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Simple Summary: Cancer cachexia is a complex metabolic condition that is often overlooked and recognized in a late irreversible phase. There is a continuing effort to discover and define a biochemical biomarker of the condition. In this study, we have chosen interleukin-13 (IL-13) and transforming growth factor beta (TGF-β), i.e., two cytokines with presumed roles in the development of cancer cachexia, and we measured their concentrations in the serum and saliva of cachectic patients with metastatic solid tumors, non-cachectic patients with metastatic solid tumors, and healthy individuals. We have demonstrated the role of saliva cytokine measurement as a potential sample for cachexia investigations as opposed to the standard approach to biomarker research, which is serum measurement. We have also found that the salivary IL-13 and TGF-ß are independent risk factors and thus could serve as potential biomarkers of the condition, a fact that warrants further research and confirmation. Cancer cachexia is a syndrome characterized by weight and muscle loss and functional impairment, strongly influencing survival in cancer patients. In this study, we aimed to establish the role of saliva cytokine measurement in cancer cachexia investigation and define two potential independent salivary biomarkers of the condition. Methods: serum and saliva specimens were obtained from 78 patients. Forty-six patients were non-cachectic, and 32 patients were cachectic (per SCRINIO group criteria), all with metastatic solid tumors. Commercial ELISA kits were used to determine the salivary and serum concentrations of interleukin 13 (IL-13) and transforming growth factor beta (TGF-β) in two patient groups and healthy controls. Laboratory values were obtained from the hospital information system, and weight and height were measured at the time of sampling. Results: A statistically significant difference was observed between the groups in saliva IL-13 concentrations but no difference in serum concentrations. Statistically significant differences were also observed between the groups in saliva and serum concentrations of TGF-β. Logistic regression analysis has identified salivary IL-13 and TGF-β as independent factors for cancer cachexia. Conclusions: We demonstrated saliva as a valuable specimen for cachexia investigation and established IL-13 and TGF-β as potential cancer cachexia biomarkers. Further research is needed to evaluate these findings. [ABSTRACT FROM AUTHOR]