Body mass index and cancer risk: An umbrella review of meta-analyses of observational studies

Background: Increasing evidence indicates that obesity is a risk factor for various tumors. We aimed to clarify the evidence for association between body mass index (BMI) and cancer risk based on existing systematic review and meta-analyses.Methods: PubMed, Embase and Web of science were systematically searched to obtain systematic review and meta-analyses reporting association between BMI and cancer incidence. The methodological quality and strength of evidence of each meta-analysis were assessed by AMSTAR and GRADE respectively. We also assessed the heterogeneity and publication bias of all included meta-analyses.Results: Finally, 43 meta-analyses with 19 cancers were identified by this umbrella review. The result revealed that underweight was inversely associated with the incidence of brain tumors while was positively related to the risk of esophageal and lung cancer. Overweight would enhance the incidence of brain tumors, kidney cancer, endometrial cancer, ovarian cancer, multiple myeloma, bladder cancer and liver cancer. Obesity was related to the increased incidence of brain tumors, cervical cancer, kidney cancer, endometrial cancer, esophageal cancer, gastric cancer, ovarian cancer, multiple myeloma, gallbladder cancer, bladder cancer, colorectal cancer, liver cancer, thyroid cancer and Hodgkin’s lymphoma. What’s more, dose-response analysis was conducted by 10 studies and the results demonstrated that per 5Kg/m2 increment of BMI was associated with 1.01 to 1.13 fold increased risk of general brain tumors, multiple myeloma, bladder cancer, pancreatic cancer, breast cancer, Non-Hodgkin’s lymphoma. And every 1Kg/m2 increment of BMI was linked to 6% and 4% increment in the risk of kidney cancer and gallbladder cancer respectively.Conclusions: The evidence presented in this umbrella review showed that overweight and obesity were associated with increased risk of most cancers. We recommend that it is better to maintain BMI within the normal range to prevent the occurrence of tumors. However, prospective studies with high quality are needed to further investigate the association between BMI and various cancer risk.