Reserve capacities of the small intestine for absorption of energy.

Previous in vitro studies showed that the small intestine has reserve capacities for absorption of nutrients. However, the size of the reserve capacity is controversial. Therefore, we measured the intestinal capacity for absorption of energy in relation to the postprandial gastric delivery of energy into the gut. In minipigs, a 150-cm length of jejunum was perfused (1-8 kcal/min) with four nutrient solutions containing 60% of energy as carbohydrate, protein, and fat, respectively, or containing 33.3% of each nutrient. In separate experiments, gastric delivery of energy to the jejunum was measured after oral administration of four meals with the same nutrient composition as the perfusion solutions. With all nutrient solutions, intestinal absorption of energy demonstrated saturation kinetics. The jejunal capacity for absorption of energy ranged from 0.66 to 0.94 kcal ⋅ m-1 ⋅ min-1. Despite large differences in nutrient composition of the four meals, equal amounts of energy (1.3 ± 0.41 kcal/min) were delivered from the stomach to the jejunum. The absorption rates of energy after meals ranged from 0.40 to 0.58 kcal ⋅ m-1 ⋅ min-1. Therefore, only 58.8 ± 2.7% of the jejunal capacity for absorption of energy was used. Additionally, the length of small intestine that would have been required for complete absorption was 42.9 ± 3.7% of the total length. Results indicate that the feedback control of gastric emptying provides at least two types of intestinal reserve capacities: a reserve in absorption (1.7 fold) and a reserve in intestinal length (2.4 fold).