Abstract / Summary
Background/Objectives: Gastric cancer is a major global health challenge with multifactorial aetiology. Vitamin D has been investigated for a potential role in gastric carcinogenesis, but evidence remains heterogeneous. This systematic review aimed to synthesise evidence on serum 25(OH)D, dietary vitamin D intake and VDR polymorphisms in relation to gastric cancer in adults and children. Methods: A systematic search following PRISMA 2020 standards was conducted in PubMed, Web of Science, Scopus and Lens.org for studies published from January 2007 to January 2026. Observational studies evaluating vitamin D exposure or VDR polymorphisms in relation to gastric cancer were eligible. Risk of bias was assessed using established instruments. Where comparable data were available, random-effects meta-analyses were performed, with REML estimation and Knapp-Hartung adjustment. Results: Twenty-four studies met the inclusion criteria, and no eligible paediatric studies were identified. Four studies evaluating serum 25(OH)D showed a non-significant tendency towards lower concentrations in gastric cancer patients (MD = -8.82 ng/mL, 95% CI -18.15 to 0.51). Five studies evaluating dietary vitamin D showed no significant association with gastric cancer risk (OR = 0.87, 95% CI 0.47-1.60). Five studies evaluating the VDR FokI polymorphism under the dominant model demonstrated increased gastric cancer susceptibility among variant carriers (OR = 1.44, 95% CI 1.03-2.01). Conclusions: Associations between vitamin D and gastric cancer varied by exposure type. Serum 25(OH)D and dietary intake showed no significant pooled associations, whereas VDR FokI was associated with increased gastric cancer susceptibility. The absence of paediatric evidence highlights an important knowledge gap.