischaemic heart disease were randomised to either receive B vitamins (5-methyltetrahydrofolate, vitamin B6 and vitamin B12 or placebo) and ω-3 fatty acids or placebo in a 2×2 factorial design. Mean duration of supplementation and of follow-up was 4.7 years. In this study, allocation to B-vitamins did not have any significant effect on cancer incidence (HR 1.19, 95% CI 0.88 to 1.60). Adding this study to our meta-analysis of total cancer incidence would, however, not change the conclusions, with a pooled effect estimate (RR 1.08, 95% CI 1.01 to 1.14).
Strengths and limitations of the study
Our review has several strengths. Our thorough systematic search makes it plausible that we have retrieved all relevant studies and can be assured not to miss important data. However, not all studies could be included in our meta-analyses due to unreported outcomes or dose of folic acid in supplements. The data extraction has been performed by individual reviewers and then controlled by others, which also strengthens our trust in this review. Additionally, most of the RCTs included are of high methodological quality, with a low risk of bias. This generally implies that we can be quite confident that the different study groups were randomised adequately and that the result from the studies can be trusted. Also, in some of the included studies, cancer cases found shortly after initiation of the study were excluded since it would be biologically implausible that these cases were related to the intervention.
The limited time of follow-up of most RCTs is a major limitation of our review because the time frame for cancer development might exceed the follow-up time in many RCTs. Actually, this review may be too conservative regarding estimating the long-term cancer risk associated with folic acid supplementation since regularly RCTs have a short follow-up time. This is also supported by our sensitivity analysis where the subgroup of studies with the longer follow-up time showed borderline increase in cancer risk with folic acid supplements, whereas the studies with shorter follow-up did not. However, when we add observational studies to broaden the evidence base and lengthen follow-up time, we do not find any additional risk difference between the groups.
Another limitation of our study is the lack of information on dietary patterns in the studied populations as the effect of supplements may vary according to the folate status of the population, for example, the proportion that are folate deficient. However, this is not thought to vary within one single study (RCTs) but may vary between studies and countries. In one of the RCTs,11
baseline dietary data were supplied in a separate paper,53
and although it showed an inverse association between high dietary intake and colorectal adenoma in the placebo group, thus indicating a protective effect, no association between dietary folate and colorectal adenoma was shown in the folic acid-supplemented group. The authors interpreted this as a beneficial effect of folate supplementation compared to deficiency but limited to some point of sufficiency, where increases provide no additional benefit. Our analysis may emphasise the need for pooling all relevant evidence into meta-analyses when appropriate and not trying to interpret conflicting results only based on one single RCT.
Another important limitation of our review is the selected populations in which the RCTs were performed, thus the findings may not be applicable to the population in general. Most participants included were treated in secondary prevention of adenomas and cardiovascular disease and may have increased cancer risk, for example, smoking is a common risk factor for cancer and cardiovascular disease. The one study with pregnant women had a higher proportion of smokers than is reported today.66
Effects of folic acid supplementation may vary among different subgroups, as demonstrated on trial level in our analysis. Our conclusions are based on RCTs with a low risk of bias, and randomisation of participants should minimise the problem of confounding.
We performed only regular meta-analyses and analysed sensitivity by dividing the results into groups. A more sophisticated way of investigating impact of different possible factors is to perform meta-regression.19
However, this is not recommended if <10 studies are included.19
Furthermore, we could not perform sensitivity analyses on some other variables that would be of interest, for instance on shorter periods of folic acid exposure that is more common in pregnancy.
More studies assessing cancer risk in the only population where folic acid supplementation is shown to be beneficial today (periconceptionally to fertile women to prevent neural tube defects) would have strengthened the analysis. However, the one long-term follow-up after such folic acid supplementation29
did not find an elevated mortality rate for any cancer (total cancers and breast cancer analysed separately) following the low supplementation dosage of 0.2 mg daily.
Our review has not addressed dietary folate, and we cannot conclude on a possible ‘source-specific’ effect. A Cochrane review under preparation is addressing this issue.67