Body Weight, Body Composition, Nutrition, and Sex in Gastric Cancer: Epidemiological Associations and Surgical Outcomes—A Narrative Review

Gastric cancer remains a major cause of cancer-related mortality worldwide, while the roles of excess body weight, body composition, nutrition, and biological sex remain incompletely defined. This narrative review separately evaluates (i) epidemiological associations with gastric cancer risk, with explicit attention to cardia versus non-cardia tumors and Helicobacter pylori as a major confounder, and (ii) perioperative and long-term outcomes after gastrectomy, including postoperative complications, treatment completion, recurrence, and survival. BMI, visceral adiposity, skeletal-muscle mass, and sarcopenic obesity are treated as distinct, non-interchangeable phenotypes. Available studies are heterogeneous and partly conflicting. Prediagnostic weight loss is particularly important because cancer-related weight loss and cachexia can lower BMI at diagnosis, creating reverse causality in prognostic analyses. Visceral adiposity and sarcopenic obesity may provide clinically relevant information beyond BMI alone. Sex-related differences in adipose distribution and hormonal exposure may also modify risk and outcome, but observational associations do not establish causality. Overall, BMI alone is an insufficient marker of risk; prospective studies should integrate premorbid weight, longitudinal weight change, body composition, nutritional status, tumor subsite, H. pylori status, and sex-specific analyses.