An experimentally-determined value for any pre-absorptive dissociation of zinc from ZnAA is necessary for the accurate compartmental modeling of zinc metabolism when provided as ZnAA compared with zinc chloride, which the investigators will perform in a future study.~The current study will allow us to determine the pre-absorptive dissociation of zinc from ZnAA, while serving as a pilot test of a novel technique to determine for the first time an individual's zinc transport kinetics. Since zinc absorption kinetics follow a saturable process, the zinc transport maximum (Tr_max) and transport rate (K_Tr) can be determined by fitting Hill transport equation to measurements of absorbed zinc at 3 or more levels of zinc intake (mg zinc/d):~Total Absorbed Zinc (TAZ) = (Zinc Intake * Tr_max) / (Zinc Intake + K_Tr)~Although population data have been used to estimate zinc transport kinetics (Tran, et al. AJCN 2004), Tr_max and K_tr have never been determined in individual subjects. The investigators plan to determine the fractional zinc absorption (FZA) from multiple levels of dietary zinc in rapid succession (Chung, et al. AJCN 2004) and, therefore, estimate an individual's transport maximum and rate, Tr_max and K_Tr. Once Tr_max and K_Tr are known, the FZA can be used to determine an unknown zinc intake from zinc chloride and any zinc dissociated from the ZnAA complex using the following equation:~Zinc Intake = (Tr_max / FZA) - K_Tr~Since zinc that dissociates from the ZnAA complex will compete for absorption with the inorganic zinc stable isotopic oral tracer (70-zinc chloride), reducing the amount of that tracer absorbed, the investigators can therefore determine the total zinc intake (the sum of the known zinc in the controlled study diet and the unknown quantity of zinc dissociated from ZnAA pre-absorption) based on the transport kinetics (FZA, Tr_max and KTr).~In other words, for the meal with added ZnAA, the zinc intake is the sum of the zinc in the test meal (3.6 mg zinc and 0.4 mg 70-Zn tracer) and the zinc that dissociates from the ZnAA complex pre-absorption. Since the intact ZnAA (i.e. that zinc that does not dissociate pre-absorption from the ZnAA complex) is absorbed by a different transport mechanism (Sauer, et al. Biometals 2017), it does not compete with the stable isotope tracer for absorption via zinc cation transporters and is, therefore, not part of the zinc intake for the purpose of these calculations.~If the hypothesized 20% dissociation is correct (i.e. 3 mg zinc dissociated from an initial 15 mg ZnAA), the total amount of ionic zinc will be 7 mg (the 3.6 mg base diet, 0.4 mg zinc stable isotope tracer, and 3 mg dissociated zinc), and the FZA will therefore be the same as that from a 7 mg test diet (the 3.6 mg base diet, 0.4 mg zinc tracer, and 3 mg zinc as zinc chloride).
Zinc may be absorbed from diet via zinc transporter mediated pathways, or, when coupled with amino acids, via amino acid transporter pathways. When zinc is coupled with amino acids in diet, it may dissociate from those amino acids in the acidic environment of the stomach prior to entering the small intestine. An experimentally-determined value for any pre-absorptive dissociation of zinc from a zinc amino acid complex (ZnAA) is necessary for the accurate compartmental modeling of zinc metabolism when provided as ZnAA compared with ionic zinc, which the investigators will perform in a future study. The current study will allow us to determine the dissociation of zinc from ZnAA, while serving as a pilot test of a novel technique to determine for the first time an individual's zinc transport kinetics.