Meta-analysis of Fruit and Vegetable Sources and Cardiovascular Outcomes

Background: A high intake of fruit and vegetables is a common trait among healthy dietary patterns and a cornerstone of nutrition interventions for cardiovascular disease (CVD) prevention. There is an international consensus that at least 5 servings per day of fruit and vegetables is beneficial to overall population health (WHO, 2003). This guidance, however, is largely founded on observational studies showing protective associations of high intakes of fruit and vegetables with CVD in health-conscious individuals (Reddy and Katan, 2004). Recommendations to consume specific types of fruit and vegetables, such as citrus fruit and green leafy vegetables, have been established mainly on the association of isolated nutrients with CVD risk reduction in observational studies or the effect of isolated nutrients on CVD risk factors in small randomized trials (Katamay et al., 2007). Given its prominence in dietary guidelines, there is a need to determine the extent of CVD risk reduction that may be attributed to the consumption of fruit and vegetables. As guidelines are shifting away from a focus on single nutrients to a focus on whole foods and dietary patterns and achieving adequate intakes continue to be a challenge worldwide and (Hall et al., 2009), there is also an advantage in establishing which types of fruit and vegetables are associated with the greatest protection in CVD incidence and mortality per serving. Although several systematic reviews and meta-analyses of prospective cohort studies have demonstrated protective associations between the intake of total fruit and vegetables and CVD risk (Aune et al., 2017; Zhan et al., 2017; Gan et al., 2015; Wang et al, 2014; Hu et al., 2007; He et al., 2007; Dauchet et al., 2006), few have focused on specific fruit and vegetables or explored a range of cardiovascular outcomes which may have different etiologies. Several large prospective cohort studies have recently been published which may strengthen the certainty of the evidence by increasing the number of observations for the relationship between different fruit and vegetables exposures and different cardiovascular outcomes. We propose to build and expand on the exiting systematic reviews and meta-analyses by conducting a comprehensive synthesis of prospective cohort studies comparing the certainty of the evidence for the different types of fruit and vegetables in relation to a range of cardiovascular outcomes. Objective: To conduct a systematic review and meta-analysis of the available prospective cohort studies using the Grading of Recommendations Assessment, Development, and Evaluation (GRADE) approach to assess the association between different types of fruit and vegetables and different cardiovascular outcomes including CVD, CHD, and stroke incidence and mortality. Design: The conduct of the proposed systematic review and meta-analysis will follow the Cochrane handbook for systematic reviews of interventions and the GRADE handbook for grading the certainty of the evidence and the strength of recommendations using the GRADE approach. The reporting will follow the Meta-Analysis Of Observational Studies in Epidemiology (MOOSE) and the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statements. Data Sources: MEDLINE, Embase, and the Cochrane Central Register of Controlled Trials (CENTRAL) will be searched using relevant search terms. Search of databases will be supplemented by manual searches of bibliographies. Study Selection: Prospective cohort studies exploring the relation of fruit and/or vegetable intake to incident cardiovascular disease for a minimum follow-up time of 1 year will be included. Data extraction: Two independent reviewers will extract relevant information including study design, sample size, subject characteristics, exposure form (i.e. total fruit and vegetables, citrus fruit, green leafy vegetables, etc.), duration/person-years of follow-up, adjustments of models and risk ratios for clinical outcomes derived from clinical event data across quantiles of exposure. Any disagreements will be reconciled by consensus with arbitration by the Principal Investigator. Risk of bias: Risk of bias (ROB) will be assessed by two independent reviewers using the Newcastle-Ottawa Scale (NOS). Up to 9 points can be awarded based on cohort selection (max 4 points), the comparability of cohort design and analysis (max 4 points), and adequacy of the outcome measures (max 3 points). Studies that receive ≥6 points will be considered as higher quality. Again, any disagreements will be reconciled by consensus with arbitration by the Principal Investigator. Outcomes: We will assess 6 outcomes: CVD incidence and mortality, CHD incidence and mortality, and stroke incidence and mortality. Data Synthesis: We will pool the natural log-transformed relative risks for the 6 cardiovascular outcomes using the inverse variance method with random effects models with data expressed as risk ratios (RR) with 95% confidence intervals (CIs). Analyses will be conducted by extreme quantiles analyses, comparing the highest with the lowest (reference) level of exposure. Data will also be scaled to allow for per serving/day analyses. Between-study heterogeneity will be assessed by the Cochran Q test and quantified by the I2 statistic with significance at P\<0.1. An I2≥50% and P\<0.1 will be considered as evidence of substantial heterogeneity. Sensitivity analyses and a priori subgroup analyses will be undertaken to explore sources of heterogeneity. Sensitivity analyses will include the systematic removal of included cohorts with the recalculation of summary estimates. A priori subgroup analyses will be conducted when ≥10 cohort comparisons are available by age, sex, location of cohort, follow-up duration, dietary assessment tool, risk of bias (NOS score). A separate subgroup analysis will also be conducted by each domain of risk of bias by NOS. Meta-regression analyses will assess the significance of categorical and continuous subgroups analyses. Dose-response analyses will be undertaken using random-effects generalized least squares trend estimation models (GLST) to assess linear relationships and spline curve modeling (the MKSPLINE procedure) to assess non-linear relationships. If ≥10 cohort comparisons are available, then publication bias will be assessed by visual inspection of funnel plots and formal testing with the Egger and Begg tests at a significance level of p≤ 0.10. If publication bias is suspected, then the investigators will attempt to adjust for funnel plot asymmetry by imputing the missing study data using the Duval and Tweedie trim and fill method. Grading the Evidence: The Grading of Recommendations Assessment, Development and Evaluation (GRADE) method will be used to assess the overall certainty of the evidence for each of the cardiovascular outcomes. This system grades the evidence as "high quality", "moderate quality", "low quality", or "very low quality". Observational studies including prospective cohort studies start at a "low quality" and then can be downgraded or upgraded based on established criteria. Criteria to downgrade will include study limitations (weight of studies show risk of bias by NOS), inconsistency (substantial unexplained inter-study heterogeneity, I2\>50%, P\<0.10), indirectness (presence of factors relating to the population, exposures, and outcomes that limit generalizability), imprecision (95% CI are wide or cross a minimally important difference of 5% \[RR 0.95- 1.05\]), and publication bias (significant evidence of publication bias or small-study effects). Criteria to upgrade will include a large magnitude effect (RR\>2 or RR\<0.5 in the absence of plausible confounders), a dose-response gradient, and attenuation by plausible confounding effects. Knowledge translation plan: The results will be disseminated through interactive presentations at local, national, and international scientific meetings and publication in high impact factor journals. Target audiences will include the public health and scientific communities with interest in nutrition, physical activity, lifestyle modification, obesity, diabetes, and cardiovascular disease. Feedback will be incorporated and used to improve the public health message and key areas for future research will be defined. Significance: The study will aid in establishing a stronger evidence-base to determine the extent to which a high fruit and vegetable intake may be associated with reduction in incident CVD. It may also bring to light categories of fruits and/or vegetables that could provide additional benefit in CVD prevention. References: World Health Organization. (2003). Fruit and Vegetable Promotion Initiative - report of the meeting. Geneva. Reddy KS and Katan MB. (2004). Diet, nutrition and the prevention of hypertension and cardiovascular disease. Public Health Nutrition. 7(1A): 167-186 Katamay SW, Esslinger KA, Vigneault M, et al. (2007) Nutrition Reviews. 65(4): 155-166. 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