Abstract Background Nutrition plays a major role in intensive care unit (ICU) treatment, influencing ICU length of stay and patient’s survival. If preferable enteral nutrition administration is not feasible, ESPEN and ASPEN guidelines recommend initiation of a supplemental parenteral route between the first and seventh day, but exact timing remains elusive. While rapid development in critical care enabled significant reduction in the mortality rate of ICU patients, this improvement also tripled the number of patients going to rehabilitation. Thus, it is quality of life after ICU that has become the subject of interest of clinicians and healthcare policy-makers. A growing body of evidence indicates that protein turnover in the early phase of critical illness may play a crucial role in the preservation of lean body mass. A negative protein balance may lead to muscle wasting that persists weeks and months after ICU stay, resulting in deterioration of physical functioning. Folliwing oncological gastrointestinal tract surgery, patients are threatened with negative protein turnover due to cancer and extensive surgical insult. Methods This is a multi-centre, single-blinded, randomised controlled trial. The study population includes patients admitted to ICU units after major oncological gastrointestinal surgery that require supplemental parenteral nutrition. After initiation of enteral nutrition, the intervention group receives remaining daily requirement via supplemental parenteral nutrition on the first day of ICU stay while the control group is not supplemented parenterally until the seventh day of ICU stay while enteral nutrition is gradually increased. Primary endpoint: long-term quality of life measured in the physical component score (PCS) of SF-36 questionnaire at 3 and 6 months after ICU admission. Discussion To our knowledge, this is the first trial to investigate the influence of early supplemental parenteral nutrition on long-term quality of life after major oncological gastrointestinal surgery. We assume that, particularly in this population of patients, early supplemental parenteral nutrition may increase the long-term quality of life. The study construction also allows establishment of patients’ PCS SF-36 score prior to surgery and mean change in PCS SF-36 score during the recovery period, which is rarely seen in studies on critically ill patients. Trial registration ClinicalTrials.gov: NCT03699371 registered on 12 October 2018.