Feasibility studies in rats fed heavy metals as multiple nutrient markers.

The nitrates of five metals (cerium, terbium, ytterbium, lutetium, and iridium) were fed to rats to determine the feasibility of their use as nonabsorbed, multiple markers for recovery, passage, and indirect apparant digestibility studies. Fecal recovery of a single oral dose was completed within 72 hours. When the salts were mixed into the diet, 48-96 hours was required to establish a steady-state concentration of markers in feces. The diurnal variation of cerium in feces was found to be considerable when it was fed twice daily as a single dose prior to each feeding. When incorporated into the diet, negligible diurnal variation in fecal concentration was noted with lutetium, and small variation was seen with other metals. In nutrient apparent digestibility studies, good agreement was generally found between direct and indirect multiple marker methods. Experiments with a daily intake marker suggest that cerium was not satisfactory as a multiple marker in which neutron activation analysis was the method of determination.