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Pancreatic cancer is one of the leading causes of cancer mortality worldwide (Rahib et al, 2014; Ferlay et al, 2015; Malvezzi et al, 2016). Known risk factors include tobacco use (Iodice et al, 2008), high levels of alcohol consumption (Michaud et al, 2010; Gapstur et al, 2011; Lucenteforte et al, 2012), obesity (Arslan et al, 2010; Genkinger et al, 2011), family history (Verna et al, 2010) and diabetes (Ben et al, 2011; Bosetti et al, 2014). Several analyses have been performed of individual nutrients and compounds on pancreatic cancer risk, but the evidence remains uncertain as to which aspect of diet is related to pancreatic cancer risk (World Cancer Research Fund/American Institute for Cancer Research, 2012). Heterocyclic amines (Anderson et al, 2002, 2005) have been suggested as risk factors for pancreatic cancer, while folate (Skinner et al, 2004; Larsson et al, 2006), β-carotene (Jeurnink et al, 2015), α-tocopherol (Stolzenberg-Solomon et al, 2009; Jeurnink et al, 2015), vitamin C (Bueno de Mesquita et al, 1991; Gong et al, 2010), selenium (Banim et al, 2013) and flavonoids (Rossi et al, 2012; Jeurnink et al, 2015) may protect against pancreatic cancer.

Dietary patterns have also been associated with pancreatic cancer risk and may provide a more relevant dietary measure of cancer potential than analysis of individual dietary components. Adherence to dietary and lifestyle recommendations such as the World Cancer Research Fund/American Institute for Cancer Research (WCRF/AICR) recommendations to reduce cancer risk have been associated with decreased overall cancer risk, but have not been definitively associated with decreased pancreatic cancer risk (Romaguera et al, 2012; Lucas et al, 2015). Although elevated consumption of red meat and low consumption of fruit and vegetables have been positively associated to the risk of pancreatic cancer (Anderson et al, 2002, 2005; Polesel et al, 2010; World Cancer Research Fund/American Institute for Cancer Research, 2012; Bosetti et al, 2013; Lucas et al, 2015), the evidence is inconsistent to draw conclusions. A common theme to these more comprehensive dietary approaches is that they are high in dietary antioxidants.

Total antioxidant capacity (TAC) is a marker of dietary antioxidant potential, and is defined as the moles of oxidants neutralised by 1 l plasma, food extracts or single molecules (Serafini et al, 2006). According to WCRF/AICR recommendations, TAC provides a more comprehensive overview of dietary patterns as opposed to analysis of single dietary antioxidants (World Cancer Research Fund/American Institute for Cancer Research, 2012). We hypothesised that dietary TAC would be inversely associated with pancreatic cancer risk in a case–control study of pancreatic cancer subjects.

Materials and Methods

We analysed data from a case–control study of pancreatic cancer conducted between 1991 and 2008 in Milan and Pordenone, Italy (Polesel et al, 2010). Cases were 326 subjects (174 men, 152 women, median age 63 years, range 34–80 years) with incident pancreatic cancer admitted to major general hospitals in the study centres. Controls were 652 subjects (348 men, 304 women, frequency-matched according to age (±5 years), sex and study centre) with a 2:1 ratio. Controls were admitted to the same teaching or general hospitals as cases for a variety of acute non-neoplastic diagnoses, including acute surgical conditions (28%), traumatic orthopaedic conditions (31%), other orthopaedic conditions (31%) and other miscellaneous conditions (10%). Over 95% of cases and controls who were approached agreed to participate. All enrolled subjects signed an informed consent, according to the recommendations of the Board of Ethics of each participating centre.

All subjects were interviewed by centrally trained interviewers using a structured questionnaire that included socio-demographic factors, lifestyle habits (including history of tobacco use, alcohol use and physical activity), anthropometric measures (e.g., self-reported height and weight at different ages), personal medical history of selected diseases (e.g., diabetes) and history of cancer in first-degree relatives. Subject’s usual diet two years before cancer diagnosis (cases) or hospital admission (controls) was assessed though a validated (Decarli et al, 1996) and reproducible (Franceschi et al, 1993) food frequency questionnaire (FFQ), which included data on 83 foods and beverages grouped into seven sections: (1) bread and cereal dishes; (2) meat, fish and other main dishes; (3) potatoes and vegetables; (4) fruit; (5) sweets, desserts and soft drinks; (6) dairy and hot beverages (including tea and coffee); and (7) alcohol consumption. Subjects indicated average weekly consumption of each item; those with intermediate use were assigned frequency of 0.5.

The most commonly used measures of TAC are the Trolox equivalent antioxidant capacity (TEAC), the total radical-trapping antioxidant parameter (TRAP) and the ferric-reducing antioxidant power (FRAP) (Prior et al, 2005). TEAC and FRAP are based on single electron transfer mechanism and measure the ability of antioxidants to scavenge to the stable radical cation ABTS•+(2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonic acid)) and to reduce Fe3+ (ferric ion) to Fe2+ (ferrous ion). Total radical-trapping antioxidant parameter measures the chain-breaking potential to reduce peroxyl radicals generated by AAPH ((2,2-azobis(2-amidinopropane) dihydrochloride)) or ABAP (2,2′azobis(2-amidinopropane) dihydrochloride) and represents a measurement of the hydrogen atom transfer mechanism (Re et al, 1999).

Using the Italian food composition database (Gnagnarella et al, 2004), an ad hoc database was developed to calculate TAC for each of the three indices (i.e., TEAC, TRAP and FRAP) based on experimental assessment of the food extracts (Pellegrini et al, 2003, 2006). A total of 64 items contribute to the assessment of TEAC, 57 to TRAP and 59 to FRAP in this study. Coffee was excluded from the TAC estimate, since the main contributors to in vitro antioxidant capacity of coffee are the Maillard reaction products (creating during the coffee roasting of beans) (Delgado-Andrade and Morales, 2005), whose absorption and antioxidant capacity in vivo have not been demonstrated (Morales et al, 2012). In addition, due to their high molecular weight, they may function in a different manner from other antioxidants. Intake of total energy was computed using an Italian food composition database (Gnagnarella et al, 2004).

Energy-adjusted TEAC, TRAP and FRAP were categorised into tertiles based on the control distribution. We estimated the corresponding odds ratios (ORs) and 95% confidence intervals (CIs) for pancreatic cancer using conditional multiple logistic regression models, controlled for study centre (Milan, Pordenone), sex, age (5-year intervals) and adjusted for year of interview (continuous), years of education (<7, 7–11, ⩾12, categorically), body mass index (BMI) (<25, 25–30, ⩾30 kg m−2, categorically), tobacco use (never smoker, ex-smoker, current smoker <15 and ⩾15 cigarettes per day, categorically), alcohol intake (non-drinkers, <28 g of ethanol per day, >28 g of ethanol per day, categorically) and diabetes (yes, no). Energy intake was adjusted for using the residual model (Willett and Stampfer, 1986). We also fitted models in strata of sex, age, BMI, tobacco smoking, alcohol use and history of diabetes, and assessed the heterogeneity of the risk estimates across strata using the Wald χ2-test.

Results

The distribution of 326 pancreatic cancer cases and 652 controls according to sex, age and other selected characteristics is shown in Table 1. No significant differences were noted for education, BMI and alcohol consumption. Compared with controls, cases were more often smokers and more frequently had a history of diabetes. When analysing the distribution of these characteristics among tertiles of the three energy-adjusted TAC indices, subjects in the higher tertiles were more frequently men, current smokers and alcohol drinkers; moreover, they had a higher intake of fruit and vegetables and a lower consumption of cereals (Supplementary Table 1).

Table 1 Distribution of 326 cases of pancreatic cancer and 652 controls according to study centre, sex, age and other selected variables. Italy, 1991–2008
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Table 2 provides the distribution of pancreatic cancer cases and controls with the corresponding ORs and 95% CIs by tertiles of energy-adjusted dietary TAC indices. Significant inverse associations were noted between TEAC and FRAP and pancreatic cancer risk; the OR for the highest tertile of TEAC compared with the lowest tertile was 0.61 (95% CI 0.39–0.94, P-value for trend 0.03), and the OR for FRAP was 0.63 (95% CI 0.41–0.99, P-value for trend 0.05). TRAP was inversely but not significantly associated with pancreatic cancer risk, with an OR of 0.78 (95% CI 0.49–1.24, P-value for trend 0.27).

Table 2 Distribution of 326 pancreatic cancer case and 652 control patients and corresponding ORsa and 95% CIs by tertiles of three energy-adjusted TAC indices. Italy, 1991–2008
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Table 3 gives the ORs for pancreatic cancer according to tertiles of TEAC and FRAP by strata of selected covariates. The association between TEAC and FRAP and pancreatic cancer risk was apparently stronger (although not significantly) in subjects with a history of tobacco use. No differences were noted across strata of sex, age, BMI, alcohol use and history of diabetes. Given the low number of diabetic subjects, we also computed continuous OR for an increment equal to 1 s.d. For TEAC, the continuous OR was 0.81 (95% CI, 0.41–1.62) among diabetic subjects and 0.91 (95% CI, 0.75–1.10) among non-diabetic subjects. The continuous OR for FRAP was 0.69 (95% CI, 0.36–1.35) among diabetic subjects and 0.93 (95% CI, 0.77–1.12) among non-diabetic subjects.

Table 3 ORs of pancreatic cancer and 95% CIs by tertiles of TEAC and FRAP by selected covariates. Italy, 1991–2008
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Discussion

The association between antioxidants and pancreatic cancer risk is complex, with overall mixed results. Dietary intake of single antioxidants such as vitamin C (Bueno de Mesquita et al, 1991; Gong et al, 2010), α-tocopherol (Stolzenberg-Solomon et al, 2009; Bravi et al, 2011; Jeurnink et al, 2015), β-carotene (Olsen et al, 1991; Jeurnink et al, 2015), flavonoids (Nothlings et al, 2008; Rossi et al, 2012; Arem et al, 2013) and selenium (Banim et al, 2013) has been associated with decreased risk of pancreatic cancer. However, the intake of individual antioxidant supplements has failed to demonstrate a protective effect in pancreatic cancer (Rautalahti et al, 1999; Heinen et al, 2012; Han et al, 2013). We hypothesised that antioxidants may have an important role in pancreatic cancer risk, but that there may be interactions between antioxidant supplements and other dietary components that abrogate a beneficial effect.

Plant-based diets, rich in dietary antioxidants, have been associated with decreased pancreatic cancer risk (Anderson et al, 2005; Polesel et al, 2010; Bosetti et al, 2013). Also the Mediterranean diet has been associated with decreased pancreatic cancer risk and is rich in antioxidants (Bosetti et al, 2013). Thus, these data are in line with our findings suggesting that a diet high in TAC is associated with decreased pancreatic cancer risk.

Antioxidants are thought to reduce oxidative DNA damage and subsequent genetic mutations, and therefore may provide a protective effect against cancer (Foksinski et al, 2007). Tobacco smoke promotes cancer by a variety of different mechanisms, including genetic mutations in tumour suppressors and oncogenes, gene promoter hypermethylation and protein kinase activation ((U.S. Department of Health and Human Services, 2010) and therefore may modify the effect of antioxidants on cancer risk (Woodson et al, 1999; Wu et al, 2015). Our data indicating that the association between dietary antioxidant and pancreatic cancer may be stronger in subjects with a history of tobacco exposure could suggest that those exposed to increased oxidative stress may benefit the most from a diet high in antioxidants (Lettieri-Barbato et al, 2013). Further studies are warranted, as our results did not reach statistical significance.

All three indices were inversely related to pancreatic cancer, but the association was not significant for TRAP. Apart from the play of chance, this might be because of the fact that TRAP is more strongly influenced by the consumption of alcoholic beverages as compared with FRAP and TEAC (Praud et al, 2015), and alcohol use is positively related to pancreatic cancer risk (Gapstur et al, 2011; Lucenteforte et al, 2012). In addition, the TRAP assay has a high specificity for antioxidant behaviour, such as peroxyl radical scavenging activity, compared with FRAP and TEAC, respectively addressing iron reduction and scavenging of the not physiological ABTS+. Trapping antioxidant parameter provides more direct evidence of the canonical antioxidant activity, but at the same time since it is more specific, the test requires evidence of a redox condition (Serafini et al, 2006).

Limitations of the study include the hospital-based case–control design. Pancreatic cancer subjects and hospitalised controls may differ from those in the general population. We attempted to minimise selection bias by selecting controls that were admitted for reasons such as trauma and acute surgical conditions, and excluding those with a cancer diagnosis. We additionally attempted to limit bias by having the same trained interviewers administering the questionnaire to both cases and controls under similar conditions. Responses to dietary questionnaires may introduce some bias for those with a recent diagnosis of cancer. To minimise this, we asked about diet in the 2 years before cancer diagnosis. We also excluded controls with long-term diagnoses that required dietary modifications. At the time of study enrolment, limited data was available on pancreatic cancer risk factors; therefore, recall bias should be minimal. Data were not available on dietary supplements, which may be contributors to TAC; however, their use was infrequent in this study population during the study period (Sette et al, 2013). Our dietary TAC assay measures in vitro antioxidant activity which may not fully represent in vivo activity, due to still unclear association between dietary and endogenous antioxidant and to the low bioavailability of flavonoids (Manach et al, 2004; Serafini et al, 2011). The FFQ was reproducible (Franceschi et al, 1993) and valid (Decarli et al, 1996), and the reproducibility of FFQ data provided by hospital controls was satisfactory (D'Avanzo et al, 1997). Strengths of the study include the ability to control for other known pancreatic cancer risk factors, and near-complete data collection for both cases and controls.

Thus, we found that a diet high in antioxidant potential, as measured by dietary TEAC and FRAP, is associated with a decreased risk of pancreatic cancer. The association may be stronger in those with a history of tobacco exposure, although this was not statistically heterogeneous. Our findings provide evidence that a diet high in dietary antioxidants may be protective against pancreatic cancer.