Objective To analyze the impact of sarcopenia on the efficacy and adverse reactions of immunotherapy combined with chemotherapy in patients with advanced gastric cancer.Methods Patients with locally advanced or metastatic gastric cancer confirmed by pathology who were not eligible for radical surgery were selected as study subjects.A body composition analyzer was used to measure the appendicular muscle mass of the patients and calculate the skeletal muscle mass index(SMI).Based on the SMI,the patients were divided into sarcopenia group and non-sarcopenia group.On the basis of nutritional intervention and comprehensive exercise therapy,the patients were administered immu-notherapy combined with chemotherapy.The efficacy and adverse reactions were evaluated.The primary endpoint was progression-free survival(PFS),and the secondary endpoints were the objec-tive response rate(ORR)and treatment-related adverse reactions.Results A total of 52 gastric cancer patients were included,with 23 in the sarcopenia group and 29 in the non-sarcopenia group.The median PFS in the non-sarcopenia group was 9.8 months(95％CI,8.9 to 12.4),and was 5.4 months in the sarcopenia group(95％CI,4.9 to 8.1).The median PFS in the non-sarcopenia group was longer than that in the sarcopenia group,and the difference was statistically significant[HR(95％CI)=0.41(0.23 to 0.73),P=0.003].The results of the multivariate Cox propor-tional hazards regression model showed that comorbidities,treatment cycles,and sarcopenia were all independent prognostic factors affecting the PFS of gastric cancer patients(P＜0.05).The ORR in the non-sarcopenia group was 48.28％(14/29),and was 17.39％(4/23)in the sarcopenia group(x2=5.276,P＜0.05).Treatment-related adverse reactions with grading ≥3 in both groups were mainly hematological toxicities.In the non-sarcopenia group,the incidence of grading ≥ 3 treat-ment-related adverse reactions was 27.59％(8/29),and the incidence of grading＜3 treatment-re-lated adverse reactions(including those with no adverse reactions)was 72.41％(21/29).In the sarcopenia group,the incidence of grading ≥3 treatment-related adverse reactions was 56.52％(13/23),and the incidence of grading＜3 treatment-related adverse reactions(including those without adverse reactions)was 43.48％(10/23).The incidence of grading ≥3 treatment-related adverse reactions in the non-sarcopenia group was lower than that in the sarcopenia group(P=0.035).Conclusion For patients with locally advanced or metastatic gastric cancer complicated with sarcopenia,the median PFS of immunotherapy combined with chemotherapy is shorter,the ORR is lower,and the incidence of treatment-related adverse reactions is increased.Therefore,ear-ly intervention for sarcopenia should be implemented to improve the quality of life of patients with advanced gastric cancer.