Scaling of dimensions of small intestines in non-ruminant eutherian mammals and its significance for absorptive mechanisms.

The mucosal surface area of small intestines in non-ruminant eutherian mammals increases approximately in proportion to the 0.6 power of body mass, whereas resting metabolic rate (RMR) increases approximately with the 0.74 power of body mass; the mass exponent for field metabolic rates (FMR) may exceed 0.8. These relationships imply that the average rate of absorption of metabolic substrates, expressed per unit area of mucosal surface, is greater in large animals than in small. In the present paper I collate data from the literature relating mucosal surface area, fluid absorption and glucose transport rates to body size. Glucose-stimulated fluid absorption per unit area of mucosal surface increases with body size, whereas transcellular, carrier-mediated glucose transport per unit area decreases with body size. In perfused jejunal segments of normal human subjects the rates of fluid absorption per unit area of mucosa are five to ten times greater than in laboratory rats. The absorbed fluid contains glucose in amounts that may greatly exceed the maximum transport capacity of the apical glucose transporter. It follows that the paracellular component of glucose absorption increases with body size. Scaling of intestinal dimensions and transport therefore provides new information about the relative contributions of transcellular and paracellular pathways to absorption of nutrients.