A compartmental model of zinc kinetics in children

BACKGROUND: Zinc deficiency may be an important cause of childhood growth failure and morbidity. However, little information is available on zinc absorption and metabolism in children due to the lack of kinetic data in this population. OBJECTIVE: The objective of this study was to evaluate the applicability to healthy children of a multi-compartmental model that recently has been used to describe zinc kinetics ir adults [Lowe et al, Am.J.Clin.Nutr. 1997; 65:1810-9]. This model consists of a central plasma compartment with absorption from, and endogenous secretion into, a gut compartment; bi-directional fluxes to two kinetically distinct tissue compartments; and additional losses in urine and to a slowly turning over tissue compartment. METHODS: Two subjects, prepubertal Caucasian girls aged 10.9 yr and 11.1 yr were studied. Each subject received 1.0 mg of an oral tracer mixed with milk, and 0.5 mg of an intravenous tracer highly enriched in zinc-67 and zinc-70 respectively. Blood, urine and stool samples were collected for 6 days. Stable isotope enrichments were measured by thermal ionization mass spectrometry and converted to tracer:tracee ratios (for plasma data) and cumulative losses (for urine and stool data). Kinetic data were fitted using the SAAM II software package (SAAM Institute, Seattle, WA). RESULTS: The model fitted the data well. All compartment masses and fluxes were estimated with a coefficient of variation of ≤ 20%. Zinc absorption from milk, estimated from model kinetics, was low at 8.1% (95% CI 7.6%-8.6%) and 16.3% (95% CI 14.7%-17.8%), respectively. Endogenous zinc secretion was 0.39 mg/d and 1.55 mg/d, respectively. The mass of the central plasma compartment was 2.48 mg/kg and 1.16 mg/kg, respectively, similar to body-weight adjusted values for adults (2.02 mg/kg). CONCLUSIONS: (1) The model adequately fits zinc kinetic data in children with good precision; (2) Zinc absorption from milk may be lower than previously described for aqueous solutions.