General considerations regarding intestinal absorption of radionuclides as a function of age

Although enhanced absorption of radionuclides from the GI tract has been confirmed in recent years in many animal species, the species-dependent variations in GI absorption by neonates make it difficult to extrapolate animal data to humans. Numerous measurements have been made in 1 to 2 day-old animals, but fewer studies have covered the entire neonatal period up to weaning. For plutonium, sufficiently good inter-species agreement was obtained to assume that its absorption by the GI tract remains significantly high until weaning. The same was reported for neptunium absorption by hamsters but a preliminary experiment with baboons seems to refuse this, and the problem of extrapolation to humans from species more closely related to man remains unsolved. The fact that effects of different factors such as the chemical form of radionuclides, the neonatal transfer of immunoglobulins and intestinal wall permeability, has not been definitely proved also makes such extrapolation difficult. Because the baboon resembles man more than other animals as regards lifespan, length of gestation, the birth of only one animal at a time, the weaning time, the length of adolescence and the time of sexual maturation, it seems to be the animal species most likely to provide suitable data for extrapolation to human. 21 refs.; 3 figs.; 3 tabs