This study aimed to investigate the impact of glucose metabolism in subcutaneous adipose tissue (SAT) and skeletal muscle (SM) variables on overall survival (OS) in patients with locally advanced gastric cancer.A retrospective study was conducted on 110 patients with advanced gastric cancer who underwent baseline [18F]FDG PET/CT. Demographic, clinical, and survival data were collected. Mean standardized uptake value (SUVmean) of SAT as well as skeletal muscle, and body composition measurement, including SAT area, SAT radiodensity, SM area, and SM radiodensity, were assessed on PET/CT. SM area was normalized for patient stature, resulting in the skeletal muscle index (SMI). Patients were stratified into subgroups with high and low SAT uptake based on the optimum cut-off value of the SAT SUVmean. The univariate, multivariate regression analysis, Kaplan-Meier survival analysis, and Spearman's correlation analysis were employed to evaluate the associations among metabolic activity and CT-derived body composition variables as well as OS.Out of 110 patients with an average age of 62.65 ± 13.25 years, 58 (52.73%) patients died during follow-up. Cox regression analysis identified TNM stage, SAT SUVmean, and SMI as independent prognostic factors for OS (all p < 0.05). Notably, patients with elevated SAT uptake exhibited significantly poorer long-term survival compared to those with low SAT uptake (p < 0.001). Correlation analyses revealed a moderate positive association between SAT SUVmean and SAT radiodensity (p < 0.001, r = 0.47), whereas a significant inverse correlation was observed between SAT SUVmean and SAT area (p < 0.001, r = -0.465). Additionally, stratification by combined SAT SUVmean and SMI profiles showed that patients with low SAT uptake and high SMI exhibited a better prognosis than those with high SAT uptake and/or low SMI (p = 0.004 and p < 0.001).Increased [18F]FDG uptake in SAT was correlated with higher SAT radiodensity as well as lower SAT area, and shortened survival in patients with advanced gastric cancer, underscoring its potential as a biomarker for adverse outcomes.© 2025. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.