10074 Background: Cancer cachexia is a complex syndrome characterized by significant weight loss and depletion of skeletal muscle mass. Despite its profound impact on the well-being of cancer patients, the effective management of cancer cachexia remains a challenging and underexplored aspect of oncology. This study aims to investigate the potential benefits of oral nutritional supplements (ONS) to cancer patients undergoing chemotherapy. Methods: This is single-arm, prospective pilot study. Patients with cancer cachexia undergoing chemotherapy were included. Patients received ONS twice a day for 8 weeks. The ONS contained 200 kcal, 22.5 gram of carbohydrates, 7.5 gram of fat, and 12.5 gram of protein per 200 mL. Primary endpoints were improvements of lean body mass and physical performance. The secondary endpoints were changes of health-care related quality-of-life (HRQoL), nutritional status, and gut microbiomes profiles. EORTC-QLQ-C30 questionnaire, body composition analysis, 4-meter-walk test, hand grip strength test, and fecal microbiome analysis were conducted at baseline and after 8 weeks. The bacterial 16S rRNA gene (V3 and V4 region) was amplified using PCR and sequenced on the Illumina MiSeq platform. The QIIME 2 pipelines were used to analyze the raw data. Results: From January 2023 to October 2023, total of 10 patients were included. There were no significant differences of the mean daily energy intake (p=0.62) and calf-circumferences (p=0.2) between baseline and after 8 weeks. However, participants exhibited a statistically significant improvement in hand-grip strength (p=0.002) and 4-meter walk test (p=0.021) after 8 weeks. In the body composition analysis using dual-energy X-ray absorptiometry, no differences were observed in body weight (p=0.43), fat mass (p=0.62), fat-free mass (p=0.27), and fat-free mass index (p=0.38). Also, no significant differences were detected in the results of HRQoL survey (p=0.85). In the microbiome analysis, no differences of microbiome composition at phylum and genus levels, α-diversity, and β-diversity were observed between baseline and after 8 weeks. Conclusions: While ONS have shown potential in enhancing physical functions of patients with cancer cachexia undergoing chemotherapy, this did not correlate with the anticipated microbiome changes or improvements in body composition. The absence of expected microbiome alterations, despite enriched protein and dietary fiber, underscores the complexity of cachexia and indicates that its management may extend beyond nutritional supplementation alone. Further research should explore the broader context of cachexia, including nutrition, metabolic processes, and the impact of chemotherapy.