ABSTRACT Surgical patients with gastrointestinal cancer often suffer from malnutrition. This study aimed to evaluate the influence of fasting time on the nutritional status of patients hospitalized with preoperative and postoperative gastrointestinal tract neoplasms. Observational, longitudinal, and prospective study conducted in the surgical unit at a public-school hospital. The patients were divided into groups: upper (UGIT) and lower (LGIT) gastrointestinal tract. Follow-up started within 72 h of hospitalization with reassessment 72 h after surgery. Data collected: sex, age, type and duration of surgery, preoperative (compared with 8 h) and postoperative (compared with 24 h) fasting time, food acceptance, Subjective Global Assessment, anthropometry, and laboratory tests. Analyses: Student t, Wilcoxon, and chi-square tests. Fifty-one patients were followed up, 29 (57%) UGIT and 22 (43%) LGIT. The preoperative fasting time was 8.2±2.8 h in UGIT and 8.1±2.2 h in LGIT groups, respectively; however, postoperative fasting times in UGIT (60.4±40.7 h) and LGIT groups (57.6±38.2 h) were longer than 24 h (P<0.001). Although eutrophic in the preoperative period, in the postoperative most patients in the UGIT and LGIT groups presented, respectively, malnutrition (71%; 59%; P<0.001), severe weight loss (79%; 80%), a significant correlation between triceps skinfold and postoperative fasting time (r= -0.306; P= 0.03), and hemoglobin and albumin values (r= 0.633; P<0.001), additionally low dietary acceptance, especially in the UGIT group. Prolonging postoperative fasting time worsened the nutritional status of surgical patients, especially in the UGIT group.