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      Cancer risk with folic acid supplements: a systematic review and meta-analysis

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          Abstract

          Objective

          To explore if there is an increased cancer risk associated with folic acid supplements given orally.

          Design

          Systematic review and meta-analysis of controlled studies of folic acid supplementation in humans reporting cancer incidence and/or cancer mortality. Studies on folic acid fortification of foods were not included.

          Data sources

          Cochrane Library, Medline, Embase and Centre of Reviews and Dissemination, clinical trial registries and hand-searching of key journals.

          Results

          From 4104 potential references, 19 studies contributed data to our meta-analyses, including 12 randomised controlled trials (RCTs). Meta-analysis of the 10 RCTs reporting overall cancer incidence (N=38 233) gave an RR of developing cancer in patients randomised to folic acid supplements of 1.07 (95% CI 1.00 to 1.14) compared to controls. Overall cancer incidence was not reported in the seven observational studies. Meta-analyses of six RCTs reporting prostate cancer incidence showed an RR of prostate cancer of 1.24 (95% CI 1.03 to 1.49) for the men receiving folic acid compared to controls. No significant difference in cancer incidence was shown between groups receiving folic acid and placebo/control group, for any other cancer type. Total cancer mortality was reported in six RCTs, and a meta-analysis of these did not show any significant difference in cancer mortality in folic acid supplemented groups compared to controls (RR 1.09, 95% CI 0.90 to 1.30). None of the observational studies addressed mortality.

          Conclusions

          A meta-analysis of 10 RCTs showed a borderline significant increase in frequency of overall cancer in the folic acid group compared to controls. Overall cancer incidence was not reported in the seven observational studies. Prostate cancer was the only cancer type found to be increased after folic acid supplementation (meta-analyses of six RCTs). Prospective studies of cancer development in populations where food is fortified with folic acid could indicate whether fortification similar to supplementation moderately increases prostate cancer risk.

          Article summary

          Article focus
          • The present systematic review aims to explore if there is an increased cancer risk associated with folic acid supplements given orally.

          Key messages
          • Meta-analysis of 10 RCTs showed no benefit but a borderline significant increase in incidence of overall cancer in the folic acid group compared to controls. Overall cancer incidence was not reported in the seven observational studies. When analysing site-specific cancers, prostate cancer was the only cancer type where an increase in risk was shown for folic acid supplements.

          • The current study does not show any evidence for augmented cancer risk for fertile women who are recommended folic acid periconceptionally in order to reduce the risk of neural tube defects.

          Strengths and limitations of this study
          • This systematic review has been performed with a thorough systematic search; and data extraction and quality assessment have been performed by at least two independent persons.

          • The limited time of follow-up of most RCTs is a limitation of our review because the time frame for cancer development might exceed the follow-up time in many RCTs.

          • The highly selected population in which most of the studies were conducted limits the applicability to the general population and women of childbearing age taking folic acid periconceptionally.

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          Most cited references40

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          Homocysteine lowering with folic acid and B vitamins in vascular disease.

          In observational studies, lower homocysteine levels are associated with lower rates of coronary heart disease and stroke. Folic acid and vitamins B6 and B12 lower homocysteine levels. We assessed whether supplementation reduced the risk of major cardiovascular events in patients with vascular disease. We randomly assigned 5522 patients 55 years of age or older who had vascular disease or diabetes to daily treatment either with the combination of 2.5 mg of folic acid, 50 mg of vitamin B6, and 1 mg of vitamin B12 or with placebo for an average of five years. The primary outcome was a composite of death from cardiovascular causes, myocardial infarction, and stroke. Mean plasma homocysteine levels decreased by 2.4 micromol per liter (0.3 mg per liter) in the active-treatment group and increased by 0.8 micromol per liter (0.1 mg per liter) in the placebo group. Primary outcome events occurred in 519 patients (18.8 percent) assigned to active therapy and 547 (19.8 percent) assigned to placebo (relative risk, 0.95; 95 percent confidence interval, 0.84 to 1.07; P=0.41). As compared with placebo, active treatment did not significantly decrease the risk of death from cardiovascular causes (relative risk, 0.96; 95 percent confidence interval, 0.81 to 1.13), myocardial infarction (relative risk, 0.98; 95 percent confidence interval, 0.85 to 1.14), or any of the secondary outcomes. Fewer patients assigned to active treatment than to placebo had a stroke (relative risk, 0.75; 95 percent confidence interval, 0.59 to 0.97). More patients in the active-treatment group were hospitalized for unstable angina (relative risk, 1.24; 95 percent confidence interval, 1.04 to 1.49). Supplements combining folic acid and vitamins B6 and B12 did not reduce the risk of major cardiovascular events in patients with vascular disease. (ClinicalTrials.gov number, NCT00106886; Current Controlled Trials number, ISRCTN14017017.). Copyright 2006 Massachusetts Medical Society.
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            Folic acid for the prevention of colorectal adenomas: a randomized clinical trial.

            Laboratory and epidemiological data suggest that folic acid may have an antineoplastic effect in the large intestine. To assess the safety and efficacy of folic acid supplementation for preventing colorectal adenomas. A double-blind, placebo-controlled, 2-factor, phase 3, randomized clinical trial conducted at 9 clinical centers between July 6, 1994, and October 1, 2004. Participants included 1021 men and women with a recent history of colorectal adenomas and no previous invasive large intestine carcinoma. Participants were randomly assigned in a 1:1 ratio to receive 1 mg/d of folic acid (n = 516) or placebo (n = 505), and were separately randomized to receive aspirin (81 or 325 mg/d) or placebo. Follow-up consisted of 2 colonoscopic surveillance cycles (the first interval was at 3 years and the second at 3 or 5 years later). The primary outcome measure was occurrence of at least 1 colorectal adenoma. Secondary outcomes were the occurrence of advanced lesions (> or =25% villous features, high-grade dysplasia, size > or =1 cm, or invasive cancer) and adenoma multiplicity (0, 1-2, or > or =3 adenomas). During the first 3 years, 987 participants (96.7%) underwent colonoscopic follow-up, and the incidence of at least 1 colorectal adenoma was 44.1% for folic acid (n = 221) and 42.4% for placebo (n = 206) (unadjusted risk ratio [RR], 1.04; 95% confidence interval [CI], 0.90-1.20; P = .58). Incidence of at least 1 advanced lesion was 11.4% for folic acid (n = 57) and 8.6% for placebo (n = 42) (unadjusted RR, 1.32; 95% CI, 0.90-1.92; P = .15). A total of 607 participants (59.5%) underwent a second follow-up, and the incidence of at least 1 colorectal adenoma was 41.9% for folic acid (n = 127) and 37.2% for placebo (n = 113) (unadjusted RR, 1.13; 95% CI, 0.93-1.37; P = .23); and incidence of at least 1 advanced lesion was 11.6% for folic acid (n = 35) and 6.9% for placebo (n = 21) (unadjusted RR, 1.67; 95% CI, 1.00-2.80; P = .05). Folic acid was associated with higher risks of having 3 or more adenomas and of noncolorectal cancers. There was no significant effect modification by sex, age, smoking, alcohol use, body mass index, baseline plasma folate, or aspirin allocation. Folic acid at 1 mg/d does not reduce colorectal adenoma risk. Further research is needed to investigate the possibility that folic acid supplementation might increase the risk of colorectal neoplasia. clinicaltrials.gov Identifier: NCT00272324.
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              Plasma homocysteine as a risk factor for vascular disease. The European Concerted Action Project.

              Elevated plasma homocysteine is a known risk factor for atherosclerotic vascular disease, but the strength of the relationship and the interaction of plasma homocysteine with other risk factors are unclear. To establish the magnitude of the vascular disease risk associated with an increased plasma homocysteine level and to examine interaction effects between elevated plasma homocysteine level and conventional risk factors. Case-control study. Nineteen centers in 9 European countries. A total of 750 cases of atherosclerotic vascular disease (cardiac, cerebral, and peripheral) and 800 controls of both sexes younger than 60 years. Plasma total homocysteine was measured while subjects were fasting and after a standardized methionine-loading test, which involves the administration of 100 mg of methionine per kilogram and stresses the metabolic pathway responsible for the irreversible degradation of homocysteine. Plasma cobalamin, pyridoxal 5'-phosphate, red blood cell folate, serum cholesterol, smoking, and blood pressure were also measured. The relative risk for vascular disease in the top fifth compared with the bottom four fifths of the control fasting total homocysteine distribution was 2.2 (95% confidence interval, 1.6-2.9). Methionine loading identified an additional 27% of at-risk cases. A dose-response effect was noted between total homocysteine level and risk. The risk was similar to and independent of that of other risk factors, but interaction effects were noted between homocysteine and these risk factors; for both sexes combined, an increased fasting homocysteine level showed a more than multiplicative effect on risk in smokers and in hypertensive subjects. Red blood cell folate, cobalamin, and pyridoxal phosphate, all of which modulate homocysteine metabolism, were inversely related to total homocysteine levels. Compared with nonusers of vitamin supplements, the small number of subjects taking such vitamins appeared to have a substantially lower risk of vascular disease, a proportion of which was attributable to lower plasma homocysteine levels. An increased plasma total homocysteine level confers an independent risk of vascular disease similar to that of smoking or hyperlipidemia. It powerfully increases the risk associated with smoking and hypertension. It is time to undertake randomized controlled trials of the effect of vitamins that reduce plasma homocysteine levels on vascular disease risk.
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                Author and article information

                Journal
                BMJ Open
                bmjopen
                bmjopen
                BMJ Open
                BMJ Group (BMA House, Tavistock Square, London, WC1H 9JR )
                2044-6055
                2012
                12 January 2012
                12 January 2012
                : 2
                : 1
                : e000653
                Affiliations
                [1 ]Norwegian Knowledge Centre for the Health Services, Oslo, Norway
                [2 ]Department of Obstetrics and Department of Gynaecology, Oslo University Hospital, Oslo, Norway
                [3 ]Institute for Clinical Medicine, Faculty of Medicine, University of Oslo, Oslo, Norway
                [4 ]Division of Surgery and Cancer Medicine, Oslo University Hospital, Oslo, Norway
                Author notes
                Correspondence to Dr Marianne Klemp; marianne.klemp@ 123456nokc.no
                Article
                bmjopen-2011-000653
                10.1136/bmjopen-2011-000653
                3278486
                22240654
                0016be0f-ddf8-494e-b870-af6fd7f0cc22
                © 2012, Published by the BMJ Publishing Group Limited. For permission to use (where not already granted under a licence) please go to http://group.bmj.com/group/rights-licensing/permissions.

                This is an open-access article distributed under the terms of the Creative Commons Attribution Non-commercial License, which permits use, distribution, and reproduction in any medium, provided the original work is properly cited, the use is non commercial and is otherwise in compliance with the license. See: http://creativecommons.org/licenses/by-nc/2.0/ and http://creativecommons.org/licenses/by-nc/2.0/legalcode.

                History
                : 19 November 2011
                : 1 December 2011
                Categories
                Oncology
                Research
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                Medicine
                Medicine

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