Gastrointestinal Absorption of Metals

Abstract

Estimating gastrointestinal absorption remains a significant challenge in the risk assessment of metals. This presentation reviews our current understanding of the gastrointestinal absorption of lead (Pb) to illustrate physiological mechanisms involved in metal absorption, new approaches that are being applied to the problem of estimating metal absorption in humans, and issues related to integrating this information into risk assessment. Absorption of metals can be highly variable in human populations because it is influenced by a variety of factors that include the chemical form of the metal, environmental matrix in which the ingested metal is contained, gastrointestinal tract contents, diet, nutritional status, age, and, in some cases, genotype. Thus, in risk assessment models, gastrointestinal absorption is best described as a variable whose distribution is determined in part by the above multiple influences. Although we cannot expect to evaluate empirically each of the above factors in human populations, we can expect to achieve a sufficiently detailed understanding of absorption mechanisms to develop conceptual and, eventually, quantitative models of absorption that account for some aspects of individual variability. A conceptual model is presented of the physiological processes involved in the transfer of ingested metals from the lumen of the gastrointestinal tract to the blood circulation. Components of the model include delivery to the site(s) of absorption; distribution among intracellular and extracellular ligands and transcellular and paracellular pathways of transfer across the gastrointestinal tract epithelium. The gastrointestinal absorption of Pb is discussed in the context of this model.

Copper and Lead

“…The bioavailability of Cu from the diet is about 65-70 % depending on a variety of factors including chemical form, interaction with other metals, and dietary components. The biological half-life of Cu from the diet is 13-33 days, with biliary excretion being the major route of elimination [150].…”

Bioavailability: Exposure, Dose and Risk Assessment

“…Estimating bioavailability in human and animal populations has been reviewed by Diamond et al [71]. Balance studies measure daily intake and excretion of a specific contaminant over periods of days or weeks.…”

Elements and Their Compounds in Indoor Environments

“…A major source of uncertainty is the estimation of gastrointestinal absorption (Diamond et al, 1998). Complex interactions between ingested particles and gut physiological processes can either increase or decrease metal bioavailability (see Mielke and Heneghan, 1991).…”

“…Complex interactions between ingested particles and gut physiological processes can either increase or decrease metal bioavailability (see Mielke and Heneghan, 1991). Absorption of metals in humans is influenced by a variety of factors including the exposure pathway, the chemical form of the metal, the matrix composition, the age of the host, contents of the gastrointestinal tract, temporal pattern of meal consumption, diet, and nutritional status (Diamond et al, 1998;Fishbein, 1991). Recent reviews indicate that no single in vitro test has emerged as the acceptable choice for estimating metal absorption in either the gastrointestinal tract (Diamond et al, 1998;Mushak, 1998;Canady et al, 1997;Ruby et al, 1999) or the respiratory system (Ansoborlo et al, 1999).…”

“…Absorption of metals in humans is influenced by a variety of factors including the exposure pathway, the chemical form of the metal, the matrix composition, the age of the host, contents of the gastrointestinal tract, temporal pattern of meal consumption, diet, and nutritional status (Diamond et al, 1998;Fishbein, 1991). Recent reviews indicate that no single in vitro test has emerged as the acceptable choice for estimating metal absorption in either the gastrointestinal tract (Diamond et al, 1998;Mushak, 1998;Canady et al, 1997;Ruby et al, 1999) or the respiratory system (Ansoborlo et al, 1999). Appropriate animal models, combined with detailed knowledge of the sample mineralogy and chemical speciation, are generally recommended for reliable estimates of metal bioavailability in a given matrix (Mushak, 1998).…”