Background Loss of body tissue resulting in undernutrition can be caused by reduced food intake, altered metabolism, ageing, and physical inactivity. The predominant cause of undernutrition before cardiac operations is unknown. First, we explored the association of reduced food intake and inactivity with undernutrition in patients before elective cardiac operations. Second, we assessed if adding these reversible, cause-based items to the nutritional screening process improved diagnostic accuracy. Methods A prospective observational study was performed. Undernutrition was defined by low fat-free mass index (LFFMI) measured by bioelectrical impedance spectroscopy and/or unintended weight loss (UWL). Reduced food intake was defined as the patient having a decreased appetite over the previous month. Patients admitted to hospital preoperatively were assumed to be less physically active than patients awaiting cardiac operations at home. Using these data, we developed a new tool and compared this with an existing cardiac surgery–specific tool (Cardiac Surgery–Specific Malnutrition Universal Screening Tool CSSM). Results A total of 325 patients who underwent open cardiac operations were included. Reduced food intake and inactivity were associated with undernutrition (odds ratio OR, 4.2; 95% confidence interval CI, 2.1–8.5 and OR, 2.0; 95% CI, 1.0–4.0). Reduced food intake and inactivity were integrated with body mass index (BMI) and UWL into a new scoring system: the Cardiac Surgery–Specific Undernutrition Screening Tool (CSSUST). Sensitivity in identification of undernourished patients was considerably higher with the CSSUST (90%) than with the CSSM (71%) (receiver operating characteristic ROC curve–based area under the curve AUC, 0.79; 95% CI, 0.73–0.86 and ROC AUC, 0.71; 95% CI, 0.63–0.80). Conclusions Results suggest that reduced food intake and inactivity partly explain undernutrition before cardiac operations. Our new cause-based CSSUST, which includes reduced food intake and inactivity, is superior to existing tools in identifying undernutrition in patients undergoing cardiac operations.