Micronutrient Status in Type 1 and Type 2 Diabetes Mellitus and Oral Carcinogenesis: A Scoping Review of Mechanistic Pathways and Evidence Gaps

Type 2 diabetes mellitus (T2DM) is associated with a modestly higher risk of oral cancer (OC) in crude analyses, while micronutrient deficiencies affect approximately 45% of people with T2DM. Several micronutrients linked to oral carcinogenesis—vitamin D, folate, vitamin B12, zinc, selenium, magnesium, and iron—are also among those most depleted in diabetes. Despite this overlap, only four published studies have assessed micronutrient status or intake, glycemic status, and an oral cancer or precancer outcome in the same participants, and none has reported incidence. Conducted and reported in accordance with PRISMA-ScR, this scoping review brings together three related evidence streams (diabetes–OC epidemiology, micronutrient–OC epidemiology, and micronutrient status in diabetes). It also catalogs the direct three-way evidence, maps converging molecular pathways, outlines expected mechanistic differences between type 1 diabetes mellitus (T1DM) and T2DM and proposes study designs to test the interaction. Three findings temper the conventional interpretation of this triad. The association between T2DM and oral cancer is no longer significant after adjusting for smoking, alcohol and adiposity. Areca nut contributes to both diabetes and oral cancer, making it a shared cause rather than a covariate. In addition, three Mendelian randomization analyses find no causal effect of circulating 25-hydroxyvitamin D, HbA1c, insulin-like growth factor 1 or adiposity on oral or oropharyngeal cancer. The triad is therefore mechanistically coherent but epidemiologically weak once confounding is addressed; for the exposures most often proposed, causal designs have already tested and not supported the link. What remains still open is a narrower, more tractable question: micronutrient status in oral cohorts that distinguish diabetes type and record areca nut (betel nut) use at the individual level. The distinction between T1DM and T2DM, routinely collapsed in the existing literature, is the one element of the hypothesis that current causal evidence supports—and the one for which observational data are entirely absent.