Relationship between body composition and rapidly exchangeable zinc pool in humans

The rapidly exchangeable zinc (Zn) pool (EZP) is metabolically important. We measured EZP and plasma Zn turnover rate (TR) of humans (5 males and 11 females, normal plasma Zn) after iv administration of ⁶⁷Zn. Inductively coupled plasma-mass spectrometry was used for measurement of isotope ratios. EZP was estimated from the 24-h "truncated" exponential kinetic model and the 24-h spot plasma Zn pool. TR was calculated as a product of the plasma Zn (i.e., central) compartment size and the initial slope of the disappearance curve. Lean body weight was measured by the bioelectrical impedance. Skeletal muscle mass was estimated from the 24-h urinary creatinine excretion using Wang et al's formula (AJCN 1996;63:863). EZP obtained from the 24-h kinetic model and the 24-h spot plasma Zn pool are identical (r= 0.994, p<0.0001). Highly significant correlations were found between EZP and body weight (r=0.925, p<0.0001), lean body weight (r=0.861, p<0.0001) and 24-h urinary creatinine excretion or skeletal muscle mass (r=0.900, p<0.0001). In contrast, correlations between EZP and fat mass (r=0.646, p=0.01) were not so high. These results suggest that the skeletal muscle mass is an important contributor for EZP. TR was correlated well with lean body weight (r=0.882, p<0.0001) and EZP (r=0.865, p<0.0001). Because TR (300 - 600 mg/d) is much higher than the excretion rate of Zn (< 10 mg/d), it is closely identical to the Zn uptake rate by tissues. It appears that we observed the same metabolically active mass both initially (TR) and 24 hours later (EZP) through the behavior of the tracer.