Background & Aims: Early enteral nutrition and tailored supply of nutrients have become standard in most of the intensive care units (ICU). So far little attention has been given to losses of energy in the stools. The purpose of this explorative study was to evaluate the energy losses of patients with loose stools, necessitating the use of a feces-collector device in a tertiary academic ICU. Methods: In a group of 13 fully enterally fed and mechanically ventilated patients with loose stools, the daily energy loss in feces was determined, using bomb calorimetry. Malabsorption was defined as an absorption capacity of 85% or less. Energy expenditure was determined with indirect calorimetry. Results: Six out of 13 (46%) patients fulfilled the criterion of malabsorption. The mean total energetic absorption capacity was 84.6±13.3%. The mean capacity of absorption of fat was 89.7±16.3%. The caloric value of energy loss had a mean of 301±259 kcal/day. Fecal fat loss proved not to be a good indicator of total fecal energy loss. A total of 4/13 patients (31%) had a net negative energy balance of over 500 kcal/day. A daily feces production of 250 g or more was a good predictor of malabsorption. Energy loss could accurately be predicted by using a factor 4.87 for the combined energetic value of protein and carbohydrates, if dry weight and fecal fat content are known. Conclusions: In this clinical study on ICU patients with loose stools, malabsorption proved to be a frequently occurring and so far unrecognized problem, contributing strongly to negative energy balances in 1/3 of the patients.