Studies on the effects of dietary zinc dose on 65 Zn absorption in vivo and on the effects of Zn status on 65 Zn absorption and body loss in young rats

1. Weanling male rats were maintained on diets containing 5, 10, 20, 40, 80 or 160 mg zinc/kg for 14 d. On day 15 they received 65 Zn either by intraperitoneal injection or in a test meal containing 20 mg Zn/kg. After dosing, the rats were again maintained on the diets they had received previously. 2. Whole-body 65 Zn retention was measured immediately after dosing and daily for a further 9 d. From regression analysis of the semi-logarithmic plots of 65 Zn retention from 0 to 192 h after 65 Zn administration, the true extent of 65 Zn absorption and the biological half-life (t1/2) of body 65 Zn stores were calculated. 3. At the end of the experiment, the rats were killed and the entire small intestines of some rats from each group were rapidly flushed out to remove food and faecal residues, frozen in liquid nitrogen and stored under an atmosphere of N 2 at –20° before separation of cytosolic Zn-binding fractions by gel filtration on Sephadex G–75. 4. The results suggest that rats which received diets that were either deficient(5 mg Zn/kg), marginal (10 mg Zn/kg) or adequate (20–80 mg Zn/kg) in Zn achieved homeostatic regulation of body Zn by changes in both the extent of Zn absorption and excretion. However, when Zn supply was excessive, increasing from 80 to 160 mg Zn/kg, no further changes were seen in Zn absorption, and homeostatic control appeared to be effected entirely by changes in rates of body Zn loss. 5. Gel chromatography of intestinal cytosol on Sephadex G-75 revealed that Zn was associated with two major fractions. The first (peak 1) had a molecular weight (MW) > 75 kdaltons and the second (peak 2), a MW of approximately 10 kdaltons and was assumed to be metallothionein. 6. There was no obvious relation between the amount of Zn bound to peak 1 and dietary Zn content. In contrast, the amount of Zn recovered in peak 2 increased linearly with increasing dietary Zn content. 7. Comparisons between the effect of dietary Zn content on Zn bound to peak 2 and 65 Zn retention may, depending on the range of Zn intakes, indicate possible roles for intestinal metallothionein in the control of Zn absorption or excretion. 8. A study of the effects of dietary dose of 65 Zn on the extent of 65 Zn absorption in rats of normal Zn status indicated a possible biphasic relation. At low doses (5–40 mg Zn/kg) 65 Zn absorption appeared to exhibit a curvilinear response to increasing 65 Zn dose, indicating possibly a saturable process. At higher doses (40–160 mg Zn/kg)