Effects of oral zinc loading on zinc metabolism in humans--I: Experimental studies

The effects of oral zinc on distribution, retention and excretion of orally administered 65Zn were studied in 50 patients with taste and smell dysfunction. The study was conducted in three phases. In the first phase all patients were studied for 21 days after receiving 3-18 microCi of 65Zn as ZnCl2 orally after an overnight fast. In the second phase, started after 21 days and continued for 290 to 440 (mean 336) days, all 50 patients received placebo for ZnSO4. In the third phase 14 patients continued on placebo while 36 received ZnSO4 (100 mg/day Zn++) for 112 to 440 (mean 307) days. Phases two and three were a controlled clinical trial of the effects of zinc on retention of 65Zn tracer. Total body retention and activity in plasma and red cells were measured for all patients throughout the study. Ten of the 36 patients treated with ZnSO4 had additional measurements of 65Zn activity in liver and thigh made using external detectors. Total body retention during the second phase placebo period was not significantly different (p greater than 0.25) for the 36 subjects subsequently treated with ZnSO4 (biological half-time (Tb) 378 +/- 12 days) (mean +/- SEM) and the 14 who were continued on placebo through the third phase of the study (Tb = 384 +/- 8 days). During the third phase patients receiving ZnSO4 showed an accelerated loss of total body 65Zn (Tb = 235 +/- 8 days) which was significantly different (p greater than 0.001) from half-time values during placebo treatment. Accelerated loss of 65Zn from the thigh was apparent immediately while that from the liver began after a mean delay of 107 days. There was no apparent effect of zinc on loss of mean 65Zn activity from red blood cells.