High and low dietary fiber consumption and cancer risk: a comprehensive umbrella review with meta-meta-analysis involving meta-analyses of observational epidemiological studies

It is a well-known fact that dietary fiber is recognized as one of the essential components of a healthy diet. The aim of this paper was to investigate the impact of dietary fiber on the incidence and mortality of various types of cancer, the current evidence in this field, and the biases of this evidence using the meta-meta-analysis method. We identified meta-analyses that particularly focused on the association between dietary fiber consumption and the risk/mortality of cancer. A structured and comprehensive computer literature search was undertaken in the electronic databases PubMed/Medline, Web of Science (WoS), and Scopus. The search yielded a total of 25 papers and 28 reports. In the pooled analysis, higher dietary fiber consumption was associated with a 22% lower cancer risk (OR = 0.78, 95% CI: 0.74-0.83, p < 0.001) and a 17% lower mortality (RR = 0.83, 95% CI: 0.78-0.90, p < 0.001). In the secondary meta-meta-analysis, it was observed that there was an inverse association between dietary fiber intake and digestive tract cancers (OR = 0.68, 95% CI: 0.62-0.76) and breast cancer (OR = 0.92, 95% CI: 0.90-0.94). Taken together, this paper suggests that promoting a high-fiber diet may be an effective strategy for the prevention and management of cancer.