Objective: Cachexia and weight loss are frequently seen in cancer patients. We investigated lipid metabolism to elucidate a metabolic basis for adequate nutrition of cancer patients. Design: Lipid metabolism was assessed by indirect calorimetry and triglyceride clearance rates after randomised injection of a lipid bolus (long-chain triglycerides (LCT) or medium-chain triglycerides (MCT) during an euglycemic clamp protocol in cancer patients. Setting: Rudolf-Virchow Krankenhaus, Berlin, Germany. Subjects: Eighteen patients were included. Twelve patients had upper gastrointestinal cancer: a weight stable cancer group (Caws, n = 6) with a body mass index (BMI) of 22.9 +/- 1.7 kg/m(2) and a weight losing cancer group (Cawl n = 6) with a mean weight loss of 7.4+/-3.1 kg or 11% of the initial body weight during the previous three months (present BMI 21.8+/-0.8 kg/m(2)). The data were compared with six control patients with benign gastrointestinal diseases (BMI = 25.0 +/- 0.8 kg/m(2). Main outcome: Cancer patients had an increased basal lipid oxidation rate that was more pronounced in Cawl (+ 92 vs + 42% in Caws; P < 0.01 and 0.05 Is controls, respectively). Utilisation of LCT was increased in cancer patients, this was most pronounced in Cawl ( + 150 +/- s + 65% in Caws; P < 0.01 and 0.05, respectively]. Metabolically, then Here no differences in the utilisation of LCT and MCT/LCT containing lipid emulsions. Conclusions: Cancer patients have an increased lipid oxidation and an enhanced utilisation of exogeneous lipids. This is most pronounced in Cawl. To prevent further weight loss or to increase body n weight, they should increase their fat intake. In contrast, fat-reduced or prudent diets seem to be inadequate for the nutrition of cancer patients.