The Association between Fish Consumption and Risk of Renal Cancer: A Meta-Analysis of Observational Studies

Background Several case-control studies and cohort studies have investigated the association between fish intake and renal cancer risk, however, they yielded conflicting results. To our knowledge, a comprehensive assessment of the association between fish consumption and risk of renal cancer has not been reported. Hence, we conducted a systematic literature search and meta-analysis to quantify the association between fish consumption and renal cancer. Methods A systematic search was performed using the PubMed, Embase, and Cochrane Library Central database for case-control and cohort studies that assessed fish intake and risk of renal cancer. Two authors independently assessed eligibility and extracted data. Fixed-effect and random-effect models were used to estimate summary relative risks (RR) and the corresponding 95% confidence intervals (CIs). Subgroup analyses, sensitivity analysis and cumulative meta-analysis were also performed. Results A total of 12 case-control studies and three cohort studies published between 1990 and 2011 were included in the meta-analysis, involving 9,324 renal cancer cases and 608,753 participants. Meta-analysis showed that fish consumption did not significantly affect the risk of renal cancer (RR=0.99, 95% CI [0.92,1.07]). In our subgroup analyses, the results were not substantially affected by study design, region, gender, and confounder adjustments. Furthermore, sensitivity analysis confirmed the stability of results. Conclusions The present meta-analysis suggested that there was no significant association between fish consumption and risk of renal cancer. More in-depth studies are warranted to report more detailed results, including stratified results by fish type, preparation method, and gender.


Introduction
Renal cancer accounts for almost 2% of all cancers worldwide, which consists of malignant tumors arising from the renal parenchyma and renal pelvis [1,2]. Renal cell carcinoma(RCC) accounts for about 90% of adult renal cancer and 3% of adult malignancies. The incidence of renal cancer has been steadily increasing worldwide in males and females, doubling over the past three decades [1][2][3]. Although cigarette smoking, obesity, and hypertension are established risk factors, the etiology of renal cancer is largely unknown [1,4].
Renal cancer is a multifactorial disease, with both hereditary and environmental components playing a role [5]. It has been found that diet is an important factor in the development of renal cancer [1,5]. Increased consumption of meat, especially red meat and processed meat were found to be associated with an increased risk of renal cancer [6]. As we know, fish is an important aspect of diet, and previous meta-analyses have investigated the association between fish consumption and the risk of several cancers. It was found that fish consumption could reduce the risk of colorectal cancer (OR=0.88; 95% CI, 0.80-0.95). However, there was no significant association between fish consumption and the risk of other cancers, such as pancreatic cancer, bladder cancer, prostate cancer, or esophageal cancer [7][8][9][10][11][12]. There are several case-control and cohort studies investigating the association between fish intake and renal cancer risk, however, they yielded conflicting results.. To our knowledge, there has not been any quantitative attempt to summarize the results on the possible fish-renal cancer risk association. Thus, we conducted a quantitative meta-analysis of currently available epidemiologic studies to verify this putative association.

Study identification
This meta-analysis was conducted following the Preferred Reporting Items for Systematic reviews and Meta-Analyses guidelines(PRISMA) [13], as well as the meta-analysis of observational studies in epidemiology (MOOSE) guidelines [14]. A literature search was carried out using Pubmed (www.ncbi.nlm.nih.gov/sites/entrez) (1966 to May 2013), Embase (www.embase.com)(1947 to May 2013), and Cochrane Library Central database(http:// onlinelibrary.wiley.com/cochranelibrary/search/)(1967 to May 2013). There was no restriction of origin and language. Search terms included: "fish" or "seafood" and ''cancer(s)'' or ''neoplasm(s)'' or ''malignancy(ies)'' and "renal" or "kidney". Furthermore, the reference lists of each comparative study included in this meta-analysis and previous reviews were manually examined to identify additional relevant studies.

Study selection
Two of the authors independently selected eligible casecontrol and cohort studies investigating the association between fish intake and renal cancer risk. Disagreement between the two reviewers was settled by discussing with the third reviewer. Inclusion criteria were: (i) used a case-control or cohort study design; (ii) evaluated the association between fish intake and renal cancer risk; (iii) presented odds ratio (OR), relative risk (RR), or hazard ratio (HR) estimates with its 95% confidence interval (CI). When there were multiple publications from the same population, only data from the most recent report was included in the meta-analysis and the remaining were excluded. Studies reporting different measures of RR like risk ratio, rate ratio, hazard ratio, and odds ratio were included in the meta-analysis. In practice, these measures of effect yield a similar estimate of RR, since the absolute risk of renal cancer is low.

Data extraction
Two of the authors independently extracted the relevant data from each included study by using a unified data form. The items included in the data form were as follows: name of first author, publishing time, country of the population studied, study design, study period, number of cancer cases and subjects, dietary assessment method, type of fish, quantity of intake, the study-specific adjusted ORs, RRs, or HRs with their 95% CIs for the highest category of fish consumption versus the lowest, confounding factors for matching or adjustments. The 2 lists from the authors were compared, and disagreements were resolved by consensus.

Methodological quality assessment
To assess the study quality, a 10-star system on the basis of the Newcastle-Ottawa Scale was used in which a study was judged on 3 broad perspectives as follows: selection (four items, one star each), comparability (one item, up to two stars), and exposure/outcome (three items, one star each). A ''star'' presents a ''high-quality'' choice of individual study. With consideration that there is a correlation between caloric intake and nutrient consumption, and possibly a direct or indirect causal relation between caloric intake and renal cancer risk, the scoring system was modified by adding an item in which a study with data analysis that used an energy-adjusted residual or nutrient-density model received an additional star [15]. Hence, the full score was 10 stars, and the high-quality study was defined as a study with≥7 awarded stars.

Data synthesis and analysis
Heterogeneity was assessed using the Cochran Q and I 2 statistics. For the Q statistic, a P value<0.10 was considered statistically significant for heterogeneity; for the I 2 statistic, heterogeneity was interpreted as absent (I 2 : 0%-25%), low (I 2 : 25.1%-50%), moderate (I 2 : 50.1%-75%), or high (I 2 : 75.1%-100%) [16]. To better investigate the possible sources of between-study heterogeneity, a meta-regression analysis was performed [17]. Some studies presented individual risk estimates according to the different types of fish, and did not report the effect of total fish consumption. In this situation, the study-specific effect size in overall analysis was calculated by pooling the risk estimates of the various fish types, using the inverse-variance method [18]. For studies that reported results separately for males and females, but not combined, we pooled the results using a fixed-effect model to obtain an overall combined estimate before combining with the rest of the studies [19]. Subgroup analyses were carried out according to (i) study design ( cohort study versus population based casecontrol study versus hospital based case-control study), (ii)geographic location (Europe versus North America versus others), (iii) gender (male versus female), (iiii) number of adjustment factors (n ≥ 7 versus n ≤ 6), adjustment for alcohol intake (yes versus no), adjustment for total energy intake (yes versus no). Pooled RR estimates and their corresponding 95 % CIs were calculated using the inverse variance method. When substantial heterogeneity was detected(I 2 ≥50%), the summary estimate based on the random-effect model (DerSimonian-Laird method) [20] was reported, which assumed that the studies included in the meta-analysis had varying effect sizes. Otherwise, the summary estimate based on the fixed-effect model (the inverse variance method) [21] was reported, which assumed that the studies included in the meta-analysis had the same effect size. We carried out sensitivity analysis by excluding one study at a time to explore whether the results were significantly influenced by a specific study. Cumulative meta-analysis was also performed to identify the change in trend of reporting risk over time. In cumulative meta-analysis, studies were chronologically ordered by publication year, then the pooled RRs were obtained at the end of each year. Publication bias was assessed using Begg and Mazumdar adjusted rank correlation test and the Egger regression asymmetry test [22,23]. All analyses were performed using Stata version 11.0 (StataCorp, College Station, TX).
Fish Intake and Renal Cancer Risk: A Meta-Analysis PLOS ONE | www.plosone.org

Literature search and study characteristics
A flow diagram that shows how we located relevant studies is presented in Figure 1. A total of 1,283 citations were identified from the three databases. On the basis of the title and abstract, we identified 17 papers. After reviewing the full text, three studies were excluded, because they were from the same population [24][25][26]. One study was identified from reference lists [27]. At last, the remaining 15 studies published between 1990 and 2011 were included in the meta-analysis, involving a total of 608,753 participants and 9,324 renal cancer cases. Of these 15 studies, seven were population-based case-control studies [28][29][30][31][32][33][34], five were hospital-based casecontrol studies [35][36][37][38][39], and the remaining three were cohort studies [27,40,41]. Four studies were conducted in North America [27,28,33,34], nine in Europe [29,30,32,[35][36][37][39][40][41], one in Asia [38], and the remaining one study was a multicenter study which was conducted in Australia, Denmark, Sweden and the United States [31]. Almost all studies adjusted for smoking status and body mass index(BMI), and about half of the included studies adjusted for alcohol drinking status (Baseline data and other details are shown in Table 1). Table  S1 summarizes the quality scores of cohort studies and casecontrol studies. The Newcastle-Ottawa Scale scores for the included studies ranged from 6 to 10, with a median 7. The median scores of cohort studies and case-control studies were 8 and 7, respectively. 11 studies were deemed to be of a high quality (≥7).

Main analysis
Because of statistically significant heterogeneity was not observed (I 2 =23.8%, p = 0.19), a fixed-effects model was chosen over a random-effects model, and we found that fish consumption did not significantly affect renal cancer risk(RR=0.99, 95% CI [0.92,1.07]). Both multivariable adjusted RR estimates with 95 % CIs of each study and combined RR are shown in Figure 2.  Table 2.

Subgroup analyses, sensitivity analysis, cumulative meta-analysis, and meta-regression analysis
When stratified the various studies by study population, we found no significant association among studies conducted in Europe (RR= 0.98, 95%CI [0.86, 1.10]), North America (RR=   (Figure 3). To better investigate the possible sources of between-study heterogeneity, a metaregression analysis was performed. Study design, geographic area, control source, publication year, control for confounding factors, which may be potential sources of heterogeneity, were tested by a meta-regression method. However, meta-regression revealed that none of the above factors were responsible for the between-study heterogeneity.

Publication bias
In the present meta-analysis, no publication bias was observed among studies using Begg's P value (P = 0.40); Egger's (P = 0.38) test, which suggested there was no evidence of publication bias (Figure 4).

Discussion
The present meta-analysis included 15 observational studies currently available (12 case-control studies and three cohort studies), involving a total of 608,753 participants and 9,324 renal cancer cases. There was no statistically significant heterogeneity among the 15 studies, so a fixed-effects model was chosen over a random-effects model. Finally, we found that fish consumption did not significantly affect the risk of renal cancer(comparing the highest with the lowest category). In our subgroup analyses, the results were not substantially affected by study design, geographic location, gender, or confounder adjustments. Cohort and case-control studies alone showed no significant association between fish consumption and the risk of renal cancer. However, we should notice that there were only three cohort studies investigating the association between fish intake and renal cancer risk. That number was rather low to draw firm conclusions. Furthermore, most of the included studies didn't reported results separately for males and females. So, future studies should reported results separately for males and females. Sensitivity analysis indicated that an omission of any studies did not alter the magnitude of observed Fish Intake and Renal Cancer Risk: A Meta-Analysis PLOS ONE | www.plosone.org effect, suggesting a stability of our findings. Cumulative metaanalysis showed that the estimates gradually became consistent, and the corresponding CIs narrowed down with the increase of the number of included studies in the order of publication year. Moreover, the results of Begg's test and Egger's test did not support the existence of significant publication bias. Fish consumption has both anticarcinoma and carcinogenic effects. As we know, fish oil is rich source of n-3 fatty acids. Previous animal model studies have shown that n-3 polyunsaturated fatty acids were linked to the reduction of the progression of cancer cells [42,43]. Multiple mechanisms are involved in this chemopreventive activity, including suppression of neoplastic transformation, cell growth inhibition and enhanced apoptosis, and antiangiogenicity [44][45][46]. On the other hand, fish consumption is positively correlated with blood levels of dioxin, polychlorinated biphenyls, cadmium, mercury, and lead [47][48][49]. Cadmium, mercury, and lead are known nephrotoxicants which will induce oxidative stress and damage to the proximal renal tubule, the location where nearly renal cancer arises [50,51]. Previous studies have shown that cadmium, mercury, and lead were associated with an increased risk of renal cancer [50,52]. Maybe the combination of anticarcinoma and carcinogenic effect leads to the nonsignificant association between fish consumption and renal caner risk found in our meta-analysis.
Although we haven't found significant association between processed fish intake and increased renal cancer risk, we should notice that there were only two studies investigating processed fish and renal cancer risk, that number was rather low to draw firm conclusion. As we know, processed fish is rich in chemical carcinogens, such as nitrites, heterocyclic amines, 2-chloro-4-methylthiobutanoic acid, and so on, which may be associated with an increased risk of renal cancer. So more studies are needed to confirm the association between processed fish consumption and the risk of renal cancer in the future.
A study of women in Sweden by Wolk et al. [41]reported a reduced risk of renal cancer with higher fatty fish (salmon, herring, sardines, and mackerel) consumption. The possible reason is that there are large differences between fatty fish and lean fish in the content of omega-3 fatty acids and vitamin D. Lower serum vitamin D levels have been found to be associated with development and progression of renal Fish Intake and Renal Cancer Risk: A Meta-Analysis cancer [53]. This was the only study investigating the association between fatty fish and the risk for development of renal cancer, so the association is needed to be confirmed by more studies in the future, especially in male population.
The strength of the present meta-analysis lies in a large sample size (608,753 participants and 9,324 renal cancer cases) and no significant evidence of publication bias. Two investigators independently performed the article identification, data extraction, and verification and resolved all discrepancies. Most studies adjusted for some important potential confounders, including age, sex, smoking status, and BMI. Furthermore, our findings were stable and robust in sensitivity analysis. However, several limitations to this meta-analysis should be noted. Firstly, as a meta-analysis of observational data, the possibility of recall and selection biases can't be ruled out. Compared with case-control studies, cohort studies are less susceptible to bias due to their nature. However, the present meta-analysis included only three cohort studies, so more prospective cohort studies are need to confirm the association in the future. Secondly, we haven't searched for unpublished studies, so only published studies were included in our meta-analysis. Therefore, publication bias may have occurred although no publication bias was indicated from both visualization of the funnel plot and Egger's test. Thirdly, most of the included studies haven't adjusted for hypertension, red and processed meat consumption, which are associated with an increased risk of renal cancer [6,54]. Lastly, different types of fish(lean fish and fatty fish, fresh fish and processed fish) may have different effects on renal cancer, however, we can't do detailed subgroup meta-analysis for a lack of data. Although we assessed processed fish and renal cancer risk, the number of included studies was rather low to draw firm conclusion. Further, different processing methods may influence the effect on renal cancer.
In conclusion, the present meta-analysis suggested that there was no significant association between fish consumption and renal cancer risk. More in-depth studies are warranted to report more detailed results, including stratified results by fish type, preparation method, and gender. Fish Intake and Renal Cancer Risk: A Meta-Analysis  Fish Intake and Renal Cancer Risk: A Meta-Analysis Supporting Information Table S1. Methodologic quality of observational studies included in the meta-analysis.