Association Between Diabetes Mellitus and Head and Neck Cancer: An Umbrella Review of Systematic Reviews and Meta-Analyses

Background/Objectives: Emerging evidence links diabetes to increased cancer risk. This study aimed to assess the association between diabetes mellitus (DM)(type 1, type 2, or gestational) and the development of head and neck cancer. Methods: An umbrella review was conducted using systematic searches in Cochrane, EBSCO, Wiley, ScienceDirect, and PubMed (January 2000–January 2024), registered in PROSPERO (CRD42024512151). Included were systematic reviews (SRs) and meta-analyses (MAs) of observational studies. Article selection followed the PRISMA guidelines; the quality and risk of bias of the selected studies were assessed with the Joanna Briggs Institute Checklist. The GROOVE tool was used to identify double counting. Two independent reviewers screened studies, with a third resolving disagreements. Results: Seven SRs were included. While DM has been widely examined in cancer research, few studies specifically targeted head and neck cancers. Of the 20 associations between various cancer sites and diabetes types, 9 (45%) showed a statistically significant positive correlation. The strongest evidence was for overall cancer risk (RR = 1.22, 95% CI: 1.16–1.29, p < 0.001). Oral cancer showed elevated risks (RRR = 1.13, p = 0.009; OR = 1.32, p < 0.001; HR = 1.73, p < 0.05; RR = 1.28, p < 0.05). Increased risks were also observed for oropharyngeal (RR = 1.18; HR = 1.53), head and neck (HR = 1.47), and nasopharyngeal cancer (OR = 1.40), all p < 0.05. Heterogeneity was low in two reviews, unreported in one, and high in four. Five SRs reported associated risk factors. Conclusions: While some associations between DM and cancer appear significant, evidence remains limited and inconsistent, particularly for oral cancer. Further standardized, high-quality research is needed to clarify the link across head and neck cancer subtypes.