Are systematic reviews addressing nutrition for cancer prevention trustworthy? A systematic survey of quality and risk of bias

Context The last 30 years have yielded a vast number of systematic reviews and/or meta-analyses addressing the link between nutrition and cancer risk. Objective The aim of this survey was to assess overall quality and potential for risk of bias in systematic reviews and meta-analyses (SRMAs) that examined the role of nutrition in cancer prevention. Data Sources MEDLINE, Embase, and the Cochrane Library databases were searched (last search performed November 2018). Study Selection Studies identified as SRMAs that investigated a nutritional or dietary intervention or exposure for cancer prevention in the general population or in people at risk of cancer and in which primary studies had a comparison group were eligible for inclusion. Screening, data extraction, and quality assessment were conducted independently by 2 reviewers. Data Extraction Altogether, 101 studies were randomly selected for analysis. The methodological quality and risk of bias were evaluated using the AMSTAR-2 and ROBIS tools, respectively. Results Most SRMAs included observational studies. Less than 10% of SRMAs reported a study protocol, and only 51% of SRMAs assessed the risk of bias in primary studies. Most studies conducted subgroup analyses, but only a few reported tests of interaction or specified subgroups of interest a priori. Overall, according to AMSTAR-2, only 1% of SRMAs were of high quality, while 97% were of critically low quality. Only 3% had a low risk of bias, according to ROBIS. Conclusions This systematic survey revealed substantial limitations with respect to quality and risk of bias of SRMAs. SRMAs examining nutrition and cancer prevention cannot be considered trustworthy, and results should be interpreted with caution. Peer reviewers as well as users of SRMAs should be advised to use the AMSTAR-2 and/or ROBIS instruments to help to determine the overall quality and risk of bias of SRMAs. Systematic Review Registration PROSPERO registration number CRD42019121116.