A Review of Mercury Bioavailability in Humans and Fish

Abstract

To estimate human exposure to methylmercury (MeHg), risk assessors often assume 95%-100% bioavailability in their models. However, recent research suggests that assuming all, or most, of the ingested mercury (Hg) is absorbed into systemic circulation may be erroneous. The objective of this paper is to review and discuss the available state of knowledge concerning the assimilation or bioavailability of Hg in fish and humans. In fish, this meant reviewing studies on assimilation efficiency, that is the difference between ingested and excreted Hg over a given period of time. In humans, this meant reviewing studies that mostly investigated bioaccessibility (digestive processes) rather than bioavailability (cumulative digestive + absorptive processes), although studies incorporating absorption for a fuller picture of bioavailability were also included where possible. The outcome of this review shows that in a variety of organisms and experimental models that Hg bioavailability and assimilation is less than 100%. Specifically, 25 studies on fish were reviewed, and assimilation efficiencies ranged from 10% to 100% for MeHg and from 2% to 51% for Hg(II). For humans, 20 studies were reviewed with bioaccessibility estimates ranging from 2% to 100% for MeHg and 0.2% to 94% for Hg(II). The overall absorption estimates ranged from 12% to 79% for MeHg and 49% to 69% for Hg(II), and were consistently less than 100%. For both fish and humans, a number of cases are discussed in which factors (e.g., Hg source, cooking methods, nutrients) are shown to affect Hg bioavailability. The summaries presented here challenge a widely-held assumption in the Hg risk assessment field, and the paper discusses possible ways forward for the field.

Keywords: assimilation; bioaccessibility; biological availability; biological transport; cooking; gastrointestinal tract; inorganic mercury; methylmercury; seafood.

Figure 1

Model for methylmercury (MeHg) and inorganic mercury (Hg(II)) uptake across fish and human intestinal epithelial cells. Exposure to Hg(II) and MeHg occurs mainly through diet. Hg in both water and biological systems is bound to ligands (“X”). MeHg in fish muscle is predominately stored as MeHg-Cys [14], which enters intestinal epithelial cells through energy dependent L-type neutral amino acid transporters (LAT). MeHgCl may also enter the cell by diffusion, or by non-specific active uptake mechanisms of MeHg complexes (not shown). MeHg-Cys dominates when complexing amino acids are present [15]. Uptake of Hg(II) is presented as the model proposed by Hoyle and Handy [16]. Hg(II) may enter through voltage-gated Na⁺ or Ca²⁺ channels, through the Na⁺K⁺2Cl⁻ cotransporter, and/or by diffusion of HgCl₂. Uptake of anionic Hg complexes (HgCl₄²⁻) may be possible by anionic exchange (not shown). The mucus lining the gut has a high affinity for Hg(II) ions, thus limiting uptake [17]. Many of the same mechanisms occur in the human gut following MeHg ingestion through consumption of fish, with LAT playing a prominent role in transporting MeHg-Cys, while other peptide transporters and the organic anion transporter (OAT) may also contribute to uptake of MeHg-Cys (as reviewed by Bridges and Zalups [18]) Arrows are indicative of direction of transport.

Figure 2

Summary of studies on AE of Hg into fish for (a) MeHg and (b) Hg(II). In both, each column represents a single species of fish, fed a single type of food. The numbers along the Y-axis indicate studies, as outlined in Table S2.

Figure 2

Summary of studies on AE of Hg into fish for (a) MeHg and (b) Hg(II). In both, each column represents a single species of fish, fed a single type of food. The numbers along the Y-axis indicate studies, as outlined in Table S2.

Figure 3

Summary of in vitro studies on bioaccessibility of Hg to humans from fish consumption for (a) MeHg and (b) total Hg. In both, each column represents a single species of fish, prepared and/or packaged in a particular way. The numbers along the Y-axis indicate studies, as outlined in Table S4.

Figure 3

Summary of in vitro studies on bioaccessibility of Hg to humans from fish consumption for (a) MeHg and (b) total Hg. In both, each column represents a single species of fish, prepared and/or packaged in a particular way. The numbers along the Y-axis indicate studies, as outlined in Table S4.

Figure 4

Summary of in vitro studies on absorption of Hg to human intestinal cells from fish consumption for (a) MeHg and (b) total Hg. In both, each column represents a single species of fish, prepared and/or packaged in a particular way, and introduced to either a monoculture of Caco-2 (intestinal epithelial-like cells) or a co-culture of Caco-2 and HT29-MTX (mucin-producing cells analogous to goblet cells). The numbers along the Y-axis indicate studies, as outlined in Table S4.