The association between body mass index and the risk of different gastrointestinal cancers: A protocol for an overview of systematic reviews.

BACKGROUND: Changes in our lifestyle over the past few decades have led to a significant increase in the worldwide prevalence of both overweight (defined as a body mass index [BMI]≥25 kg/m) and obesity (BMI≥30 kg/m), thus leading to numerous harmful consequences for an individual's health. Several meta-analyses support the link between obesity and different gastrointestinal cancers, but substantial heterogeneity exists between studies. We will synthesize published systematic reviews to evaluate the association between body mass index (BMI) and the incidence and mortality of different gastrointestinal cancers.

METHODS: PubMed, EMBASE, and the Cochrane Library will be systematically searched for systematic reviews, meta-analyses, and pooled analyses in July, 2018. Two review authors will independently screen titles and abstracts for relevance, assess full texts for inclusion, carry out data extraction, and appraise methodological quality using AMSTAR checklist and reporting quality using PRISMA statement. The association between BMI and different gastrointestinal cancers will be estimated by computing the pooled relative risk (RR) and its 95% confidence interval (CI), which will be calculated from the adjusted RR, odds ratio, or hazard ratio, and 95% CI offered in the studies. Heterogeneity between studies will be assessed with the I statistic as a measure of the proportion of total variation in estimates that is due to heterogeneity, where I values of 25%, 50%, and 75% correspond to cut-off points for low, moderate, and high degrees of heterogeneity. The random effects model will be used as the pooling method when significant heterogeneity existed and the fixed effect model will be used when no heterogeneity was observed. Possible publication bias will be tested by Begg and Egger test.

DISCUSSION: This overview of systematic reviews will provide an accessible, comprehensive synthesis with which to inform clinicians and the development of guidelines for the management of the individuals with high BMI.

ETHICS AND DISSEMINATION: Only published secondary data will be used in this study, and therefore ethics approval is not required.PROSPERO registration number: CRD42018107334.