Background/aim: The purpose of this study was to determine sarcopenia, sarcopenic obesity and phase angle (PA) and the influence of chemotherapy (CT) on anthropometric measurements and and the PA in in geriatric patients with gastrointestinal (GI) cancer. Material and methods: The anthropometric measurements, calf circumference (CC), upper midarm circumference (UMAC), and hand grip strength (HGS), have been measured to understand muscle function of 153 patients (mean age of 70.5 ± 5.6 years, 28.8% female, 71.2% male). Sarcopenia and PA measurements have been evaluated by bioelectrical impedance analyses. The same evaluations were checked again after 1 cycle of CT (min: 4, max: 6 weeks). Results: Patient population consisted of colorectal (51,6%), gastric (26.8%), pancreas (11.8%), liver (7.2%), and biliary tract cancer (2%). UMAC (28.5 ± 4.4 before, 28.1 ± 4.9, P = 0.034 after CT), and HGS measurements (27.5 ± 8.6 before, 26.8 ± 8.8 after CT, P = 0.007) have significantly decreased after CT. CC measurement < 31 cm at first visit was seen in 13.1% of patients, but the ratio raised to 20.3% after CT (χ², P = 0.003). Severe sarcopenia was determined in 33% of all patients, and 30.0% of them have been considered as sarcopenic obese. Conclusion: Sarcopenia and sarcopenic obesity were prevalent in this group patients. The CT caused a decrease in muscle functions, UMAC, and CC. Patients should be followed up carefully for sarcopenia, sarcopenic obesity, and nutritional aspect and it would be proper to intervene before sarcopenia has not occurred yet.