Exchangeable zinc pool size in infants is related to key variables of zinc homeostasis

The exchangeable zinc pool (EZP) is the sum of the combined pools that exchange with zinc in the plasma within 48-72 h and is thought to be critical for zinc-dependent biological processes. The size of the EZP in adults has been found to be positively related to dietary zinc intake, daily absorbed zinc and fecal excretion of endogenous zinc. In this study, we examine data on EZP size in relation to variables of zinc homeostasis in normal infants on different feeding regimens. Data from 45 male infants who participated in one of four different studies contribute to the analysis. The feeding regimens include exclusive breastfeeding (n = 9; age, 2-5 mo); breastfeeding plus modest supplementation with cow's milk-based formula (n = 16; age, 3-4 mo); exclusive formula feeding (n = 4; age, 3-4 mo) and exclusive breastfeeding plus complementary foods (n = 16; age, 7 mo). Fractional absorption was determined by fecal monitoring after oral administration of zinc-stable isotopes. Urine enrichment 4-8 d posttracer was used to determine endogenous fecal zinc (7-mo-old infants excepted) and EZP size. Univariate correlations and multivariate regression analyses were performed between EZP and age, weight, dietary zinc intake, fractional absorption, total absorbed zinc and endogenous fecal zinc. Results include no significant relationship between EZP size and age, body weight or fractional absorption but a positive relationship with daily absorbed zinc and endogenous fecal zinc excretion. We conclude that the amount of absorbed zinc is not tightly regulated, and that endogenous fecal zinc is dependent on and responsive to the zinc status of the organism.