65Zn-labeled tissue zinc for determination of endogenous fecal zinc excretion in growing rats.

Experiments were conducted to study whether selected body tissues other than blood plasma provide suitable endogenous zinc sources to determine endogenous fecal zinc excretion and zinc absorbability by the radioisotope dilution technique. In 16 rats with a mean weight of 172 g on the day of 65Zn injection, the apparent digestibility of zinc (12.8 mg Zn/kg diet dry matter) averaged 72.4 +/- 6.2% during a 6-day balance period starting 9 days postinjection. Based on the specific radioactivity of zinc in plasma after 10 days, two-thirds of the fecal zinc were of endogenous origin and the corresponding zinc absorbability averaged about 91%. Zinc absorbabilities computed on the basis of the specific radioactivity of zinc in the small intestine, pancreas and/or kidneys after 15 days were closely comparable to the mean derived with plasma. Other tissues examined were found less suitable to reflect the specific radioactivity of endogenous fecal zinc.