Objective: This study aims to explore the difference of pre- and postoperative energy expenditure in patients undergoing major elective liver resection and relate this to their preoperative aerobic capacity. Study design: The study will be a prospective observational study with thorough pre- and postoperative measurements of energy expenditure. Energy expenditure will be measured using the doubly labelled water method, as well as by indirect calorimetry. To assess aerobic capacity, cardiopulmonary exercise testing will be performed pre- and postoperatively. Additionally, accelerometers will be used to evaluate pre- and postoperative physical activity levels. Study population: Patients aged ≥18 years undergoing major liver resection (≥3 segments) will be asked to participate. The inability to perform cardiopulmonary exercise testing, neo-adjuvant chemotherapy, and cirrhotic liver are reasons for exclusion. Main study parameters/endpoints: The main study parameter is the difference of energy expenditure pre- and postoperatively, as measured with doubly labelled water and indirect calorimetry. Secondary endpoints: Additionally, as secondary endpoints, aerobic fitness, physical activity level, and postoperative complications will be assessed.