Background: In recent years, the number of patients with cancer has increased. These patients are prone to sarcopenia as a result of the decrease in muscle mass and muscle weakness that occur in cancer cachexia. Amino Index Cancer Screening is carried out to evaluate cancer cachexia risk by examining amino acid concentration and analyzing amino acid balance. Methods: We conducted a retrospective chart review of consecutive patients with unresectable advanced gastrointestinal cancer (stage IV) receiving chemotherapy treatment (December 2012–September 2015) in an outpatient or in-hospital setting at our institution (N = 46). Data included characteristics, psoas muscle area per computed tomography, and biochemical blood test and serum amino acid profiles. Method 1: Comparison of biomarkers between 2 groups: psoas muscle index change rate (ΔPMI) decrease vs increase. Method 2.1: Correlation between ΔPMI and biomarkers. Method 2.2: Multiple regression of ΔPMI and biomarkers. Results: EAA/TAA ratio (essential amino acids/total amino acids) in the decrease group was significantly higher than that in the increase group. Among all parameters, serum C-reactive protein (CRP), leucine, and isoleucine were negatively related to ΔPMI (correlation coefficients = −0.604, −0.540, −0.518; P = .004, .011, .016, respectively). On multiple regression analysis, serum CRP value was strongly related to ΔPMI (r2 = 0.452, β = −0.672, P = .001). Conclusions: Higher serum EAA/TAA ratio and CRP were associated with depletion in psoas muscle area, which led to a diagnosis of sarcopenia, in patients with advanced gastrointestinal cancers. These parameters at baseline could be predictors of cancer cachexia.