Total true efficiency of zinc utilization: determination and homeostatic dependence upon the zinc supply status in young rats.

Total true efficiency of zinc utilization and its component factors, true absorption and metabolic efficiency, were studied in relation to dietary zinc supply in a 15-day experiment with 36 weanling rats in six groups. After day 3, the initial supply of 18.2 ppm zinc in the diet for all groups was changed to 5.6, 10.6, 18.2, 39, 70 and 141 ppm in groups 1 to 6, respectively. During the 6-day balance period after day 9, zinc intake was proportional to dietary zinc concentration. Mean daily rates of weight gain and apparent zinc retention increased from a near zero level in group 1 to a high plateau from group 4 to 6. True absorption rate steadily rose and reached about twice that of apparent absorption in group 6. This difference in rates was due to endogenous zinc excretion. Percent total utilization and percent true absorption steadily declined from a mean of about 100 in groups 1 and 2 to nearly 15 and 30, respectively, in group 6. Metabolic efficiency, reflecting true retention of absorbed zinc, similarly declined as percent true absorption did. The major conclusion was that not only the absorptive but also the metabolic efficiency greatly contributed to zinc homeostasis and total true efficiency of zinc utilization.