Search tips
Search criteria 


Logo of nihpaAbout Author manuscriptsSubmit a manuscriptNIH Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
Eur J Cancer. Author manuscript; available in PMC Nov 17, 2012.
Published in final edited form as:
PMCID: PMC3500543
A U-shaped association between plasma folate and pancreatic cancer risk in the European Prospective Investigation into Cancer and Nutrition
Shu-Chun Chuang,1,2* Rachael Stolzenberg-Solomon,3* Per Magne Ueland,4,5 Stein Emil Vollset,6,7 Øivind Middtun,8 Anja Olsen,9 Anne Tjønneland,9 Kim Overvad,10 Marie-Christine Boutron-Ruault,11,12 Sophie Morois,11,12 Franςoise Clavel-Chapelon,11,12 Brigit Teucher,13 Rudolf Kaaks,13 Cornelia Weikert,14 Heiner Boeing,15 Antonia Trichopoulou,16,17 Vassiliki Benetou,16 Androniki Naska,16 Mazda Jenab,18 Nadia Slimani,18 Isabelle Romieu,18 Dominique S. Michaud,1,19 Domenico Palli,20 Sieri Sabina,21 Salvatore Panico,22 Carlotta Sacerdote,23,24 Rosario Tumino,25 Guri Skeie,26 Eric J. Duell,27 Laudina Rodriguez,28 Esther Molina-Montes,29,30 José Marí Huerta,30,31 Nerea Larrañaga,30,32 Aurelio Barricarte Gurrea,30,33 Dorthe Johansen,34 Jonas Manjer,34 Weimin Ye,35,36 Malin Sund,37 Petra HM Peeters,38 Suzanne Jeumink,39,40 Nicholas Wareham,41 Kay-Tee Khaw,42 Francesca Crowe,43 Elio Riboli,1 Bas Bueno-de-Mesquita,39,40* and Paolo Vineis1,2,24*
1School of Public Health, Imperial College London, London, UK
2MRC/HPA Centre for Environment and Health, Imperial College London, London, UK
3Division of Cancer Epidemiology and Genetics, National Cancer Institute, National Institutes of Health, Department of Health and Human Services, MD, USA
4Section of Pharmacology, Institute of Medicine, University of Bergen, Bergen, Norway
5Laboratory of Clinical Biochemistry, Haukeland University Hospital, Bergen, Norway
6Department of Public Health and Primary Health Care, University of Bergen, Bergen, Norway
7Division of Epidemiology, Norwegian Institute of Public Health, Bergen, Norway
8Bevital AS, Bergen, Norway
9Institute of Cancer Epidemiology, the Danish Cancer Society, Copenhagen, Denmark
10Department of Epidemiology, School of Public Health, Aarhus University, Denmark
11INSERM, Center for Research in Epidemiology and Public Health, U1018, F-94805, Villejuif, France
12Paris South University, UMRS1018, F-94805, Villejuif, France
13Division of Cancer Epidemiology, German Cancer Research Center, Germany
14German Institute of Human Nutrition, Potsdam-Rehbrüecke Arthur-Scheunert-Alle 114-116, 14558 Nuthetal, Germany
15Department of Epidemiology, Deutsches Institut für Ernährungsforschung, Potsdam-Rehbrücke, Germany
16WHO collaborating Center for Food and Nutrition Policies, Department of Hygiene, Epidemiology and Medical Statistics, University of Athens Medical School, Athens, Greece
17Hellenic Health Foundation, Athens, Greece
18International Agency for Research on Cancer, Lyon, France
19Brown University, Providence, RI, USA
20Molecular and Nutritional Epidemiology Unit, Cancer Research and Prevention Institute – ISPO, Florence, Italy
21Nutritional Epidemiology Unit, Fondazione IRCCS Istituto Nazionale dei Tumori, Italy
22Dipartimento di Medicina Clinica e Sperimentale, Universita di Napoli, Federico II, Naples, Italy
23CPO-Piemonte, Torino, Italy
24HuGeF Foundation, Torino, Italy
25Cancer Registry and Histopathology Unit, “Civile M.P. Arzeeo” Hospital ASP 7, Ragusa, Italy
26Institutt for Samfunnsmedisin, Universitetet I Tromsø, Tromsø, Norway
27Unit of Nutrition, Environment and Cancer, Cancer Epidemiology Research Programme, Institut Catala d’Oncologia (ICO), Barcelona, Spain
28Jefa de Seccion de Informacion Sanitaria, Servicio de Salud Sanitarios, Principado de Asturias, Asturias, Spain
29Andalusian School of Public Health, Granada, Granada, Spain
30Ciber Epidemiologia y Salud Pública (CIBERESP), Spain
31Department of Epidemiology, Murcia Regional Health Authority, Murica, Spain
32Public Health Department of Gipuzkoa, Basque Government, Spain
33Navarre Public Health Institute, Pamplona, Spain
34Department of Surgery, Skåne University Hospital Malmö, Lund University, Malmö, Sweden
35Department of Medical Epidemiology and Biostatistics, Karolinska Institut, Stockholm, Umea, Sweden
36The Medical Biobank at Umea University, Umea, Sweden
37Departments of Surgical and Perioperative Science, Surgery and Public Health and Clinical Medicine, Nutrition Research, Umea University, Umea, Sweden
38Julius Center for Health Science and Primary Care, University Medical Center, Utrecht, The Netherlands
39The National Institute for Public Health and the Environment (RIVM), Bilthoven, The Netherlands
40Department of Gastroenterology and Hepatology, University Medical Centre Utrecht (UMCU), Utrecht, The Netherlands
41MRC Epidemiology Unit, Cambridge, UK
42University of Cambridge, Cambridge, UK
43Cancer Epidemiology Unit, Nuffield Department of Clinical Medicine University of Oxford, Oxford, UK
Corresponding author: Paolo Vineis, MD, MPH, Chair in Environmental Epidemiology, School of Public Health, Imperial College London, St Mary’s Campus, Norfolk Place W2 1PG, London, p.vineis/at/
*Equally contributed to the paper
Folate intake has shown an inverse association with pancreatic cancer; nevertheless, results from plasma measurements were inconsistent. The aim of this study is to examine the association between plasma total homocysteine, methionine, folate, cobalamin, pyridoxal 5′-phosphate, riboflavin, and flavin mononucleotide and pancreatic cancer risk in the European Prospective Investigation into Cancer and Nutrition (EPIC). We conducted a nested case-control study in the EPIC cohort, which has an average of 9.6 years of follow-up (1992–2006), using 463 incident pancreatic cancer cases. Controls were matched to each case by center, sex, age (+/−1y), date (+/−1y) and time (+/−3h) at blood collection, and fasting status. Conditional logistic regression was used to calculate the odds ratios (OR) and 95% confidence intervals (CI), adjusting for education, smoking status, plasma cotinine concentration, alcohol drinking, body mass index and diabetes status. We observed a U-shaped association between plasma folate and pancreatic cancer risk The ORs for plasma folate ≤5, 5–10, 10–15 (reference), 15–20, and >20 nmol/L were 1.6 (95% CI=0.7–3.5), 1.4 (0.9–2.1), 1.0 (reference), 0.8 (0.5–1.2), and 1.3 (0.9–2.0), respectively. Methionine was associated with an increased risk in men (per quintile increment: OR=1.2 95% CI=1.0–1.4) but not in women (OR=0.9, 95% CI=0.8–1.1; p for heterogeneity<0.01). Our results suggest a U-shaped association between plasma folate and pancreatic cancer risk in both men and women. The positive association that we observed between methionine and pancreatic cancer may be sex dependent and may differ by time of follow-up. However, the mechanisms behind the observed associations warrant further investigation.
Keywords: folate, one-carbon metabolism, pancreatic cancer, EPIC
Pancreatic cancer is the 5th most common cause of death from cancer in Europe in 2008 (95,200 deaths, representing 5.5% total cancer deaths)1. There is no effective screening test for the malignancy; it is often diagnosed at an advanced stage, which leads to a 5-year survival rate as low as 6% in Europe2. Few risk factors for pancreatic cancer have been consistently identified. Cigarette smoking3, obesity4, diabetes mellitus5 and chronic pancreatitis6 increase the risk of pancreatic cancer.
Evidence has been mounting for folate being a potentially important micronutrient in the prevention of cancer. Prospective studies in Finland7 and Sweden8 showed an inverse association between folate intake and pancreatic cancer, while the US NHS and the HPFS studies9 showed the inverse association was from foods but not from supplements. The PLCO study10 further suggested that the inverse association was only observed in women but not in men. The World Cancer Research Fund (WCRF) systematically reviewed the literature and concluded that folate intake probably protects against pancreatic cancer (2007)11.
To date, there have been two studies investigating the association between blood biomarkers of one-carbon metabolism and pancreatic cancer. In a cohort of Finnish male smokers (ATBC cohort, 126 cases), a significant twofold reduction in pancreatic cancer risk was observed in participants with higher serum folate (>4.45 ng/mL) and pyridoxal 5′-phosphate (PLP, the coenzyme form of vitamin B6, >39.46 nmol/L), compared to those with lower concentrations (≤3.33 ng/mL for folate and ≤26.34 nmol/L for PLP)12. However, the results from a pooled analysis based on four US cohorts (208 cases) showed no association13; this lack of association between folate concentrations and risk of pancreatic cancer may be because very few participants had less than adequate serum folate concentrations due to the folic acid grain fortification of wheat flour that has been mandatory in the US since 1998.
The aim of this study was to assess the association between nutrition related one-carbon metabolites, namely plasma total homocysteine (tHcy), methionine, folate, cobalamin, PLP, riboflavin, and flavin mononucleotide (FMN), and pancreatic cancer in a case-control study nested within the European Prospective Investigation into Cancer and Nutrition (EPIC).
The EPIC cohort
EPIC is a multicenter prospective cohort study, which recruited 520,000 healthy volunteers from 23 centers in 10 countries (Sweden, Denmark, Norway, the Netherlands, United Kingdom, France, Germany, Spain, Italy, and Greece) between 1992 and 2000. The cohort was described in detail previously14. In brief, the study population included volunteers aged 25 to 70 years at the time of recruitment. Informed consent forms were filled at each local center and the study was approved by the Institutional Review Board at International Agency for Research on Cancer (IARC) and the local ethical committees. Lifestyle questionnaires included questions on dietary items, education, occupation, previous illness, alcohol and tobacco consumption, and physical activity.
Ascertainment of cases and control selection
The follow-up was based on population cancer registries in Denmark, Italy, the Netherlands, Norway, Spain, Sweden, and the United Kingdom. In France, Germany, and Greece, a combination of health insurance records, cancer and pathology registries, and active follow-up through participants and their next-of-kin were used. Mortality data were also obtained at regional or national level. All participants were followed from recruitment (1992–2000) until cancer development, death, emigration or the end of the follow-up period (1992–2006).
Over 15 years of follow-up, 638 incident pancreatic cancer (C25) cases were identified. One hundred and seventy cases were excluded due to lack of blood samples (N=122), benign tumor (N=1), carcinoma in situ (N=1), uncertain whether it is a primary or metastatic tumor (N=1), neuroendocrine tumors (N=15), and secondary tumors (N=30). For each incident case, one control was selected by incidence density sampling and matched by center, gender, age at blood collection (+/−1y), date (+/−1y) and time (+/−3h) at blood collection, and fasting status. Five cases and four controls were further excluded because the blood samples were not sufficient for the assay. Thus, the current analysis included 463 incident pancreatic cancer cases and 464 controls.
Biological Samples and Laboratory Analyses
The blood samples were collected according to a standardized protocol14. Measurements of 25 markers, directly or indirectly involved in one-carbon metabolism, were made at Bevital A/S (, Bergen, Norway. Plasma concentrations tHcy, methionine, PLP, riboflavin, FMN, and cotinine (a nicotine metabolite and established marker for tobacco smoke exposure) were determined by mass spectrometry based methods15, 16, while microbiological methods were used to determine concentrations of folate17, and cobalamin18.
Samples were analyzed in batches of 86 and quality control included 6 calibration samples, 2 control samples, and 1 blank sample in each batch. Samples from case and control participants were kept at −80°C and analyzed in random order. The within and between day coefficients of variance (CV) were about 3–9% for folate, vitamin B12, PLP, riboflavin, and FMN, and about 0.9% for within-day CV and 2% for between-day CV for tHcy and methionine19. All staff at the Bevital laboratory were blinded to the case-control status of the blood samples.
Statistical Analysis
Quintiles of each one-carbon nutrient biomarker were categorized based on distributions in the controls. Conditional logistic regression was used to estimate odds ratios (OR) and 95% confidence intervals (CI) for pancreatic cancer, with the lowest quintile serving as the reference category. ORs were also calculated by increasing quintiles as ordinal variable. All models included the following variables: education (no degree/primary school, technical or professional school, secondary school, university degree, not specified/missing), smoking status (never, former, current, unknown), cotinine plasma concentration, baseline alcohol drinking (never/former, current less than 6, 6–18, and greater than 18 g/day), body mass index (BMI, kg/m2, continuous), and self-reported diabetes status at baseline (Yes vs. No). We examined effect modification by sex, region (northern-, central-, southern Europe), smoking status, cotinine plasma concentration (<5 indicating non-smoking, 5–85 indicating passive smoking, >=85 nmol/L indicating current smoking), lifetime alcohol drinking, BMI, and physical activity (low/medium and high/very high) in stratified analyses.
We also evaluated the potential effect of subclinical disease on the association between the biomarkers and pancreatic cancer with time stratified analysis (<=4 vs. >4 years20, 21 since blood was drawn). When necessary, we broke the matching sets and used unconditional logistic regression models, adjusted for the matching variables as well as the potential confounding variables described above. P-values for heterogeneity across sex and follow-up years were assessed by χ2 statistics.
All analyses were performed using SAS 9.1. All tests were two sided and statistical significance was assessed at the level of 0.05.
Table 1 shows the characteristics of the cases and controls. The study population includes about the same number of men and women. Almost half of the cases and controls were from Northern European countries (44%). The average follow-up was 9.6 years. Men tended to be smokers, to drink more, and to be more often overweight than women.
Table 1
Table 1
Characteristics of cases and controls.
The current analyses focused on the associations for the key one-carbon metabolites, tHcy, methionine, folate, PLP, cobalamin, riboflavin, and FMN, and risk of pancreatic cancer, overall and by sex (Table 2). Methionine was positively associated with pancreatic cancer risk in men (for each quintile increment: OR=1.2, 95% CI=1.0–1.4) but not in women (OR=0.9, 95% CI=0.8–1.1); heterogeneity between sex was detected (p<0.01). Plasma folate tended to have a U-shaped dose-response relationship with pancreatic cancer risk: comparing the 4th to the 1st quintile, an inverse association was observed (OR=0.5, 95% CI=0.3–0.8), while no clear association was observed for the 5th vs. 1st quintile (OR=0.8, 95% CI=0.5–1.4). The effect size was about the same for both men and women (p for heterogeneity=1.0). Folate deficiency or high folate (defined as >20 nmol/L) were associated with higher risk of pancreatic cancer, but this was not statistically significant. As plasma concentrations of methionine result from the combination of dietary intake and the metabolism of folate intake, we calculated the folate/methionine ratio and examined its association with pancreatic cancer. Similar to the plasma folate, a U-shaped dose-response relationship was observed with the lowest OR in the 4th quintile (OR=0.5, 95% CI=0.3–0.8 and the 5th quintile OR=0.7, 95% CI=0.4–1.1, data not shown). Plasma PLP was inversely associated with pancreatic cancer in women (for each quintile increment, OR=0.8, 95% CI=0.7–1.0) but not in men (OR=1.1, 95% CI=0.9–1.2); however, the test for heterogeneity between men and women was not statistically significant (p for heterogeneity=0.6).
Table 2
Table 2
Odds ratios and 95% confidence intervals for biomarker levels (quintiles) and risk of pancreatic cancer by sex
No heterogeneity in the association between one-carbon metabolism biomarkers and pancreatic cancer risk was evident according to follow-up time except for methionine (Table 3). Using sex-specific quintiles, methionine was positively associated with risk of pancreatic cancer in men who had more than 4 years of follow-up between blood sample collection and diagnosis of pancreatic cancer (each quintile increment, OR=1.4, 95% CI=1.1–1.7) but not among men with shorter period of follow-up (<=4 years: OR=1.0, 95% CI=0.8–1.3). On the other hand, methionine was inversely associated with pancreatic cancer risk in women, especially among women with shorter follow-up time (OR=0.7, 95% CI=0.5–0.9).
Table 3
Table 3
Odds ratios and 95% confidence intervals for methionine levels (sex-specific quintiles) and risk of pancreatic cancer by years of follow-up before cancer diagnosis
No heterogeneity across smoking status, cotinine concentration, alcohol drinking, BMI, or physical activity was detected (data not shown). Blood samples from Oxford and Norway cohorts were exposed at ambient temperatures for up to 48 hours, and metabolites might be partly degraded due to such handling22. Excluding the two centers did not change the results materially.
Overall, no clear pattern of association was observed for one-carbon metabolites and pancreatic cancer. Our results suggest a weak inverse, U-shaped association between pancreatic cancer risk and folate and an inverse relation with PLP in women. Methionine was inversely associated with the risk of pancreatic cancer in women who had shorter period of follow-up but positively associated in men, especially for those with longer follow-up times.
The previous studies12, 13 clarified that the decreasing trend of serum folate and PLP may be only among non-multivitamin supplement users. The information for multivitamin use is not available in the EPIC study; however, in a subset of the EPIC cohort (~36,000 subjects), information on supplement use was collected23. Results from the sub-cohort indicated that participants from more northern countries (Denmark, Norway, Sweden, and UK) reported higher supplement use than more southern countries (Greece, Italy, Spain) and women reported a higher use than men24. Results from the present study showed no evidence of regional (data not shown) or gender differences in the folate- and PLP-pancreatic cancer associations although the PLP effects appeared to be stronger in women.
Inadequate folate intake has been associated with several cancer risks but results have been inconsistent. The US folic acid fortification program has shown great success on increasing the plasma folate status in the general population25. The plasma folate is correlated better with folic acid supplements than with natural folate (bioavailability)26. A tolerable upper intake of 1000 μg/day of folate has been set in the US and Europe because high folate levels could mask megaloblastic anemia associated with vitamin B12 deficiency27. Moreover, there is rising concern for a dual function of folate, i.e. while folate deficiency could increase the risk of neoplastic transformation, folate supplementation could promote the progression of existing cancerous or pre-cancerous lesions20, 28. Our observation suggested a U-shaped dose-response relationship. Although the effect was not significantly different from the null, those with high plasma folate concentrations (>20 nmol/L) had a similar risk as those with moderate deficiency (5–10 nmol/L) comparing to those with adequate level (10–15 nmol/L).
This is the first report on plasma methionine and pancreatic cancer risk. Serum methionine was previously reported to be inversely associated with lung cancer; in that study, the effects were consistent across sex and years of follow-up29. Dietary methionine intake based on food questionnaires was inversely associated with pancreatic cancer risk in a Swedish cohort study30. The ATBC7 and the US Health Professional and Nurses’ Health cohort9 reported a null association between methionine intake and pancreatic cancer risk. Concerning other cancer sites, the effect of methionine intake has been inconsistent.
In our study, plasma methionine was positively associated with pancreatic cancer in men, especially for those with longer follow-up times but no such association was observed among women. We are uncertain what might explain this difference by gender. It is possible that the association could be explained by residual confounding from other risk factors, such as smoking and drinking, because the distribution of risk factors was different in men and women (Table 1). However, no heterogeneity across smoking (p=0.6) or drinking status (p=0.2) was detected. Methionine status could also be a marker of certain dietary patterns, behaviours, or disease status that could explain our association.
We could not exclude the possibility that the observed associations were due to residual confounding from other dietary components or dietary patterns or risk behaviours. Cigarette smoking is by far the most consistent risk factor for pancreatic cancer. Furthermore, alcohol intake is known to interfere with folate metabolism and could be a possible risk factor for pancreatic cancer. Nevertheless, the observed associations did not differ across smoking status, the average lifetime alcohol consumption, BMI, and physical activity. Adjusting and not adjusting for diabetes and restricting to non-diabetic participants did not change the results materially, which might imply that diabetes status is not an important confounder or effect modifier of the associations we have observed. The observed sex-dependent methionine-pancreatic cancer associations could be explained by different residual confounding between sexes. Whether there is gender-specific susceptibility needs further investigation.
Our results have to be interpreted with caution due to several limitations. First, we cannot rule out the possibility of chance findings because multiple associations were tested. In addition, the methionine-pancreatic cancer association patterns changed after 4 years of follow-up for both men and women; this could suggest reverse causality, i.e. participants changed their behaviours because of pre-diagnostic cancer-related symptoms. We do not have pancreatitis information in our dataset, and this might be an intermediate between alcohol and pancreatic cancer. Changes in lifestyle and risk factors for pancreatic cancer over time could alter the associations between metabolite concentrations and pancreatic cancer risk. Neither did we have information on one-carbon nutrient metabolites at different points in time during the follow-up period, which limits our ability to address the potential changes in dietary intakes and of levels of metabolites over time within individuals. On the other hand, the larger sample size compared to previous studies, and the diversity of the EPIC population gave us greater power and the opportunity to test for potential heterogeneity across different strata. The prospective nature of dietary assessment and blood collection and the long follow-up of the current study enabled us to test whether the observed lower levels of B-vitamins are a result of cancer development. In addition, folic acid fortification is infrequent in Europe (except in the UK where it is voluntary).
In conclusion, our results suggest a U-shaped association between folate and pancreatic cancer risk in both men and women, and an inverse association between PLP and pancreatic cancer risk in women, but not in men. The positive association that we observed between methionine and pancreatic cancer may be sex dependent and may differ by time period. However, these effects warrant further investigation.
The project is supported by World Cancer Research Fund (WCRF, grant number: 2008/51). The EPIC cohort is supported by the Europe Against Cancer Program of the European Commission (SANCO). The individual centres also received funding from: Denmark: Danish Cancer Society; France: Ligue centre le Cancer, Institut Gustave Roussy, Mutuelle Générale de l’Education Nationale, Institut National de la Santé et de la Recherche Médicale (INSERM); Greece: Hellenic Ministry of Health, the Stavros Niarchos Foundation and the Hellenic Health Foundation ; Germany : German Cancer Aid, Cerman Cancer Research Center, and Federal Ministry of Education and Research (Grant 01-EA-9401) ; Italy: Italian Association for Research on Cancer and the National Research Council; The Netherlands: Dutch Ministry of Public Health, Welfare and Sports (VWS), Netherlands Cancer Registry (NKR), LK Research Funds, Dutch Prevention Funds, Dutch ZON (Zorg Onderzoek Nederland), World Cancer Research Fund (WCRF), Statistics Netherlands; Norway: Helga – Nordforsk center of excellence in food, nutrition and health; Spain: Health Research Fund (FIS) of the Spanish Ministry of Health (Exp 96/0032) and the participating regional governments and institutions; Sweden: Swedish Cancer Society, Swedish Scientific Council, and Regional Government of Skane; UK: Cancer Research UK and Medical Research Council.
Conflict of interests:
None of the authors declared conflict of interests.
1. Ferlay J, Parkin DM, Steliarova-Foucher E. Estimates of cancer incidence and mortality in Europe in 2008. Eur J Cancer. 2010 Mar;46(4):765–81. [PubMed]
2. Sant M, Allemani C, Santaquilani M, et al. EUROCARE-4. Survival of cancer patients diagnosed in 1995–1999. Results and commentary. Eur J Cancer. 2009 Apr;45(6):931–91. [PubMed]
3. Vrieling A, Bueno-de-Mesquita HB, Boshuizen HC, et al. Cigarette smoking, environmental tobacco smoke exposure and pancreatic cancer risk in the European Prospective Investigation into Cancer and Nutrition. Int J Cancer. 2010 May 15;126(10):2394–403. [PubMed]
4. Arslan AA, Helzlsouer KJ, Kooperberg C, et al. Anthropometric measures, body mass index, and pancreatic cancer: a pooled analysis from the Pancreatic Cancer Cohort Consortium (PanScan) Arch Intern Med. 2010 May 10;170(9):791–802. [PMC free article] [PubMed]
5. Huxley R, nsary-Moghaddam A, Berrington de GA, Barzi F, Woodward M. Type-II diabetes and pancreatic cancer: a meta-analysis of 36 studies. Br J Cancer. 2005 Jun 6;92(11):2076–83. [PMC free article] [PubMed]
6. Lowenfels AB, Maisonneuve P. Epidemiology and risk factors for pancreatic cancer. Best Pract Res Clin Gastroenterol. 2006 Apr;20(2):197–209. [PubMed]
7. Stolzenberg-Solomon RZ, Pietinen P, Barrett MJ, et al. Dietary and other methyl-group availability factors and pancreatic cancer risk in a cohort of male smokers. Am J Epidemiol. 2001 Apr 1;153(7):680–7. [PubMed]
8. Larsson SC, Hakansson N, Giovannucci E, Wolk A. Folate intake and pancreatic cancer incidence: a prospective study of Swedish women and men. J Natl Cancer Inst. 2006 Mar 15;98(6):407–13. [PubMed]
9. Skinner HG, Michaud DS, Giovannucci EL, et al. A prospective study of folate intake and the risk of pancreatic cancer in men and women. Am J Epidemiol. 2004 Aug 1;160(3):248–58. [PubMed]
10. Oaks BM, Dodd KW, Meinhold CL, et al. Folate intake, post-folic acid grain fortification, and pancreatic cancer risk in the Prostate, Lung, Colorectal, and Ovarian Cancer Screening Trial. Am J Clin Nutr. 2010 Feb;91(2):449–55. [PubMed]
11. World Cancer Research Fund/American Institute for Cancer Research. Food, Nutrition, Physical Activity, and the Prevention of Cancer: a Global Perspective. Washington, DC: AICR; 2007. Pancreas; pp. 271–274.
12. Stolzenberg-Solomon RZ, Albanes D, Nieto FJ, et al. Pancreatic cancer risk and nutrition-related methyl-group availability indicators in male smokers. J Natl Cancer Inst. 1999 Mar 17;91(6):535–41. [PubMed]
13. Schernhammer E, Wolpin B, Rifai N, et al. Plasma folate, vitamin B6, vitamin B12, and homocysteine and pancreatic cancer risk in four large cohorts. Cancer Res. 2007 Jun 1;67(11):5553–60. [PubMed]
14. Riboli E, Hunt KJ, Slimani N, et al. European Prospective Investigation into Cancer and Nutrition (EPIC): study populations and data collection. Public Health Nutr. 2002 Dec;5(6B):1113–24. [PubMed]
15. Midttun O, Hustad S, Ueland PM. Quantitative profiling of biomarkers related to B-vitamin status, tryptophan metabolism and inflammation in human plasma by liquid chromatography/tandem mass spectrometry. Rapid Commun Mass Spectrom. 2009 May;23(9):1371–9. [PubMed]
16. Ueland PM, Midttun O, Windelberg A, et al. Quantitative profiling of folate and one-carbon metabolism in large-scale epidemiological studies by mass spectrometry. Clin Chem Lab Med. 2007;45(12):1737–45. [PubMed]
17. Molloy AM, Scott JM. Microbiological assay for serum, plasma, and red cell folate using cryopreserved, microtiter plate method. Methods Enzymol. 1997;281:43–53. [PubMed]
18. Kelleher BP, Broin SD. Microbiological assay for vitamin B12 performed in 96-well microtitre plates. J Clin Pathol. 1991 Jul;44(7):592–5. [PMC free article] [PubMed]
19. Windelberg A, Arseth O, Kvalheim G, Ueland PM. Automated assay for the determination of methylmalonic acid, total homocysteine, and related amino acids in human serum or plasma by means of methylchloroformate derivatization and gas chromatography-mass spectrometry. Clin Chem. 2005 Nov;51(11):2103–9. [PubMed]
20. Van Guelpen B, Hultdin J, Johansson I, et al. Low folate levels may protect against colorectal cancer. Gut. 2006 Oct;55(10):1461–6. [PMC free article] [PubMed]
21. Meinhold CL, Dodd KW, Jiao L, et al. Available carbohydrates, glycemic load, and pancreatic cancer: is there a link? Am J Epidemiol. 2010 Jun 1;171(11):1174–82. [PMC free article] [PubMed]
22. Juzeniene A, Thu Tam TT, Iani V, Moan J. 5-Methyltetrahydrofolate can be photodegraded by endogenous photosensitizers. Free Radic Biol Med. 2009 Oct 15;47(8):1199–204. [PubMed]
23. Slimani N, Kaaks R, Ferrari P, et al. European Prospective Investigation into Cancer and Nutrition (EPIC) calibration study: rationale, design and population characteristics. Public Health Nutr. 2002 Dec;5(6B):1125–45. [PubMed]
24. Skeie G, Braaten T, Hjartaker A, et al. Use of dietary supplements in the European Prospective Investigation into Cancer and Nutrition calibration study. Eur J Clin Nutr. 2009 Nov;63(Suppl 4):S226–S238. [PubMed]
25. Dietrich M, Brown CJ, 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 Aug;24(4):266–74. [PubMed]
26. Drogan D, Klipstein-Grobusch K, Wans S, et al. Plasma folate as marker of folate status in epidemiological studies: the European Investigation into Cancer and Nutrition (EPIC)-Potsdam study. Br J Nutr. 2004 Sep;92(3):489–96. [PubMed]
27. Folate in: Institute of Medicine Food and Nutrition Board. Dietary Reference Intake for Thiamin, Riboflavin, Niacin, Vitamin B6, Folate, Vitamin B12, Pantothenic Acid, Biotin, and Choline. Washington, DC: Natioanl Academy Press; 1998. pp. 196–305. [PubMed]
28. Kim YI. Folate and colorectal cancer: an evidence-based critical review. Mol Nutr Food Res. 2007 Mar;51(3):267–92. [PubMed]
29. Johansson M, Relton C, Ueland PM, et al. Serum B vitamin levels and risk of lung cancer. JAMA. 2010 Jun 16;303(23):2377–85. [PubMed]
30. Larsson SC, Giovannucci E, Wolk A. Methionine and vitamin B6 intake and risk of pancreatic cancer: a prospective study of Swedish women and men. Gastroenterology. 2007 Jan;132(1):113–8. [PubMed]