Fruit, Vegetables, and Folate: Cultivating the Evidence for Cancer Prevention

Abstract

See “High levels of folate from supplements and fortification are not associated with increased risk of colorectal cancer,” by Stevens VL, McCullough ML, Sun J, et al, on page 98; and “Nonlinear reduction in risk for colorectal cancer by fruit and vegetable intake based on meta-analysis of prospective studies,” by Aune D, Lau R, Chan DSM et al, on page 106. Since the 1980s, the concept that a diet high in fruits and vegetables could reduce the risk of cancer has been promoted widely.1Doll R. Peto R. The causes of cancer: quantitative estimates of avoidable risks of cancer in the United States today.J Natl Cancer Inst. 1981; 66: 1191-1308Crossref Scopus (3971) Google Scholar A role for diet in preventing colon cancer held particular appeal in view of the direct contact of the colonic mucosa with nutrients and toxins from foods.2Chan A.T. Giovannucci E.L. Primary prevention of colorectal cancer.Gastroenterology. 2010; 138: 2029-2043Google Scholar Several mechanisms were suggested to account for these putative anti-neoplastic properties. Fiber derived from fruits, vegetables, and grains was proposed to dilute or adsorb fecal carcinogens, modulate colonic transit time, alter bile acid metabolism, reduce colonic pH, or increase the production of short-chain fatty acids.3Kritchevsky D. Epidemiology of fibre, resistant starch and colorectal cancer.Eur J Cancer Prev. 1995; 4: 345-352Google Scholar Fruits and vegetables are also rich sources of micronutrients, including folate, which have been actively investigated as chemopreventive agents. More recently, emerging evidence has underscored the importance of the interplay between diet and the microbial ecology of the colon.4Guarner F. Malagelada J.R. Gut flora in health and disease.Lancet. 2003; 361: 512-519Google Scholar Based on early human findings linking high intake of fruits and vegetables with a lower risk of colon cancer,5Trock B. Lanza E. Greenwald P. Dietary fiber, vegetables, and colon cancer: critical review and meta-analyses of the epidemiologic evidence.J Natl Cancer Inst. 1990; 82: 650-661Scopus (480) Google Scholar the National Cancer Institute launched the 5-A-Day Program in 1991 with the goal of increasing the average US consumption of fruits and vegetables to ≥5 servings a day. However, initial human evidence was derived largely from case-control studies, and results from prospective cohorts began to emerge showing nonexistent or weak associations,6Fuchs C.S. Giovannucci E.L. Colditz G.A. et al.Dietary fiber and the risk of colorectal cancer and adenoma in women.N Engl J Med. 1999; 340: 169-176Google Scholar, 7Michels K.B. Giovannucci E. Joshipura K.J. et al.Prospective study of fruit and vegetable consumption and incidence of colon and rectal cancers.J Natl Cancer Inst. 2000; 92: 1740-1752Google Scholar including a pooled analysis of 14 studies.8Koushik A. Hunter D.J. Spiegelman D. et al.Fruits, vegetables, and colon cancer risk in a pooled analysis of 14 cohort studies.J Natl Cancer Inst. 2007; 99: 1471-1483Google Scholar Thus, an international expert panel convened in 2007, concluded that there was limited evidence to support that increasing fruits and vegetable intake reduced risk of colorectal cancer, reversing their earlier opinion from 10 years prior that evidence of benefit was convincing.9World Cancer Research Fund/American Institute for Cancer ResearchFood, Nutrition, Physical Activity, and the Prevention of Cancer: a Global Perspective. AICR, Washington, DC2007Google Scholar The obvious explanation for the inconsistency between the largely positive results from case-control studies and the disappointing null findings of cohort studies is that case-control studies, in which most dietary information is collected after diagnosis of cancer, are more prone to bias. Generally, individuals with cancer are more likely to recall perceived unhealthy dietary behaviors compared with healthy control subjects, whose willingness to participate in studies may be associated with more health-conscious behaviors. However, methodologic bias does not tell the entire story. Recent analyses of large cohorts have also reached different conclusions from prior prospective studies, largely agreeing with the positive case-control studies. Most notably, the European Prospective Investigation Into Cancer and Nutrition study conducted among 500,000 individuals from 10 European populations, did demonstrate a 14%–25% reduced risk of colorectal cancer among individuals with the highest intake of fiber or fruits and vegetables.10Bingham S.A. Day N.E. Luben R. et al.Dietary fibre in food and protection against colorectal cancer in the European Prospective Investigation into Cancer and Nutrition (EPIC): an observational study.Lancet. 2003; 361: 1496-1501Google Scholar, 11van Duijnhoven F.J. Bueno-De-Mesquita H.B. Ferrari P. et al.Fruit, vegetables, and colorectal cancer risk: the European Prospective Investigation into Cancer and Nutrition.Am J Clin Nutr. 2009; 89: 1441-1452Google Scholar So what might explain these disparate results? In this issue of Gastroenterology, 2 studies delve more deeply into the relationship between fruits, vegetables, folate, and colorectal cancer, and suggest that potential associations are more complex than initially considered. Specifically, assessing human evidence for an association requires more than a simple comparison of disease incidence among populations with high versus low intake of fruits, vegetables, and folate. Rather, studies should carefully consider the specifics of how the diet–cancer relationship is analyzed, where associations are examined, both in terms of anatomic subsites of the colorectum and the geographic locations of the populations under study, and when dietary exposure is assessed in relation to cancer outcomes. In their meta-analysis of 19 prospective studies, Aune et al12Aune D. Lau R. Chan D.S.M. et al.Nonlinear reduction in risk for colorectal cancer by fruit and vegetable intake based on meta-analysis of prospective studies.Gastroenterology. 2011; 141: 106-118Abstract Full Text Full Text PDF Scopus (193) Google Scholar suggest that significant associations between cancer incidence and dietary information may largely depend on how one examines the evidence. Consistent with prior studies,8Koushik A. Hunter D.J. Spiegelman D. et al.Fruits, vegetables, and colon cancer risk in a pooled analysis of 14 cohort studies.J Natl Cancer Inst. 2007; 99: 1471-1483Google Scholar they did not find an overall benefit in their analyses that assumed a linear relationship between fruits and vegetable consumption and colorectal cancer. By contrast, modeling a nonlinear relationship using fractional polynomial models yielded a highly significant association. Specifically, most of the risk reduction associated with fruits and vegetables intake was attributable to increasing intake above a “threshold” of 100 g/d, with relatively little additional benefit associated with higher levels. As a reference, a typical apple weighs >200 g. Thus, for the general population, these results suggest that there is little benefit to increasing the consumption of fruits and vegetables beyond the levels associated with eating a reasonably balanced diet. Conversely, these data also suggest that failure to consume a minimum level of fruits and vegetables may be considered a modest risk factor for cancer. These findings are consistent with the results of the Polyp Prevention Trial, a randomized trial that showed that diets with extremely large amounts of fruits and vegetables (as well as high fiber and low fat) failed to reduce the risk of recurrent adenoma when compared with a usual American diet that included ≥200–400 g/d of fruits and vegetables.13Schatzkin A. Lanza E. Corle D. et al.Lack of effect of a low-fat, high-fiber diet on the recurrence of colorectal adenomas Polyp Prevention Trial Study Group.N Engl J Med. 2000; 342: 1149-1155Google Scholar Taken together, we can reasonably conclude that increasing consumption of fruits and vegetables does not have a strong anti-cancer benefit for the general population, with any realistic effect limited to individuals with extremely low baseline levels of consumption. Aune et al's results also suggest that where—both anatomically and geographically—one looks for an association between fruits and vegetables intake and colorectal cancer matters. They found that higher intake of fruits and vegetables was associated with a modest, yet significant, reduction in risk of colon, but not rectal, cancer. Such a biological difference is plausible in light of emerging evidence of distinct risk factors and molecular profiles associated with cancers that arise in the colon compared with the rectum.14Iacopetta B. Are there two sides to colorectal cancer?.Int J Cancer. 2002; 101: 403-408Google Scholar, 15Wei E.K. Giovannucci E. Wu K. et al.Comparison of risk factors for colon and rectal cancer.Int J Cancer. 2004; 108: 433-442Google Scholar, 16Dimberg J. Samuelsson A. Hugander A. et al.Differential expression of cyclooxygenase 2 in human colorectal cancer.Gut. 1999; 45: 730-732Google Scholar Thus, prior studies with limited ability to examine fruits and vegetables in relation to tumor site may have been unable to detect modest associations. Aune et al12Aune D. Lau R. Chan D.S.M. et al.Nonlinear reduction in risk for colorectal cancer by fruit and vegetable intake based on meta-analysis of prospective studies.Gastroenterology. 2011; 141: 106-118Abstract Full Text Full Text PDF Scopus (193) Google Scholar observed also a significant difference in findings depending on the geographic location of the population under study. Specifically, inverse associations between fruit and vegetable intake were primarily observed in studies of populations from Europe but not the United States or Asia. Assuming measurement error does not significantly vary by geography, these results could be explained by several factors. First, fruits and vegetables may exert distinct biological effects according to genetic or environmental factors unique to specific populations. Second, the mean intake of fruits and vegetables in the reference category of “low” intake was markedly lower in the studies from Europe (155 g/d) compared with the United States (200 g/d) and Asia (217 g/d). Thus, European studies included a sharper contrast between individuals with “high” intake of fruits and vegetables and individuals with truly “low” levels of intake. By contrast, in the United States and Asian studies, a significant proportion of individuals in the “low” category actually consumed a reasonable amount of fruits and vegetables that might exceed the “threshold” level needed to minimize cancer risk. Last, the stronger effect of fruits and vegetables observed by Aune et al in the European studies may be in part because of the generally greater contribution of fruits and vegetables to overall folate intake in Europe compared with the United States, wherein there is a higher prevalence of use of folate-containing multivitamins. Folate, an essential micronutrient for DNA synthesis, repair, and methylation, has been promoted as the leading phytochemical candidate responsible for the anti-cancer benefit of fruits and vegetables. Thus, if folate is the primary mediator of the anti-cancer effect of fruits and vegetables, it may be more challenging to detect a small incremental benefit of increasing intake of these foods in a more folate-replete population such as in the United States. Although this latter explanation is plausible, the role of folate as a chemopreventive agent has become less certain with the results of randomized, controlled trials conducted among individuals with a history of colorectal adenoma. In 2 trials, folic acid, the synthetic form of folate, did not reduce the risk of recurrent adenomas,17Logan R.F. Grainge M.J. Shepherd V.C. et al.Aspirin and folic acid for the prevention of recurrent colorectal adenomas.Gastroenterology. 2008; 134: 29-38Google Scholar, 18Wu K. Platz E.A. Willett W.C. et al.A randomized trial on folic acid supplementation and risk of recurrent colorectal adenoma.Am J Clin Nutr. 2009; 90: 1623-1631Google Scholar, 19Cole B.F. Baron J.A. Sandler R.S. et al.Folic acid for the prevention of colorectal adenomas: a randomized clinical trial.JAMA. 2007; 297: 2351-2359Google Scholar with a third trial actually observing an increased risk of advanced adenoma and prostate cancer.19Cole B.F. Baron J.A. Sandler R.S. et al.Folic acid for the prevention of colorectal adenomas: a randomized clinical trial.JAMA. 2007; 297: 2351-2359Google Scholar, 20Figueiredo J.C. Grau M.V. Haile R.W. et al.Folic acid and risk of prostate cancer: results from a randomized clinical trial.J Natl Cancer Inst. 2009; 101: 432-435Google Scholar Taken together with animal studies demonstrating that excess folate during later stages of carcinogenesis could promote carcinogenesis,21Kim Y.I. Folate, colorectal carcinogenesis, and DNA methylation: lessons from animal studies.Environ Mol Mutagen. 2004; 44: 10-25Google Scholar concerns have been raised that folic acid supplementation or even high levels of dietary folate could actually increase the risk of colorectal cancer. The potential mechanism underlying this hypothesis is that additional provision of folate above adequate levels could tip the balance of DNA precursors toward hypermethylation in cancerous cells. In the United States, this is a considerably worrisome prospect because folic acid fortification in enriched flour began in 1996,22Dietrich M. Brown C.J. Block G. The effect of folate fortification of cereal-grain products on blood folate status, dietary folate intake, and dietary folate sources among adult non-supplement users in the United States.J Am Coll Nutr. 2005; 24: 266-274Google Scholar temporally associated with a rise in the rates of colorectal cancer in the late 1990s.23Mason J.B. Dickstein A. Jacques P.F. et al.A temporal association between folic acid fortification and an increase in colorectal cancer rates may be illuminating important biological principles: a hypothesis.Cancer Epidemiol Biomarkers Prev. 2007; 16: 1325-1329Google Scholar Recently, this has led to calls to reconsider programs of mandatory folic acid fortification of the food supply despite its established benefit for the prevention of neural tube defects.24Mason J.B. Cole B.F. Baron J.A. et al.Folic acid fortification and cancer risk.Lancet. 2008; 371 (author reply 1335–1336): 1335Google Scholar In this issue of Gastroenterology, a separate study, by Stevens et al25Stevens V.L. McCullough M.L. Sun J. et al.High levels of folate from supplements and fortification are not associated with increased risk of colorectal cancer.Gastroenterology. 2011; 141: 98-105Google Scholar helps to allay concerns about a potential pro-carcinogenic role of folate. In a prospective analysis of 99,523 US participants enrolled in the Cancer Prevention Study II Nutrition Cohort, the investigators examined the association between folate and its various forms (e.g. dietary vs supplement; natural vs synthetic) and colorectal cancer incidence from 1999 through 2007, a period entirely within the post-fortification period. Overall, they observed that folate derived from any form—including folate derived from supplements or folic acid—was not associated with an increase in colorectal cancer. In fact, total folate was associated with significant 19% lower risk, consistent with studies conducted in the pre-fortification era.26Kim D.H. Smith-Warner S.A. Spiegelman D. et al.Pooled analyses of 13 prospective cohort studies on folate intake and colon cancer.Cancer Causes Control. 2010; 21: 1919-1930Google Scholar These results are remarkably consistent with our recent findings in 2 large, prospective, cohort studies that included 87,861 women enrolled in the Nurses' Health Study (NHS) and 47,290 men enrolled in the Health Professionals Follow-up Study (HPFS).27Lee J.E. Willett W.C. Fuchs C.S. et al.Folate intake and risk of colorectal cancer and adenoma: modification by time.Am J Clin Nutr. 2011; 93: 817-825Google Scholar In these 2 cohorts, we observed that total folate intake in the post-fortification era was similarly not associated with an increased risk of colorectal cancer. Notably, the results of Stevens et al also provide additional evidence that when during the multistep model of carcinogenesis one examines the folate–cancer association is central to our understanding of the complex role of this nutrient for cancer prevention. In their analysis, inverse associations between folic acid or total folate and colorectal cancer was significantly influenced by follow-up time, with lower risk limited to cases diagnosed in 2002–2007, 3–8 years after assessment of intake. Moreover, inverse associations were particularly strong among those who had ever undergone endoscopy, a population presumably less likely to already have baseline adenomas or early cancer. This suggests that folate may have a weaker or nonexistent effect on already established lesions. These findings are consistent with our results in the NHS and HPFS, which demonstrated that folate intake 12–16 years before diagnosis was significantly inversely associated with colorectal cancer (relative risk 0.69; 95% confidence interval [CI], 0.51–0.94 for ≥800 compared with <250 μg folate per day), but intake in the more recent past was not. For colorectal adenoma, we observed a strong inverse association with intake 4–8 years before diagnosis (odds ratio, 0.68; 95% CI, 0.60–0.78 for ≥800 compared with <250 μg folate per day). Based on current understanding of the prolonged dwell-time of the adenoma before carcinoma develops, our results are consistent with a short-term influence of folate on adenoma initiation. Taken together, the results in Cancer Prevention Study II, NHS, and HPFS suggest that folate may inhibit the earliest stages of colorectal carcinogenesis, with a diminished to nonexistent influence on progressively more advanced lesions (Figure 1). The importance of when one is exposed to folate demonstrated by these studies also helps to explain the discrepant results between epidemiologic studies and the null results of the 3 randomized intervention trials of folic acid in adenoma recurrence.29Figueiredo J.C. Mott L.A. Giovannucci E. et al.Folic acid and prevention of colorectal adenomas: A combined analysis of randomized clinical trials.Int J Cancer. 2010; 129: 192-203Google Scholar First, the intervention studies examined the effect of folic acid in patients with established neoplasia (adenoma). Thus, if folate is indeed effective only in inhibition of the earliest stages of carcinogenesis, the colon of patients with prior adenoma may already have preexisting subclinical neoplasia or a “field defect” that folic acid is unable to reverse.28Giovannucci E. Ogino S. DNA methylation, field effects, and colorectal cancer.J Natl Cancer Inst. 2005; 97: 1317-1319Google Scholar Second, as observed for intake of fruits and vegetables in the control arm of the Polyp Prevention Trial, the control arm in these intervention trials may have already consumed the minimum level of folate needed to circumvent pro-carcinogenic pathways. In support of this explanation, trial participants with low plasma folate concentrations at baseline that were randomized to folic acid did experience a decrease in adenoma recurrence.18Wu K. Platz E.A. Willett W.C. et al.A randomized trial on folic acid supplementation and risk of recurrent colorectal adenoma.Am J Clin Nutr. 2009; 90: 1623-1631Google Scholar, 29Figueiredo J.C. Mott L.A. Giovannucci E. et al.Folic acid and prevention of colorectal adenomas: A combined analysis of randomized clinical trials.Int J Cancer. 2010; 129: 192-203Google Scholar In summary, these 2 studies apply more rigorous scrutiny to the association between fruits, vegetables, folate, and cancer, highlighting several key points. First, extremely high consumption of fruits and vegetables does not seem to have a substantial anti-cancer benefit. However, diets characterized by extremely low levels of these foods could pose an increase in risk that can be easily minimized with modestly increasing intake. Second, together with our data from 2 other large cohorts,27Lee J.E. Willett W.C. Fuchs C.S. et al.Folate intake and risk of colorectal cancer and adenoma: modification by time.Am J Clin Nutr. 2011; 93: 817-825Google Scholar these studies confirm that folic acid administered at current levels does not seem to increase the risk of cancer. This should reassure us that efforts to fortify the food supply to prevent neural tube defects are not causing harm. Third, there remains some evidence that increasing long-term folate intake, particularly among folate-deficient populations, may augment existing colorectal cancer prevention efforts. Fourth, these studies highlight the complexity of diet–cancer relationships and support the importance of accounting for baseline nutritional status, timing of intervention, and examining endpoints that span the spectrum of neoplasia (eg, initial adenoma vs recurrent adenoma vs cancer) and anatomic subtype (eg, colon vs rectum). In future studies, as we improve our understanding of the molecular heterogeneity of colorectal cancer, it will be important to examine diet in relation to molecular subtypes of cancer in which specific mechanistic pathways may be more susceptible to dietary intervention.30Schernhammer E.S. Ogino S. Fuchs C.S. Folate and vitamin B6 intake and risk of colon cancer in relation to p53 expression.Gastroenterology. 2008; 135: 770-780Abstract Full Text Full Text PDF Scopus (50) Google Scholar, 31Ogino S. Chan A.T. Fuchs C.S. et al.Molecular pathological epidemiology of colorectal neoplasia: an emerging transdisciplinary and interdisciplinary field.Gut. 2010; 60: 397-411Google Scholar Finally, data supporting a benefit for fruits and vegetables in prevention of cardiovascular disease continue to accumulate.32Appel L.J. Moore T.J. Obarzanek E. et al.A clinical trial of the effects of dietary patterns on blood pressure DASH Collaborative Research Group.N Engl J Med. 1997; 336: 1117-1124Google Scholar, 33Hung H.C. Joshipura K.J. Jiang R. et al.Fruit and vegetable intake and risk of major chronic disease.J Natl Cancer Inst. 2004; 96: 1577-1584Google Scholar Thus, although increasing consumption of fruits and vegetables may have a relatively minor, if not negligible, impact on colorectal cancer incidence,2Chan A.T. Giovannucci E.L. Primary prevention of colorectal cancer.Gastroenterology. 2010; 138: 2029-2043Google Scholar there remain cogent reasons to continue recommending higher intake for the general population. High Levels of Folate From Supplements and Fortification Are Not Associated With Increased Risk of Colorectal CancerGastroenterologyVol. 141Issue 1PreviewFolate intake has been inversely associated with colorectal cancer risk in several prospective epidemiologic studies. However, no study fully assessed the influence of the high levels of folate that are frequently consumed in the United States as a result of mandatory folate fortification, which was fully implemented in 1998, and the recent increase in use of folate-containing supplements. There is evidence that consumption of high levels of folic acid, the form of folate used for fortification and in supplements, has different effects on biochemical pathways than natural folates and might promote carcinogenesis. Full-Text PDF Nonlinear Reduction in Risk for Colorectal Cancer by Fruit and Vegetable Intake Based on Meta-analysis of Prospective StudiesGastroenterologyVol. 141Issue 1PreviewThe association between fruit and vegetable intake and colorectal cancer risk has been investigated by many studies but is controversial because of inconsistent results and weak observed associations. We summarized the evidence from cohort studies in categorical, linear, and nonlinear, dose–response meta-analyses. 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