Folate consumption and cancer risk: a confirmation and some reassurance, but we're not out of the woods quite yet

Folate intake and the risk of endometrial cancer: A meta-analysis

Background

Folate may involve in various aspects of carcinogenesis. However, the relationship between folate intake and risk of many cancers, including endometrial cancer, is still inconclusive. We conducted a meta-analysis to systematically review the association.

Methods

Relevant studies were searched through three electronic databases (PubMed, Embase, and Web of Science) up to April 4, 2016. Population based prospective or case-control studies involving in investigating folate intake and risk of endometrial cancer were considered as eligible. Three investigators independently extracted data. Controversies were reconciled by discussing with a fourth investigator. Effect sizes of studies were pooled via a random effects model. Thereafter to explore the origin of heterogeneity among results of studies, a mixed effects model was employed with study design and dose of folate intake taken as covariates.

Results

Nine case-control studies and five cohort studies were included in the current meta-analysis. The result pooled from the highest category suggested a marginal negative association between folate intake and risk of endometrial cancer (OR=0.89 95% CI: 0.76-1.05). Based on the mixed effects model, in the highest category, the risk showed an increasing trend along with increment of folate intake (5% risk increase per 100μg/d, P=0.01).

Conclusion

A marginally negative association was observed between folate intake and endometrial cancer, which might subject to a threshold effect. More finely designed perspective studies or randomized trials are still needed to confirm the association.

Safety of folic acid

There is a large body of literature demonstrating the efficacy of maternal folic acid intake in preventing birth defects, as well as investigations into potential adverse consequences of consuming folic acid above the upper intake level (UL). Recently, two authoritative bodies convened expert panels to assess risks from high intakes of folic acid: the U.S. National Toxicology Program and the UK Scientific Advisory Committee on Nutrition. Overall, the totality of the evidence examined by these panels, as well as studies published since the release of their reports, have not established risks for adverse consequences resulting from existing mandatory folic acid fortification programs that have been implemented in many countries. Current folic acid fortification programs have been shown to support public health in populations, and the exposure levels are informed by and adherent to the precautionary principle. Additional research is needed to assess the health effects of folic acid supplement use when the current upper limit for folic acid is exceeded.

Causes, Consequences and Public Health Implications of Low B-Vitamin Status in Ageing

The potential protective roles of folate and the metabolically related B-vitamins (vitamins B12, B6 and riboflavin) in diseases of ageing are of increasing research interest. The most common cause of folate and riboflavin deficiencies in older people is low dietary intake, whereas low B12 status is primarily associated with food-bound malabsorption, while sub-optimal vitamin B6 status is attributed to increased requirements in ageing. Observational evidence links low status of folate and the related B-vitamins (and/or elevated concentrations of homocysteine) with a higher risk of degenerative diseases including cardiovascular disease (CVD), cognitive dysfunction and osteoporosis. Deficient or low status of these B-vitamins alone or in combination with genetic polymorphisms, including the common MTHFR 677 C → T polymorphism, could contribute to greater disease risk in ageing by causing perturbations in one carbon metabolism. Moreover, interventions with the relevant B-vitamins to optimise status may have beneficial effects in preventing degenerative diseases. The precise mechanisms are unknown but many have been proposed involving the role of folate and the related B-vitamins as co-factors for one-carbon transfer reactions, which are fundamental for DNA and RNA biosynthesis and the maintenance of methylation reactions. This review will examine the evidence linking folate and related B-vitamins with health and disease in ageing, associated mechanisms and public health implications.

Folic Acid Supplementation in Postpolypectomy Patients in a Randomized Controlled Trial Increases Tissue Folate Concentrations and Reduces Aberrant DNA Biomarkers in Colonic Tissues Adjacent to the Former Polyp Site

BACKGROUND

Low folate status is associated with an increased risk of colorectal carcinogenesis. Optimal folate status may be genoprotective by preventing uracil misincorporation into DNA and DNA hypomethylation. Adenomatous polyps have low folate status compared with normal colonic mucosa, and they are surrounded by histologically normal mucosa that also is of low folate status.

OBJECTIVE

In a randomized controlled trial conducted at a single Dublin hospital between April 2002 and March 2004, we assessed the effect of folic acid supplementation on tissue folate, uracil misincorporation into DNA, and global DNA hypomethylation in colonocytes isolated from sites of adenomatous polyps and from histologically normal tissue adjacent and 10-15 cm distal to them.

METHODS

Twenty patients with adenomatous polyps on initial colonoscopy and polypectomy were randomly assigned to receive either 600 μg folic acid/d [n = 12, 38% men, mean age 64.3 y, and body mass index (BMI, in kg/m(2)) 26.6] or placebo (n = 8, 50% men, mean age 68.4 y, and BMI 27.2) for 6 mo, and then repeat the colonoscopy. Blood and colonocyte tissue folate concentrations were measured with the use of a microbiological assay. Uracil misincorporation and global DNA hypomethylation were measured in colonocytes with the use of modified comet assays.

RESULTS

Over time, folic acid supplementation, compared with placebo, increased tissue folate (mean ± SEM) from 15.6 ± 2.62 pg/10(5) cells to 18.1 ± 2.12 pg/10(5) cells (P < 0.001) and decreased the global DNA hypomethylation ratio from 1.7 ± 0.1 to 1.0 ± 0.1 (P < 0.001). The uracil misincorporation ratio decreased by 0.5 ± 0.1 for the site adjacent to the polyp over time (P = 0.05).

CONCLUSION

A response to folic acid supplementation, which increased colonocyte folate and improved folate-related DNA biomarkers of cancer risk, was seen in the participants studied. Exploratory analysis points toward the area formerly adjacent to polyps as possibly driving the response. That these areas persist after polypectomy in the absence of folate supplementation is consistent with a potentially carcinogenic field's causing the appearance of the polyp.

One-carbon metabolism dietary factors and distal gastric cancer risk in chinese women

BACKGROUND

Previous studies on the association between one-carbon dietary factors and gastric cancer risk have been inconsistent.

METHODS

We investigated this association using data from a prospective study, the Shanghai Women's Health Study (1997-2010), including 323 distal gastric cancer cases identified from 73,009 Chinese women. HRs and 95% confidence intervals (CI) were estimated using Cox proportional hazard regression after adjusting for confounders.

RESULTS

Overall, no statistically significant association of gastric cancer was observed with dietary intake of folate, methionine, or B vitamins. However, when stratified by menopausal status, higher intake of riboflavin was associated with decreased gastric cancer risk in premenopausal women with HR of 0.35 (95% CI, 0.17-0.73), 0.48 (0.24-0.97), 0.28 (0.12-0.65), and 0.23 (0.07-0.91), respectively, for the quintiles 2 to 5 intake groups compared with the lowest quintile intake (P for trend = 0.02). Among premenopausal women, highest intake of folate was associated with increased gastric cancer risk (HR, 2.62; 95% CI, 1.04-6.59). There were no statistically significant associations observed among postmenopausal women.

CONCLUSIONS

These results suggest that dietary factors involved in one-carbon metabolism are associated with gastric cancer risk among premenopausal women.

IMPACT

Riboflavin may be a protective factor and folate may be a risk factor for premenopausal gastric cancer.

EURRECA-Estimating folate requirements for deriving dietary reference values

In most countries, the dietary folate intake associated with adequate status of red cell folate and/or serum folate provides the basis for formulating reference values. One of the major challenges in setting dietary reference values for folate, however, is the need to account for the differences in bioavailability between the natural forms of the vitamin and the synthetic form, folic acid, albeit to date, few countries in Europe take bioavailability into consideration. A series of systematic reviews that included only those studies which used the most robust measures of both folate intake and folate status were carried out by the EURRECA Network of Excellence to examine the relationships between folate intake, status, and a number of health outcomes relevant to specific stages of the lifecycle. This review summarizes the available evidence and the issues to consider in the setting of dietary reference values for folate.

Folic acid fortification and colorectal cancer risk

The U.S. has been reported as the only country experiencing a decline in incidence rates of colorectal cancer (CRC), despite increasing prevalence of CRC major risk factors, including the Western dietary pattern and obesity. This paper presents a hypothesis that improved folate status in the U.S. is the factor that could most likely explain the seemingly contradictory phenomenon, although a momentary increase in CRC incidence rates was observed in the later 1990s with the initiation of nationwide folic acid fortification. To corroborate this hypothesis, time trends in CRC incidence rates and death rates in the U.S. were plotted by age, race, and gender based on data from the Surveillance, Epidemiology, and End Results (SEER); data were analyzed by simultaneously addressing the following four critical factors: (1) a long induction period between improved folate status and potential protection of CRC; (2) a change in the U.S. Food and Drug Administration regulation in 1973 on the dose of folic acid allowed in supplements; (3) differential impacts of 1973 regulatory change and 1990s mandatory fortification by race; and (4) changes in CRC screening over time in the U.S. Although this type of analysis precludes a definitive conclusion, available evidence suggests that the increase in CRC incidence rates in the later 1990s is unlikely due to folic acid fortification and, assuming a time lag of a decade or longer to see a benefit on CRC, folate appears to be one of the most promising factors that could explain the downward trend of CRC incidence rates in the U.S.

Biomarker responses to folic acid intervention in healthy adults: a meta-analysis of randomized controlled trials

BACKGROUND

The task of revising dietary folate recommendations for optimal health is complicated by a lack of data quantifying the biomarker response that reliably reflects a given folate intake.

OBJECTIVE

We conducted a dose-response meta-analysis in healthy adults to quantify the typical response of recognized folate biomarkers to a change in folic acid intake.

DESIGN

Electronic and bibliographic searches identified 19 randomized controlled trials that supplemented with folic acid and measured folate biomarkers before and after the intervention in apparently healthy adults aged ≥18 y. For each biomarker response, the regression coefficient (β) for individual studies and the overall pooled β were calculated by using random-effects meta-analysis.

RESULTS

Folate biomarkers (serum/plasma and red blood cell folate) increased in response to folic acid in a dose-response manner only up to an intake of 400 μg/d. Calculation of the overall pooled β for studies in the range of 50 to 400 μg/d indicated that a doubling of folic acid intake resulted in an increase in serum/plasma folate by 63% (71% for microbiological assay; 61% for nonmicrobiological assay) and red blood cell folate by 31% (irrespective of whether microbiological or other assay was used). Studies that used the microbiological assay indicated lower heterogeneity compared with studies using nonmicrobiological assays for determining serum/plasma (I(2) = 13.5% compared with I(2) = 77.2%) and red blood cell (I(2) = 45.9% compared with I(2) = 70.2%) folate.

CONCLUSIONS

Studies administering >400 μg folic acid/d show no dose-response relation and thus will not yield meaningful results for consideration when generating dietary folate recommendations. The calculated folate biomarker response to a given folic acid intake may be more robust with the use of a microbiological assay rather than alternative methods for blood folate measurement.

Low colonocyte folate is associated with uracil misincorporation and global DNA hypomethylation in human colorectum

Low folate status is a risk factor for colon carcinogenesis; mechanisms proposed to account for this relationship include uracil misincorporation into DNA and global DNA hypomethylation. We investigated whether such biomarkers are related to folate status in isolated colonocytes from colonoscopy patients. In cases with adenomatous polyps (n = 40) or hyperplastic polyps (n = 16), colonocytes were isolated from biopsies from the polyp, from a site adjacent to the polyp, and from normal mucosa 10-15 cm distal to the polyp. In polyp-free controls (n = 53), biopsies were taken from ascending, transverse, and descending areas of colon. Within adenoma cases, there was a trend (P-trend < 0.001) of decreasing colonocyte folate (pg/10⁵ cells, mean ± CI) from the site distal to the polyp (16.9 ± 2.4), to the site adjacent to the polyp (14.7 ± 2.3), to the polyp (12.8 ± 2.0). Correspondingly, there were increases in uracil misincorporation (P-trend < 0.001) and global DNA hypomethylation (P-trend = 0.012) across the 3 sites. Colonocyte folate concentrations were significantly correlated with RBC folate concentrations, but only in individuals with generally lower (≤484 μg/L) RBC folate status (r = 0.54; P = 0.006; n = 24), and were also significantly lower in normal mucosa of cases with adenomatous polyps than in controls matched for colonic segment. In conclusion, localized folate deficiency in specific areas of colon might create carcinogenic fields and affect the development of colorectal polyps through uracil misincorporation and DNA hypomethylation; alternatively, the polyp itself might deplete folate in the surrounding tissue. Folate supplementation trials aimed at colon cancer prevention should target individuals with suboptimal folate status.

Dietary folates and cancer risk in a network of case-control studies

BACKGROUND

Folate deficiency leads to DNA damage and inadequate repair, caused by a decreased synthesis of thymidylate and purines. We analyzed the relationship between dietary folate intake and the risk of several cancers.

PATIENTS AND METHODS

The study is based on a network of case-control studies conducted in Italy and Switzerland in 1991-2009. The odds ratios (ORs) for dietary folate intake were estimated by multiple logistic regression models, adjusted for major identified confounding factors.

RESULTS

For a few cancer sites, we found a significant inverse relation, with ORs for an increment of 100 μg/day of dietary folate of 0.65 for oropharyngeal (1467 cases), 0.58 for esophageal (505 cases), 0.83 for colorectal (2390 cases), 0.72 for pancreatic (326 cases), 0.67 for laryngeal (851 cases) and 0.87 for breast (3034 cases) cancers. The risk estimates were below unity, although not significantly, for cancers of the endometrium (OR = 0.87, 454 cases), ovary (OR = 0.86, 1031 cases), prostate (OR = 0.91, 1468 cases) and kidney (OR = 0.88, 767 cases), and was 1.00 for stomach cancer (230 cases). No material heterogeneity was found in strata of sex, age, smoking and alcohol drinking.

CONCLUSIONS

Our data support a real inverse association of dietary folate intake with the risk of several common cancers.

C1 metabolism and CVD outcomes in older adults

CVD is the most common cause of death in people over 65 years. This review considers the latest evidence for a potential protective effect of C1 donors (folate and the metabolically related B-vitamins) in CVD. Such an effect may or may not be mediated via the role of these nutrients in maintaining plasma homocysteine concentrations within a desirable range. Despite predictions from epidemiological studies that lowering plasma homocysteine would reduce cardiovascular risk, several secondary prevention trials in at-risk patients published since 2004 have failed to demonstrate a benefit of homocysteine-lowering therapy with B-vitamins on CVD events generally. All these trials were performed in CVD patients with advanced disease; thus current evidence suggests that intervention with high-dose folic acid is of no benefit in preventing another event, at least in the case of heart disease. The evidence at this time, however, is stronger for stroke, with meta-analyses of randomised trials showing that folic acid reduces the risk of stroke, particularly in people with no history of stroke. Genetic studies provide convincing evidence to support a causal relationship between sub-optimal B-vitamin status and CVD. People homozygous for the common C677T variant in the gene encoding the folate-metabolising enzyme, methylenetetrahydrofolate reductase (MTHFR), typically have a 14–21% higher risk of CVD. Apart from folate, riboflavin is required as a co-factor for MTHFR. New evidence shows that riboflavin intervention results in marked lowering of blood pressure, specifically in patients with the MTHFR 677TT genotype. This novel gene–nutrient interaction may provide insights as to the mechanism that links C1 metabolism with CVD outcomes.