BACKGROUND: The prognostic significance of body composition in metastatic colorectal cancer (mCRC) patients receiving systemic chemotherapy is increasingly recognized. This study aimed to explore the relationship between various body composition metrics measured using bioelectrical impedance analysis (BIA) and patient outcomes in patients with mCRC. METHODS: In a retrospective cohort of 164 mCRC patients, body composition was assessed using BIA before first-line chemotherapy. Metrics evaluated included the extracellular water/total body water ratio (ECW/TBW), skeletal muscle mass, body fat mass, protein, and mineral levels. This study examined the relationship between these parameters and survival outcomes and their impact on chemotherapy-induced toxicity and relative dose intensity. RESULTS: Patients with a higher ECW/TBW ratio had significantly lower overall survival (OS) (p <
0.001) across quartiles (median OS: Q1 = 38.6 months, Q4 = 19.1 months) and a significantly lower progression-free survival (median progression-free survival: Q1 = 10.5 months, Q4 = 8.3 months
p = 0.03). Multivariate analysis identified ECW/TBW as an independent predictor of OS (hazard ratio: 2.12
95 % confidence interval: 1.36-3.23
p <
0.001). Subgroup analysis indicated a significant interaction between ECW/TBW quartiles and the effectiveness of anti-endothelial growth factor receptor and anti-vascular endothelial growth factor therapies on overall survival (p for interaction <
0.001). Body composition was significantly associated with chemotherapy relative dose intensity and hematologic adverse events, particularly thrombocytopenia, which was significantly correlated with ECW/TBW, skeletal muscle mass, lean body mass, and protein levels. CONCLUSION: This study highlights the value of comprehensive body composition assessment using BIA in predicting outcomes for patients with mCRC, supporting its incorporation into clinical practice for enhanced patient care.