Differences between animals and humans in the gastrointestinal absorption of barium

When a particular animal model can be demonstrated to differ substantially in certain aspects from human function or physiology, such differences must be accounted for when extrapolating the animal data to potential effects in humans. Significant differences exist between newborn rats and infant humans in the mechanism by which a variety of compounds are absorbed from the intestine. Consequently, newborn rats are not an appropriate experimental model for intestinal absorption of many substances, including barium, in infant humans. This is because the intestinal epithelium of newborn rats is very immature to permit the absorption of macromolecules such as gamma‐globulins from maternal milk in order to acquire passive immunity. A number of studies demonstrate that the intestines of newborn rats are extremely permeable to a wide variety of other chemicals, and that this unselective permeability has no direct counterpart in human infants. By contrast, the intestinal epithelium of infant humans is mature at bi...