Patients with a short bowel malabsorb dietary nutrients with loss of calories and weight. Malabsorbed carbohydrates are fermented by colonic bacteria to short-chain fatty acids, which are absorbed and supply energy. The maximum energy-consumption capacities in patients with short bowel were individually measured on 40:40% carbohydrate:fat diets. 8 patients with colon in continuity and 6 patients with jejunostomies were placed on isocaloric 60:20% or 20:60% carbohydrate:fat diets and faecal excretions of calories, carbohydrates, fat, nitrogen, and fluids were compared. The high-carbohydrate low-fat diet reduced faecal loss of energy by 2.0 MJ/day compared to the low-carbohydrate high-fat diet in patients with colon in continuity, and absorption of energy increased from 49 to 69% (p < 0.001). Faecal excretions of carbohydrates were low and not influenced by the change in carbohydrate intakes (26 g/day and 28 g/day, respectively) whereas faecal fat (46 g/day and 106 g/day) was highly dependent on dietary intakes and accounted for differences in faecal loss of energy. In contrast, patients with jejunostomies excreted equal amounts of calories on the high-carbohydrate diet (4.8 MJ/day) and the high-fat diet (5.9 MJ/day; p = 0.08); and the percentage of calories absorbed was not different (55% and 48%, respectively; p = 0.21). Furthermore, in patients without colon the excretions of carbohydrates (80 g/day and 42 g/day on high-carbohydrate and low-carbohydrate diets, respectively) and fat (69 g/day and 35 g/day on high-fat and low-fat diets, respectively) were proportional to the amounts ingested. The large intestine is important in the digestion of carbohydrates and hence in the salvage of calories in patients with short bowel and severe malabsorption.