No association between fruit, vegetables, antioxidant nutrients and risk of renal cell carcinoma
Previous epidemiologic studies that have examined the relationship between renal cell carcinoma (RCC) risk and intakes of plant foods and antioxidant nutrients have yielded inconsistent results. We therefore examined the associations between intakes of fruit, vegetables, carotenoids, flavonoids, vitamin E and vitamin C and RCC risk in the Alpha-Tocopherol, Beta-Carotene Cancer Prevention (ATBC) Study cohort. At baseline, 27,062 male Finnish smokers aged 50–69 years completed a 276-item dietary questionnaire that included questions on frequency of consumption and portion size. During up to 19 years of follow-up, 255 men developed RCC. Cox proportional hazards models were utilized to estimate relative risks (RR) and 95% confidence intervals (CI). Despite a large range in intake, no association was observed between fruit, vegetables or antioxidant nutrients and RCC risk. For example, multivariate RRs and 95% CIs for the highest versus the lowest quartile of intake were 0.79 (0.55–1.14), 1.23 (0.85–1.79), 1.09 (0.74–1.60), 0.83 (0.57–1.21), 1.09 (0.73–1.64) and 0.99 (0.67–1.46) for fruit, vegetables, total carotenoids, total flavonoids, total vitamin E and vitamin C, respectively (all p values for trend > 0.05). Our results indicate that diet may not play a large role in the etiology of RCC in male smokers, although further examination of these associations in nonsmokers, women and diverse racial populations is warranted.
The incidence of renal cell carcinoma (RCC) is increasing both in the United States and globally.1, 2 The rise in incidence is due in part to more frequent use of ultrasonography, computed tomography, and magnetic resonance imaging, which inadvertently detect kidney cancers.3 However, the incidence of advanced and metastatic disease has also increased, suggesting that the rise is not entirely due to earlier detection.2
Established risk factors for RCC include age, race, gender, smoking and obesity, with most studies also demonstrating a positive association with hypertension.2, 4 Dietary factors also appear to play a role, with higher fruit and vegetable intake linked to lower risks of this disease in several studies.5–10 Fruits and vegetables are rich sources of free-radical scavenging antioxidant nutrients, including carotenoids, flavonoids, and vitamin C. Vitamin E is also a powerful antioxidant, although its major food sources are not fruits and vegetables. Rather, vitamin E is found predominantly in vegetable oils and nuts.11 Because oxidative stress is hypothesized to play a role in the development of RCC,12 higher dietary intakes and/or blood levels of these nutrients may protect against the initiation and progression of this malignancy. Studies that have examined the relationship between RCC and individual antioxidant nutrients, however, have yielded inconsistent results.6, 7, 9, 10, 13–16
We investigated associations of fruits, vegetables and individual antioxidant nutrients with subsequent risk of RCC in a large cohort study of male smokers. With 255 incident RCC cases available for analysis, this is one of the largest prospective studies to date examining these associations.
Material and Methods
Study design and subjects
Details of the Alpha-Tocopherol, Beta-Carotene Cancer Prevention (ATBC) Study have been described previously.17 Briefly, the ATBC Study was a randomized, double-blind, placebo-controlled, primary prevention trial that tested whether daily supplementation with β-carotene (20 mg) and/or vitamin E (50 mg DL-α-tocopheryl acetate) reduced the incidence of lung and other cancers. The study population included 29,133 Caucasian male smokers from southwestern Finland, aged 50–69 years, who smoked 5 or more cigarettes per day at baseline. Subjects with a history of cancer, those with a serious disease that would limit long-term participation in a trial, or those using vitamin E, A, or β-carotene supplements in excess of predefined limits were excluded. All subjects were enrolled between 1985 and 1988 and were actively followed until trial closure on April 30, 1993 (median follow-up time = 6.1 years); passive case ascertainment continued thereafter.
The Institutional Review Boards of both the National Public Health Institute of Finland and the US National Cancer Institute approved the study, and written informed consent was obtained from each participant before randomization.
Baseline data collection
At baseline, subjects were asked to provide demographic, smoking and occupational information, to give a history of medical examinations and diseases, and to complete a dietary questionnaire. Height, weight and blood pressure were measured by trained personnel. The dietary questionnaire asked participants to report usual frequency of consumption and portion size over the past year for 276 foods and beverages, and was accompanied by a color picture booklet designed to assist with portion size estimation. Daily nutrient intakes were calculated using the food composition database from the National Public Health Institute, Finland. The food use questionnaire was developed specifically for use in the ATBC Study and was validated in a pilot study that collected 12 two-day food consumption records distributed evenly over a period of 6 months from 190 subjects; Pearson correlation coefficients (unadjusted) were 0.41, 0.64, 0.58, 0.47 and 0.62 for vitamin A, vitamin E, vitamin C, vegetables and fruits, respectively.18 A total of 27,111 men (93% of the full cohort) successfully completed the dietary questionnaire. Overnight fasting serum samples were collected from virtually all participants at baseline and stored at −70°C. Concentrations of α-tocopherol, β-carotene and retinol were determined using high-performance liquid chromatography in the chemistry laboratory of the National Public Health Institute (Helsinki, Finland),19 whereas cholesterol levels were measured using an enzymatic assay (CHOD-PAP method, Boehringer Mannheim, Germany).
RCC cases (ICD-9 code 189.0) were identified through the Finnish Cancer Registry, which provides almost 100% case ascertainment nationwide.20 Cancers of the renal pelvis, ureter or urethra (n = 49) were not included in the present analysis. Two study physicians reviewed the medical records from hospitals and pathology laboratories independently to confirm the diagnosis of RCC. If the two disagreed, a third physician assigned a final diagnosis. A total of 255 incident cases of RCC were diagnosed between randomization and April 30, 2004.
Follow-up time for the 27,062 subjects included in this analysis was calculated from the date of study entry until RCC diagnosis, death or April 30, 2004. Cox proportional hazards models were used to estimate age-adjusted and multivariate relative risks (RR) and 95% confidence intervals (CI), with the lowest quartile of dietary intake or serum nutrients serving as the referent category. p-Values for linear trend were based on the median intake of each nutrient quartile, which were subsequently modeled as continuous variables. A base model was specified a priori and included known or suspected risk factors for RCC, including age, cigarettes smoked per day, number of years of smoking, alcohol consumption (g/day), body mass index (kg/m2) and measured systolic and diastolic blood pressure (mm Hg) as continuous variables, and trial intervention group (α-tocopherol vs. no α-tocopherol; β-carotene vs. no β-carotene), education (primary, high school, vocational or university), self-reported history of hypertension (yes or no) and regular leisure-time physical activity (sedentary, moderate intensity activities or high intensity activities) as categorical variables. Dietary intakes were further adjusted for energy using the standard multivariate approach (models using the residual method21 yielded similar results). Serum α-tocopherol was additionally adjusted for serum cholesterol because the latter is the major carrier of vitamin E in the bloodstream. Intakes of dietary fat, cholesterol and red meat, as well as place of residence and height, were evaluated as confounders but did not change risk estimates by more than 10%. A comprehensive antioxidant index, including dietary intakes of carotenoids, flavonoids, tocopherols and vitamin C, was created using principal components analysis; this approach has been described previously.22
Effect modification of associations between RCC and intakes of fruit and vegetables, total carotenoids, total flavonoids, total vitamin E and vitamin C by age (<57, ≥57 years), cigarettes/day (<20, ≥20), years smoked (<36, ≥36 years), clinical hypertension (based on measured blood pressure at baseline: <140/90, ≥140/90 mm Hg), self-reported history of hypertension (yes or no), alcohol consumption (<11, ≥11 g/day) and BMI (<25, ≥25 kg/m2) was assessed in stratified analyses and by including relevant cross-product terms in multivariate models. Cut points were determined using median values for age, cigarettes/day, years smoked and alcohol consumption. The cut point for BMI was determined based on the standard definition of overweight.
Lag analyses in which RCC cases diagnosed within the first 5 years of follow-up were removed were carried out in order to assess the impact of preclinical disease on the associations of interest. The proportional hazards assumption was verified by including a cross-product term between each nutrient and follow-up time in multivariate models (all p-values > 0.05). All reported p-values are two-tailed, with a statistical significance level of α = 0.05. Statistical analyses were performed using the SAS software version 9.1 (SAS Institute, Inc., Cary, NC).
Men with higher fruit and vegetable intake smoked less and for fewer years, were moderately heavier, had higher baseline serum levels of β-carotene, and were more highly educated and physically active than men with low intakes (Table 1). Fruit and vegetable consumption was positively associated with total energy and red meat intakes and inversely associated with dietary fat, cholesterol and alcohol consumption.
Table 1. Baseline characteristics (means and proportions) by quartiles of total fruit and vegetable intake in the ATBC study
RCC was not significantly associated with fruit or vegetable intake (Table 2), dietary intakes or serum levels of individual antioxidant nutrients (Table 3), or a comprehensive dietary antioxidant index (Table 3). Risks were unchanged when supplemental vitamin E and vitamin C were combined with dietary intakes of these nutrients (data not shown). A lag analysis in which 74 cases diagnosed within the first 5 years of follow-up were excluded showed similarly null associations when compared to the full cohort. For example, the RR and 95% CI for the highest versus the lowest quartile of total carotenoids, total vitamin E and total fruits and vegetables were 1.08 (0.68–1.72), 1.15 (0.70–1.87) and 0.88 (0.56–1.39), respectively (all p values for trend > 0.05).
Table 2. Age-adjusted and multivariate rr and 95% CI for RCC according to quartiles of fruit and vegetable intake in the ATBC study (1985–2004)
Table 3. Age-adjusted and multivariate RR and 95% CI for RCC according to quartiles of dietary and serum antioxidant nutrients in the ATBC study (1985–2004)
There was no variation in the risks of RCC associated with fruit and vegetable, total carotenoid, total flavonoid, total vitamin E or vitamin C intakes across categories of smoking dose, smoking duration, history of hypertension, or alcohol intake (data not shown). A significant interaction was observed, however, between total vitamin E intake and BMI (p = 0.01), with men in the normal range BMI group exhibiting a suggestively increased risk of RCC with higher vitamin E intake (RR and 95% CI for highest vs. lowest quartile = 1.36 [0.68–2.72], p trend = 0.12); this was not observed among men in the overweight and obese BMI category (RR and 95% CI = 0.96 [0.58–1.58], p trend = 0.96). A borderline significant interaction (p = 0.05) was observed between fruit and vegetable intake and age such that younger men exhibited a decreased risk of RCC at higher levels of total fruit and vegetable intake (RR and 95% CI for highest vs. lowest quartile = 0.55 [0.30–0.99], p trend = 0.02), with no such effect noted in older men (RR and 95% CI = 1.42 [0.86–2.35], p trend = 0.17).
Our findings are consistent with many other prospective studies that have reported no association between increased fruit and vegetable consumption and RCC risk.23–28 Case–control studies have yielded more conflicting findings.6–9, 13–15, 29–33 In contrast to most reports, a study that combined data from the Nurses' Health Study and the Health Professionals Follow-up Study and included a total of 248 RCC cases showed that higher fruit and vegetable consumption was associated with a significantly reduced risk of RCC in men (RR and 95% CI for ≥6 vs. <3 servings per day = 0.45 [0.25–0.81]), but not in women.10 Furthermore, there was an interaction with smoking status such that the protective effect was only evident in nonsmoking males. Our cohort was comprised exclusively of smokers, which might have precluded the detection of any modest yet beneficial effects of fruit and vegetable intake on RCC risk.
It is unclear why beneficial associations between fruit and vegetable intake and RCC risk were observed in younger but not older men in our cohort. Our study is not the first to demonstrate such a finding; in a case-control study conducted in Italy, Galeone et al. noted a significant inverse association between vegetable fibre intake and RCC risk in subjects <60 years of age, with no such relation apparent in older individuals.34 We are not aware of any heterogeneity in the biology of RCC across age subgroups. Although such differences may indeed exist, it is more likely that our finding is due to chance.
Although oxidative stress appears to play a role in the development of RCC,12 antioxidants have been inconsistently related to RCC in past prospective and case–control studies.6–10, 13–16, 35, 36 Most of these studies investigated α-tocopherol, vitamin C and individual carotenoids, but did not examine flavonoids or combinations of individual antioxidant nutrients. We found no evidence for a protective effect of any antioxidant nutrient on RCC risk.
Strengths of our study include its prospective design, which limits recall bias, use of a validated dietary questionnaire that was created specifically for the ATBC study, availability of pre-randomization serum β-carotene, retinol and α-tocopherol concentrations for virtually all participants, the large number of incident RCC cases, and our ability to control for a large number of important confounders.
A possible limitation is that our results may not be generalizable to females, nonsmokers or non-White ethnicities. We also relied on a single measure of dietary intake and serum nutrient concentrations, which may not adequately reflect lifetime dietary habits. In addition, FFQs may only be valid when asking about food intake over the past few days; when compared with food records or 24-hr recalls, their validity can be somewhat poor, especially for energy intake.37 The correlation coefficients in our study, however, ranged between 0.50 and 0.76 for individual antioxidants, suggesting good agreement between data from the FFQ and food consumption records. Random misclassification in FFQ data could have biased our results toward the null, especially if the true association between fruits, vegetables and/or antioxidant nutrients and RCC risk was modest.
In conclusion, our results suggest that increased consumption of fruit, vegetables and antioxidant nutrients does not reduce the risk of RCC in adult male smokers, although the lack of effect in our study may be partially attributable to misclassification of dietary intakes associated with the use of FFQs. Future studies should continue to address whether higher intakes of plant foods and antioxidants might be beneficial for RCC in nonsmokers, women and diverse racial populations.