The effect of chemotherapy on nutritional status and weakness in geriatric gastrointestinal system cancer patients.

•This study aimed to evaluate patients with geriatric gastrointestinal system (GIS) cancer in terms of nutritional status and weakness and determine the changes caused by chemotherapy (CT).•A total of 153 patients with GIS cancer who were aged ≥65 y (mean age, 70.5 ± 5.6 y; 44 female and 109 male) were evaluated.•Based on Mini Nutritional Assessment scores, 37.9% of patients were malnourished and 34.6% were at risk of malnutrition during the first visit. After CT, the same evaluation showed that 46.4% of patients were malnourished and 29.4% were at risk of malnutrition. Even a single course of CT significantly increased the rate of malnutrition in patients with geriatric GIS cancer (P = 0.001).•Patient groups with the highest rates of weakness were those diagnosed with biliary tract, hepatic, and colorectal cancer (33.3%, 27.3%, and 20%, respectively). Weakness was significantly increased after one course of CT in patients who previously received CT (P = 0.039).•Data on the frequency of malnutrition and frequency of weakness and the effect of CT on these parameters were obtained from patients with geriatric GIS cancer, which is a unique group. Based on the data, nutritional therapy in this group of patients should be seen as an integral part of oncologic therapy, and patients should be followed closely for nutritional status because malnutrition is common in patients with GIS cancer. Even a single course of CT worsens patients' nutritional status.